stackexchange_physics_asteroids.jsonl

{"unit_id": "stackexchange:physics.stackexchange:asteroids:73654:0000", "text": "Question: Why are there no asteroids or meteoroids with relativistic speeds?\n\nCosmic rays can have energies going into the $10^{20}$ eV domain. [Asteroids](http://en.wikipedia.org/wiki/Asteroid) and [meteoroids](http://en.wikipedia.org/wiki/Meteoroid) originating in the solar system are probably limited in their speed because they all started out from the same lump of matter having more or less the same speed, but what about rocks from other galaxies ? Could they reach relativistic speeds in relation to the solar system? Are there astronomical events that could be interpreted as a collision between such an asteroid and a planet or star?\r\n\r\nJust a mole of hydrogen atoms with that kind of energy would have three times the energy of the Chicxulub impact event, so I hope there's a reason for them not existing!\n\nAccepted Answer:\n\nFirst, the speed of other galaxies isn't too helpful. For example, the radial velocity of the Andromeda galaxy relatively to us is 300 km/s, i.e. 0.1% of the speed of light only. Moreover, internally, everything in that galaxy moves by pretty much the same speed and is confined to the vicinity of that galaxy which makes us pretty sure that no piece will reach us before Andromeda galaxy will.\r\n\r\n**More importantly, macroscopic systems in outer space aren't moving with the same huge speeds near the speed of light as the cosmic rays essentially because of the second law of thermodynamics.**\r\n\r\nWhen cosmic rays are accelerated to high speeds, we may treat them statistically and the high energy of these elementary particles may be assigned a high temperature. But the physical systems with many degrees of freedom prefer to evolve to the most likely, high-entropy configurations. That's why the excess energy (e.g. in a supernova) tends to divide between the elementary particles chaotically. \r\n\r\nIn particular, the individual particles' kinetic energy results from velocities that have a random direction. At these huge temperatures (kinetic energies per particle), the atoms are unbound (well above the ionization energy) and macroscopic matter doesn't exist. So the likelihood that a large object will move towards the Earth at a near-luminal velocity is as unlikely as the possibility that the numerous atoms in the large objects are assigned velocities with the exactly equal direction although the directions are being chosen from an isotropic, random distribution. \r\n\r\nAfter some time for \"thermalization\" (interactions between atoms are allowed to change the system with the conservation laws' being the only absolutely constraints), the greater object you consider, the less likely it is that all the atoms in that object will be doing the same thing. This is a form of the second law of thermodynamics.\r\n\r\nThe previous paragraph prevents the creation of \"coherent macroscopic cosmic rays\", macroscopic objects that would move in the same direction, from the thermal chaos of hot environments such as the supernova. But even if some astrophysical process managed to eject a chunk of matter at these high speeds, the previous paragraph will still guarantee that we won't receive it on Earth. Instead, the individual atom of that speedy object would still have some residual mutual velocities so the object would split into individual atoms and we would observe cosmic rays only once again.\r\n\r\nI could summarize the situation in this way: to shoot a large object by a huge speed from a very distant celestial body to the Earth requires one to have the same and huge radial velocities of all atoms but almost vanishing relative transverse velocities. The likelihood of that goes to zero exponentially in any thermal environment.\r\n\r\nIf one assumes that the source of the initial speed is not thermal, then one must accept that the projectiles will derive their speed from their broader environments – speeds of astrophysical bodies that already exist – and those are simply of order 0.1 percent of the speed of light as in the Andromeda example or lower. These modest speeds boil down to the inhomogeneities during the structure formation and, ultimately, to inflation, or to the planetary speeds derivable for a star. Whenever you want to locally violate the modesty of the speeds, e.g. by a gravitational collapse, you don't \"shoot\" any new particles before the collision and when the collision happens, the excess energy of the collision also inevitably creates a high temperature and we're back to the previous paragraph.\r\n\r\nSo near-luminal macroscopic bodies aren't observed. I would even go further and despite my being a believer that life in the Universe is extremely rare, I would say that an object moving by a near-luminal speed would prove the existence of an advanced civilization. I should be a bit careful: the [gravitational slingshot][1] is a process that allows the speed to be enhanced even naturally. But even if the source of the speed were a gravitational slingshot and the speed would be really high, like 99.9999% of the speed of light, chances would be high that some intelligence was behind the optimization of the gravitational slingshot because it's extremely unlikely for such an outcome to occur naturally.\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Gravity_assist\n\nAnswer (score=19):\n\nWhy don't we observe any relativistic asteroids? \r\n\r\nThe answer to this question would not be complete without mentioning the [virial theorem][1]. \r\n\r\nConsidering our galaxy as a system of $N$ gravitating objects, according to the virial theorem, twice the average total kinetic energy of all objects, plus the average total potential energy of these objects, adds up to zero. In simple terms this means that **on average all objects tend to move with speeds of about 0.7 times their escape velocity**. \r\n\r\nAs we, and any objects in our proximity, are bound to the Milky Way with an escape velocity of about 550 km/s, typical kinetic velocities have values of roughly 390 km/s. That is much smaller than the speed of light that is close to 300,000 km/s. Moreover, everything around us moves roughly in the same direction, following a circular flow pattern around the core of the Milky Way. So relative velocities between objects that encounter each other are typically smaller than the viral theorem suggests. Asteroids tend to hit earth's atmosphere with speeds of about 20 km/s. The [fastest impact ever observed][2] occurred at less than 30 km/s, or less than 0.01 % of the speed of light.\r\n\r\nIn principle you can consider the Milky Way and galaxies like Andromeda that are gravitationally bound to each other (the [local group of galaxies][3]) also as one system to which the viral theorem can be applied. This gives only slight changes in the gravitational binding, and hence slight changes in the average kinetic energy.\r\n\r\nBeyond the local group, galaxies and all objects therein are speeding away from us due to the Hubble expansion. \r\n \r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Virial_theorem\r\n  [2]: http://www.nature.com/news/us-meteorite-was-fastest-on-record-1.12095\r\n  [3]: http://en.wikipedia.org/wiki/Local_Group\n\nAnswer (score=1):\n\nRelativistic rocks from other galaxies are in all probability out there.  They'll never hit us, though, because they're far away and heading in the wrong direction, just like everything else we can see that moves at those velocities.\n\nAnswer (score=1):\n\nA macroscopic body at over 900 km/s is a very unlikely event in the Solar system (may I skip explanations?), but an astronomic body “happy” enough to collide with a neutron star will certainly impact the surface at very high speed (values depend on mass of the star). BTW, relativistic collisions between neutron stars, or of a neutron star and a black hole, are a long-researched topic in numerical general relativity.\r\n\r\nIn spite of neutron star’s strong gravity, I do not expect so many bodies with high chances to crash into the star to make such impacts a frequent event. There should be few “relativistic asteroids” in the system of a neutron star at all. Planetary systems around neutron stars are known (see https://en.wikipedia.org/wiki/PSR_B1257%2B12 ) although rare, but even if there are many rocky bodies orbiting the star, most of them should have pericenters too far from the star, whereas a close encounter is necessary to gain a relativistic speed. I do not know typical conditions to infer how quickly can such orbit decay to a lower orbit with higher speed.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 0, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "special-relativity", "astronomy", "meteorites", "meteoroids"], "page_start": null, "page_end": null, "quality_flags": [], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 73654, "title": "Why are there no asteroids or meteoroids with relativistic speeds?", "url": "https://physics.stackexchange.com/questions/73654/why-are-there-no-asteroids-or-meteoroids-with-relativistic-speeds", "share_url": "https://physics.stackexchange.com/q/73654", "content_license": null, "owner": {"display_name": "yippy_yay", "user_id": 27232, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/27232/yippy-yay"}}, "answers": [{"answer_id": 73656, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/73654/why-are-there-no-asteroids-or-meteoroids-with-relativistic-speeds/73656#73656", "share_url": "https://physics.stackexchange.com/a/73656", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Luboš Motl", "user_id": 1236, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1236/lubo%c5%a1-motl"}}, {"answer_id": 73665, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/73654/why-are-there-no-asteroids-or-meteoroids-with-relativistic-speeds/73665#73665", "share_url": "https://physics.stackexchange.com/a/73665", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Johannes", "user_id": 1268, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1268/johannes"}}, {"answer_id": 73689, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/73654/why-are-there-no-asteroids-or-meteoroids-with-relativistic-speeds/73689#73689", "share_url": "https://physics.stackexchange.com/a/73689", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Loren Pechtel", "user_id": 20467, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/20467/loren-pechtel"}}, {"answer_id": 132207, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/73654/why-are-there-no-asteroids-or-meteoroids-with-relativistic-speeds/132207#132207", "share_url": "https://physics.stackexchange.com/a/132207", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Incnis Mrsi", "user_id": 56960, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/56960/incnis-mrsi"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:144406:0000", "text": "Question: What is the speed of the fastest moving body in our solar system?\n\nOn [Wikipedia](http://en.wikipedia.org/wiki/Speed#Examples_of_different_speeds) I saw that the average orbital speed of planet Earth around the Sun is a whopping $29 783\\text{ m/s}$, and it made me wonder are there bodies (planets, meteorites, asteroids) that move faster? \r\n\r\nMy question is not about small photons or other (small-ish) particles and their speed (speed of light), or even about [solar winds](http://en.wikipedia.org/wiki/Solar_wind#Components_and_speed) ($750\\text{ km/s}$) but about meteorites, planets or other materials and their speed around the sun or other fixed point.\n\nAccepted Answer:\n\nA comet doesn't need to *impact* the sun in order to come very close to solar escape velocity at perihelion. There is a class of comets known as [sungrazers](http://en.wikipedia.org/wiki/Sungrazing_comet) that pass very close to the sun. Although small ones evaporate on their first pass near the sun, larger ones can survive several orbits, and be considered periodic comets.\r\n\r\nThere is a class of sungrazing comets called the [Kreutz family](http://en.wikipedia.org/wiki/Kreutz_Sungrazers) that has a very low perihelion and a reasonably high aphelion (150-200 AU) making them the best candidates that I know of for \"fastest object in the solar system\" when they pass near the sun. The comet [Lovejoy (C/2011 W3)](http://en.wikipedia.org/wiki/C/2011_W3_\\(Lovejoy\\)) has an aphelion around 157 AU and a perihelion of 0.00555 AU (within the solar corona, note that the sun itself has a photosphere radius of 0.00465 AU!). As such it passed by the sun in December 2011 at a speed of 536 km/s, within a couple percent of the escape velocity at that height, which is 565 km/s. The [Great Comet of 1843](http://en.wikipedia.org/wiki/Great_Comet_of_1843), another Kreutz-family comet, reportedly passed even lower without disintegrating, 0.00546 AU, giving it a speed of 570 km/s.\r\n\r\nPulsar did a fine job of working out the math, so I won't duplicate it here, except to emphasize the point that once your aphelion is tens of thousands of times higher than your perihelion, aphelion stops making much of a difference. If you're 100km above the surface of the sun and travelling at hundreds of km/s, the difference between the speed you need to go to get 100 AU out, and the speed you need to go to get 1000 AU out, is miniscule, and both are very close to escape velocity.\n\nAnswer (score=47):\n\nThe maximum speed of an object that orbits the Sun at a certain distance $r$ is known as the [escape velocity](http://en.wikipedia.org/wiki/Escape_velocity):\r\n$$\r\nv_\\text{esc} = \\sqrt{\\frac{2GM_\\odot}{r}},\r\n$$\r\nwhere $M_\\odot$ is the mass of the Sun. If the object would have a greater speed, it would eventually leave the solar system. So I'd say that the absolute maximum possible speed of any object in the solar system would be the escape velocity at the radius of the Sun $R_\\odot$:\r\n$$\r\nv_\\max = \\sqrt{\\frac{2GM_\\odot}{R_\\odot}},\r\n$$\r\nwhich, as you can find in the wiki article, is $617.5\\;\\text{km/s}$. A comet that slams into the Sun, which occasionally happens, would have a speed close to this maximum. Alas, it's also the last speed it'll have before it meets its doom :-)\r\n\r\n----------\r\n**Update**\r\n\r\nIf you want to know the fastest object in the solar system that didn't crash into the Sun, then the best candidates are [sungrazing comets](http://en.wikipedia.org/wiki/Sungrazing_comet), i.e. comets with very eccentric orbits that pass very close to the Sun. One particular group are the [Kreutz Sungrazers](http://en.wikipedia.org/wiki/Kreutz_Sungrazers). The comet C/2011 W3 (Lovejoy) mentioned by hobbs in the comments belongs to this group, but there was another of these comets that passed the Sun even closer: [the Great Comet of 1843](http://en.wikipedia.org/wiki/Great_Comet_of_1843). \r\n\r\nThis comet has a perihelion of only 0.005460 AU (where 1 [Astronomical Unit](http://en.wikipedia.org/wiki/Astronomical_unit) is 149 597 871 km). This means it came to within less than 121 000 km of the surface of the Sun, and amazingly it survived (most comets break up when they come this close). So what is its velocity at perihelion?\r\n\r\nThe general formula is (see [this link](http://en.wikipedia.org/wiki/Apsis#Mathematical_formulae))\r\n$$\r\nv_p = \\sqrt{\\frac{\\mu}{a}\\frac{1+e}{1-e}},\r\n$$\r\nwith\r\n$$\r\na = \\frac{r_p + r_a}{2}\r\n$$\r\nthe semi-major axis, $r_p$ and $r_a$ the peri- and aphelion, \r\n$$\r\ne = \\frac{r_a-r_p}{r_a+r_p}\r\n$$\r\nthe eccentricity, and $\\mu = GM_\\odot$ the [standard gravitational parameter](http://en.wikipedia.org/wiki/Standard_gravitational_parameter) of the Sun. So we can rewrite this as\r\n$$\r\nv_p = \\sqrt{\\frac{2GM_\\odot}{r_p}\\left(\\frac{r_a}{r_a+r_p}\\right)}.\r\n$$\r\nAs you can see, this reduces indeed to the formula for the escape velocity if $r_a$ goes to infinity. For our comet, $r_p = 0.005460$ AU and $r_a = 156$ AU, and we find\r\n$$\r\nv_p = 570\\;\\text{km/s}.\r\n$$\n\nAnswer (score=12):\n\nWhen there aren't comets falling into the sun, Mercury is hard to beat.  [This NASA fact sheet][1] lists Mercury's orbital velocity around the sun as varying from $38.86$ to $58.98$ km/sec, not so much greater than Earth (less than a factor $2$, even at maximum).\r\n\r\n\r\n  [1]: http://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html\n\nAnswer (score=5):\n\nThe asteroid \"1566 Icarus\" has a perihelion distance of 0.187 au and a semi-major axis of $a=1.078$ au, an orbital period of 1.119 years and eccentricity $e=0.827$.\r\n\r\nUsing\r\n$$v_{\\rm peri} = \\sqrt{\\frac{GM}{a}\\frac{(1+e)}{(1-e)}},$$\r\nwhere $M$ is a solar mass, then its fastest speed is 93.5 km/s.\r\n\r\nSo this does not come close to Comet Lovejoy (mentioned in other comments), but beats Mercury, and is perhaps the fastest object we can continue to study on a regular basis, since Comet Lovejoy disintegrated. Doubtless there will be other small chunks of rock that might beat this.\n\nAnswer (score=2):\n\nKepler's Three Laws of Planetary Motion are particularly helpful when addressing this question. They state that (in informal language)\r\n\r\n 1. The shape of a planet's orbit in an ellipse, with the Sun at one focus of the ellipse.\r\n 2. As planets move around their elliptical orbits, the imaginary line drawn from the planet to the Sun sweeps out equal regions of equal area in equal amounts of time.\r\n 3. The square of the period of a planets orbit, $T^2$ is equal to the cube of planet's orbit's semi-major axis (a^3)\r\n\r\nAlthough not immediately obvious, Laws 2 and 3 combined both imply that as a satellite (planet, asteroid, comet, or otherwise) approaches closer to the sun, it can be expected to have a faster velocity.\r\n\r\nSpecifically, if we look just at the eight planets, and Law 3, $$T^2\\propto a^3$$ which, when solved for period states that $$T\\propto \\sqrt{a^3}$$   So using the equation above, let's say planet $A$ travels in some orbit around the sun, and the semi-major axis has a length of $a$. If planet $B$ travels in and orbit, with a semi-major axis of $4a$ then the period has now increased by a factor of 8, even though the semimajor axis (and approximately the circumference, if the orbit has an eccentricity close to 0) only increased by a factor of 4. So as you move away from the sun, your period increases more than your distance, which means your orbital velocity is decreasing.  Just look at this graph below, taken from enchantedlearning.com.\r\n\r\n![enter image description here][1]\r\n\r\nYou can see a clear relationship between velocity, and distance away from the sun.\r\n\r\nNow let's look at interlopers to our solar system, like comets.  Compared to planets, most comets tend to have eccentricities very close to 1 (which means their orbits are very elliptical).  Some comets even have eccentricities greater than one, which means they're on one-time hyperbolic orbits around the sun.  As these comets approach perihelion (the close approach to the Sun) Kepler's Second Law tells us that thevelocity of the satellite increases.  The most extreme examples are sun-grazing comets, which have very close approaches to the Sun.  In fact, comet ISON was moving so quickly last November when it approached perihelion that had a) you been able to see the comet in daylight and b) Coment ISON not met an untimely demise you would have actually seen it change position in the sky (relative to background starts) _by the hour_.\r\n\r\n\r\n  [1]: https://i.sstatic.net/sqs2o.gif\n\nAnswer (score=1):\n\nThe fastest moving object that does not get destroyed by crashing into the sun would be the apollo asteroids that get very close to the sun.  For example Icarus gets going pretty fast at perihelion, (0.18665203 AU from the sun) at just under 100 km/sec.\n\nAnswer (score=1):\n\nThis question has received some excellent responses. As the person asking seems keen to get a larger variety of responses, I am going to give this question another twist by enquiring about the maximum speed relative to Earth:\r\n\r\n> Earth is a planet, which means it cleans its orbit around Sun \r\n> from material objects. What is the maximum speed at which such \r\n> object can hit earth's atmosphere?\r\n\r\nEarth's orbit around Sun is very close to circular. Equating the centripetal force required to keep Earth in this orbit to the gravitational force exerted by Sun, it follows that Earth orbits Sun with a kinetic energy equal to half the energy needed to escape Sun. \r\n\r\nAn object that orbits Sun along an extremely elongated elliptical path and reaches closest approach to Sun somewhere along Earth's path, has at that point (perihelion) a kinetic energy equal to the energy needed to escape from Sun. \r\n\r\nAs kinetic energy scales quadratically with speed, it follows that Earth's speed along it's orbit around Sun equals $\\frac12\\sqrt2$ times the local escape velocity. This escape velocity, the  velocity required to escape from a location along Earth's orbit around Sun, equates to a marathon (a wee bit more than 42 km) per second. It follows that Earth orbits Sun at a speed of 29.8 km/s. \r\n\r\nIf at closest approach the object moves in opposite direction to Earth, collision will be head-on and one has to add both speeds to get the total speed. This total speed equals 71.9 km/s.\r\n\r\nThis, however, does not equate to the speed at impact, as the gravitational attraction to Earth accelerates the object towards impact. So, to arrive at the speed at impact we have to add Earth's escape velocity (11.2 km/s) to the above derived velocity. \r\n\r\n**The resulting maximum velocity at impact is 83.1 km/s. Solar system objects can not hit us at larger speed.**\n\nAnswer (score=0):\n\nDepending on what you are looking for, here are some possible candidates for the fastest bodies in the solar system:\r\n\r\n 1. Comets from outside the solar system that fall into the Sun, just\r\n    before impact\r\n 2. Comets with a periodic elliptical orbit around the sun, at the\r\n    moment of their closest approach to the sun\r\n 3. Mercury, with a mean orbital velocity of 47.9 km/sec\r\n 4. Metis (the inner-most moon of Jupiter), with a mean orbital velocity\r\n    of 31.6 km/sec\r\n 5. Since Metis orbits within Jupiter's main ring, one can assume some\r\n    of the particles of the ring that are closer to Jupiter have a\r\n    higher orbital velocity than Metis\r\n\r\nIf you want anything faster, you have to get into cosmic rays and such, which you said you weren't interested in.\n\nAnswer (score=-3):\n\nAs I understand the question, comets (or any thing coming from outside the solar system) **can not** be considered.  This leaves only the asteroids and other debris that still circle the sun at distance **r**.  If this mass starts \"falling\" towards the sun, it will attain a velocity given by Pulsar's equation, if it is corrected by replacing the term (1/Rsun) with (1/Rsun - 1/r)", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 1, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "solar-system", "speed", "comets"], "page_start": null, "page_end": null, "quality_flags": [], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": 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{"unit_id": "stackexchange:physics.stackexchange:asteroids:25645:0000", "text": "Question: How do the terms comet, asteroid, meteoroid, meteor and meteorite differ?\n\nThese terms are frequently used interchangeably by the uninitiated to mean approximately a “space rock”. In practical terms, how do their meanings differ?\n\nAccepted Answer:\n\nAt the risk of being snarky (each definition is from wikipedia)...\r\n\r\n[Comet][1] -  A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles.\r\n\r\n[Asteroid][2] - Asteroids (from Greek ἀστήρ 'star' and εἶδος 'like, in form') are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as small objects in the outer Solar System were discovered, their volatile-based surfaces were found to more closely resemble comets, and so were often distinguished from traditional asteroids\r\n\r\n[Meteor][3] - A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's (or another body's) atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite. Many meteors appearing seconds or minutes apart are called a meteor shower. The root word meteor comes from the Greek meteo¯ros, meaning \"high in the air\".\r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Comet\r\n  [2]: http://en.wikipedia.org/wiki/Asteroid\r\n  [3]: http://en.wikipedia.org/wiki/Meteoroid\n\nAnswer (score=3):\n\nI am sure the above answers will serve well for the OP, but I tend to remember it simply by this : \r\n\r\n*Asteroid*: A relatively small, inactive body, composed of rock, carbon or metal, which is orbiting the Sun.\r\n\r\n*Comet*: A relatively small, sometimes active object, which is composed of dirt and ices. Comets are characterised by dust and gas tails when in proximity to the Sun. Far from the Sun it is difficult to distinguish an asteroid from a comet.\r\n\r\n*Meteoroid*: A small particle from an asteroid or comet orbiting the Sun.\r\n\r\n*Meteor*: A meteoroid that is observed as it burns up in the Earth's atmosphere - a shooting star.\r\n\r\n*Meteorite*: A meteoroid that survives its passage through the Earth's atmosphere and impacts the Earth's surface.\r\n\r\nhttp://www.nearearthobjects.co.uk/\n\nAnswer (score=2):\n\nFirst the simple distinction: both comets and asteroids are in orbit around the Sun, and sufficiently far away from the Earth that they move quite slowly through the sky.  You need to look really closely (with a telescope) to see them move against the starry background.  Meteors start out also in orbit around the Sun, but they are very small so we can't see them until they enter the Earth's atmosphere. They are travelling very fast relative to the Earth, which makes them heat up, so that we view them as rapidly moving streaks of light across the sky. A meteor takes a second or two at most to cross the sky, while a comet or asteroid takes weeks or months.  There are also artificial satellites in orbit around the Earth. These move at an intermediate speed, taking about 5 or 6 minutes to cross the sky.\r\n\r\nSo, it's very easy to tell the difference based on the speeds they travel: meteors fast, satellites intermediate, comets and asteroids slow.\r\n\r\nNow the hard part: comets vs. asteroids. Comets are made up mostly of ice. As they near the Sun, they are warmed and the ice sublimates, forming a tail which is swept away by the solar wind. Asteroids have already visited the region of the Sun many times in the past, and have lost most of their ice, so they don't form tails.  In a telescope, they look just like stars, except that they move from one day to the next, like a planet.\n\nAnswer (score=2):\n\nThe three classes of objects grade into each other with no clear dividing line. Classically, a *comet* gives off a lot of gas and dust, even forming a noticeable tail. Once the volatiles are mostly used up, an inactive comet can be \"mistaken\" for an *asteroid*, except perhaps on a close pass to the Sun when it heats up enough to outgas again. There are many asteroids out there (particularly those that swing through the inner Solar System) that probably started life with a lot of ice and would have been called comets, but are now almost entirely rock and dust. \r\n\r\nThere is no clear line between an asteroid and a *meteoroid*. Both are rocky/dusty; it's a matter of size as to how you refer to it. However, there is no clear-cut dividing line. A big one is usually referred to as an asteroid, while smaller stuff is a meteoroid. You could draw the line on whether it's big enough to likely survive a trip through the Earth's atmosphere (as a _meteor_) and arrive on the surface as a _meterorite_, but that also depends on the composition, initial speed, and trajectory, not just diameter. And of course, it would depend on which planet (gravity and atmospheric density) it's heading towards.\n\nAnswer (score=1):\n\nAn asteroid is a small, natural object in orbit around the Sun, which has no history of ever displaying cometary appearance.  \"Planets\" are large enough to never be considered asteroids.  The International Astronomical Union also defines \"dwarf planets\" which are smaller than planets, but bigger than \"asteroids.\"  The qualification \"natural\" must be added because the number of man-made objects in orbit around the Sun are also never considered asteroids.\r\n\r\nIf an asteroid is ever observed to have a coma—a surrounding cloud, or a tail—a coma extended to have a linear appearance, it is officially categorized as a comet, and ceases to be officially categorized as an asteroid.  Scientifically, we know that the usual reason for this appearance is an icy composition, but it is much more difficult to determine composition than to observe appearance so most objects categorized as comets remain categorized solely on appearance.\r\n\r\nA meteor is a small (natural) object which is (or could be) observed entering the atmosphere.  The countless meteors visible any given night are almost always smaller than any cataloged asteroid.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 2, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "terminology", "meteors", "meteorites", "meteoroids"], "page_start": null, "page_end": null, "quality_flags": [], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 25645, "title": "How do the terms comet, asteroid, meteoroid, meteor and meteorite differ?", "url": "https://physics.stackexchange.com/questions/25645/how-do-the-terms-comet-asteroid-meteoroid-meteor-and-meteorite-differ", "share_url": "https://physics.stackexchange.com/q/25645", "content_license": null, "owner": {"display_name": "YUASK", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, "answers": [{"answer_id": 25646, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/25645/how-do-the-terms-comet-asteroid-meteoroid-meteor-and-meteorite-differ/25646#25646", "share_url": "https://physics.stackexchange.com/a/25646", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Larian LeQuella", "user_id": 2715, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/2715/larian-lequella"}}, {"answer_id": 25648, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/25645/how-do-the-terms-comet-asteroid-meteoroid-meteor-and-meteorite-differ/25648#25648", "share_url": "https://physics.stackexchange.com/a/25648", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Arun", "user_id": 8962, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/8962/arun"}}, {"answer_id": 25649, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/25645/how-do-the-terms-comet-asteroid-meteoroid-meteor-and-meteorite-differ/25649#25649", "share_url": "https://physics.stackexchange.com/a/25649", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Geoff Gaherty", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, {"answer_id": 122492, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/25645/how-do-the-terms-comet-asteroid-meteoroid-meteor-and-meteorite-differ/122492#122492", "share_url": "https://physics.stackexchange.com/a/122492", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Phil Perry", "user_id": 41712, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/41712/phil-perry"}}, {"answer_id": 25647, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/25645/how-do-the-terms-comet-asteroid-meteoroid-meteor-and-meteorite-differ/25647#25647", "share_url": "https://physics.stackexchange.com/a/25647", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Sonia", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:26091:0000", "text": "Question: How does the central peak in moon craters form?\n\nThe central peak in many of the moon's large craters are visible with a telescope and they seem a little odd to me. Can someone explain how they form.\r\n\r\n\r\n![enter image description here][1]\r\n\r\n\r\n  [1]: https://i.sstatic.net/hi25I.jpg\n\nAccepted Answer:\n\nIf you drop something into a pool of water, you will get a rebound effect in the middle where the object was dropped, and then waves will spread out around it.  This rebound effect in the middle is the same phenomenon that causes central peaks in craters.  The difference is just the scale:  An impact that forms a >~15-km-diameter crater on the moon will cause the rock to act like the liquid to the point that you get the rebound effect and form a central peak.\r\n\r\nSmaller craters on the moon will not have central peaks, and larger craters above ~120 km will form a peak-ring.\r\n\r\nThe transition diameter for these features -- a simple, bowl-shaped crater; a \"complex\" crater with a central peak; a peak-ring crater -- is inversely proportional to gravity.  So, on Earth, the transition diameters are smaller -- you only need to get a ~3-4-km-diameter crater before you can form central peaks.  On Mars, the transition diameter is around 6 km.  To a lesser extent, target material strength will affect the transition diameter, as well.\r\n\r\nBut in the end, the central peaks are formed by rock rebounding, being pushed back up by the strength of the underlying rock after the initial impact event.  Central peak formation happens within minutes of the impact itself, even in craters 10s-km across.\n\nAnswer (score=0):\n\nYou can actually simulate this yourself. Pour some flour on a sheet of paper, then drop a marble on it. Try different thicknesses for the flour, different marble sizes, different heights for the drop, etc.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 3, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astrophysics", "collision", "moon"], "page_start": null, "page_end": null, "quality_flags": [], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 26091, "title": "How does the central peak in moon craters form?", "url": "https://physics.stackexchange.com/questions/26091/how-does-the-central-peak-in-moon-craters-form", "share_url": "https://physics.stackexchange.com/q/26091", "content_license": null, "owner": {"display_name": "Christopher", "user_id": 3925, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/3925/christopher"}}, "answers": [{"answer_id": 26092, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/26091/how-does-the-central-peak-in-moon-craters-form/26092#26092", "share_url": "https://physics.stackexchange.com/a/26092", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Stuart Robbins", "user_id": 5902, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/5902/stuart-robbins"}}, {"answer_id": 26093, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/26091/how-does-the-central-peak-in-moon-craters-form/26093#26093", "share_url": "https://physics.stackexchange.com/a/26093", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Florin Andrei", "user_id": 4210, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/4210/florin-andrei"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:739831:0000", "text": "Question: Why did NASA need to observationally confirm whether DART successfully redirected Dimorphos?\n\nNASA's DART impactor made a head-on collision with the asteroid Dimorphos on September 26, 2022. A real-time video feed gave immediate confirmation of the direct hit. But according to [this press release](https://www.nasa.gov/press-release/nasa-confirms-dart-mission-impact-changed-asteroid-s-motion-in-space), NASA had to observe Dimorphos for two more weeks before being able to confirm that Dimorphos's trajectory was indeed noticeably altered (as planned).\r\n\r\nWhy? It seems to me that determining the collision's effect on Dimorphos's orbit would be a very simple exercise in Newtonian mechanics. I assume that Dimorphos's total mass was well-known from its orbital dynamics with Didymos. I know that its internal composition wasn't well understood, but is that really so important for understanding its post-collision dynamics? Conservation of momentum means that the subsequent overall motion of Dimorphos's center of mass should not be affected by the details of its internal composition.\r\n\r\nI know that the collision ejected some material off of Dimorphos's surface, so there's a bit of a semantic question as to whether after the collision, the term \"Dimorphos\" should refer to \"all of that material that made up Dimorphos *before* the collision\" or \"what's left on the largest connected component of that material *after* the collision\". But it doesn't seem to me that this would make a big difference regarding Dimorphos's overall dynamics. It seems to me that approximating the collision as a perfectly inelastic collision between two point particles would probably give a pretty good model. Even if the impactor did knock off a significant fraction of Dimorphos's mass (which seems unlikely), then it seems to me that this outcome would count as \"significantly changing its trajectory\" almost by definition.\r\n\r\nWas there ever really any genuine uncertainty whether DART would redirect Dimorphos *given that* DART directly impacted Dimorphos? What kind of plausible internal composition of Dimorphos could have led to a failure to be redirected?\r\n\r\n**Edit to clarify question scope:** As is often the case, many people are interpreting the title of my question too literally. (My understanding is that Stack Exchange's convention is that the \"official\" version of an SE question is found in the question body, and the purpose of the question's title is to draw attention rather to precisely state the question.) I'm not trying to have a general philosophical debate about how much you should trust theory vs. experiment. Nor am I trying to understand why NASA actually *did* observationally confirm the redirection, as a lot of complicated non-physics factors enter into that decision. (So any speculation about NASA's political incentives, etc. are out of scope for this question.) I'm just asking, very concretely, what were the main sources of scientific uncertainty in the extent to which Dimorphos would be redirected given a successful collision, and how those uncertainties would affect the extent of redirection. \"The composition of Dimorphos\" would not be a concrete enough uncertainty; I'd like to know *how* the composition of Dimorphos would change the redirection. Of the many comments and answer to this question so far, only John Doty's answer addresses my question within the scope that I intended it.\n\nAccepted Answer:\n\nThe spacecraft had a large amount of energy, but not a lot of momentum. Most of the impulse delivered to the target was due to the momentum of the ejecta. Energy scales as $mv^2$, but momentum scales as $mv$. For a given energy, cut the ejecta velocity in half, eject four times as much, and deliver twice the impulse. But whether the energy produces a small quantity of fast ejecta or a large quantity of slow ejecta depends the the material properties of the target. These were poorly known.\n\nAnswer (score=15):\n\nBecause sometimes things don't quite turn out as the theory expects, and the only way to know is to experiment.\r\n\r\nAa an example, take the main result of this experiment: the orbital period was changed by ~32min (https://www.nasa.gov/press-release/nasa-confirms-dart-mission-impact-changed-asteroid-s-motion-in-space).\r\n\r\nIf we take for correct the calculations in https://physics.stackexchange.com/questions/731677/dart-crash-on-dimorphos-computation-of-orbital-period-change and we add to that an estimate of the effect due to reshaping outlined in https://iopscience.iop.org/article/10.3847/PSJ/ac7566, taking the larger estimates, we arrive at a change of 18 minutes, well short of the real world 32.\r\n\r\nI'm no physicist and I haven't read all the details of either calculation, so there may well be some assumption made both times that could be integrated (there probably is), but I think it well shows how much value there is in actually preforming the experiment and not trusting theory blindly.\n\nAnswer (score=2):\n\nBiliards or snooker are games where you can perfectly calculate the mechanics, nonetheless there is still a great amount of variability.\r\n\r\nIn the case of DART the impact point might have been solid rock or rubble slipping sideways and dispersing part of the momentum or something in between. Another factor of uncertainty is the angle, it is very difficult to hit with great precision a rotating body.\n\nAnswer (score=0):\n\n[This article](https://www.nature.com/articles/d41586-023-00601-4) confirms John Doty's answer that the biggest unknown was indeed how much ejecta the DART impact would throw out from Dimorphos:\r\n> Researchers estimate that this spray of rubble meant Dimorphos’ added momentum was almost four times that imparted by DART.\r\n\r\nMy take is that strictly speaking - putting aside epistemological questions about the nature of proof - it wasn't really necessary for NASA to observe the aftermath of the collision in order to determine *whether* DART significantly modified Dimorphos's trajectory; that would have happened even for a completely inelastic (head-on) collision with no ejecta at all. But NASA did need to observe the aftermath in order to determine by *how much* the impact changed Dimorphos's trajectory.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 4, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-4.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "momentum", "orbital-motion", "collision"], "page_start": null, "page_end": null, "quality_flags": [], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 739831, "title": "Why did NASA need to observationally confirm whether DART successfully redirected Dimorphos?", "url": "https://physics.stackexchange.com/questions/739831/why-did-nasa-need-to-observationally-confirm-whether-dart-successfully-redirecte", "share_url": "https://physics.stackexchange.com/q/739831", "content_license": null, "owner": {"display_name": "tparker", "user_id": 92058, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/92058/tparker"}}, "answers": [{"answer_id": 739833, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/739831/why-did-nasa-need-to-observationally-confirm-whether-dart-successfully-redirecte/739833#739833", "share_url": "https://physics.stackexchange.com/a/739833", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "John Doty", "user_id": 264639, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/264639/john-doty"}}, {"answer_id": 739924, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/739831/why-did-nasa-need-to-observationally-confirm-whether-dart-successfully-redirecte/739924#739924", "share_url": "https://physics.stackexchange.com/a/739924", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "bracco23", "user_id": 352178, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/352178/bracco23"}}, {"answer_id": 739890, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/739831/why-did-nasa-need-to-observationally-confirm-whether-dart-successfully-redirecte/739890#739890", "share_url": "https://physics.stackexchange.com/a/739890", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "FluidCode", "user_id": 250611, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/250611/fluidcode"}}, {"answer_id": 754619, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/739831/why-did-nasa-need-to-observationally-confirm-whether-dart-successfully-redirecte/754619#754619", "share_url": "https://physics.stackexchange.com/a/754619", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "tparker", "user_id": 92058, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/92058/tparker"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:25162:0000", "text": "Question: What is the current status of Pluto?\n\n[Pluto](http://en.wikipedia.org/wiki/Pluto) has been designated a planet in our solar system for years (ever since it was discovered in the last century), but in 2006 it was demoted.\r\n\r\nWhat caused this decision? And is there a chance that it could be reversed?\r\n\r\nEdit: well, http://www.dailygalaxy.com/my_weblog/2017/03/nasas-new-horizon-astronomers-declare-pluto-is-a-planet-so-is-jupiters-ocean-moon-europa.html is interesting; this is science, so anything could (potentially) change.\n\nAccepted Answer:\n\nPluto is now classified as a [dwarf planet][1].  The main difference between a planet and a dwarf planet has to do with the requirement that a planet clear out the material in and near its orbit.  Planets do this, dwarf planets do not.\r\n\r\nThe reclassification was triggered by the discovery of many additional object (the Edgeworth-Kuiper Belt) out beyond the orbit of Neptune.  Some of the objects are nearly as big as (and is a few cases, possibly bigger than) Pluto and in very similar orbits.  Thus it was realized that Pluto was just the largest of a large number of objects in the outer solar system.\r\n\r\nThis is simply science at work.  At the local university, we have an Astronomy textbook from the 1800's that lists the 12 planets:  Mercury, Venus, Earth, Mars, Ceres, Pallas, Juno, Vesta, Jupiter, Saturn, Uranus, and Neptune.  However, as more objects were detected between Mars and Jupiter, it was realized this was a new class of object and the middle four were downgraded from planet status to asteroids.  It is the same process at work today out in the outer solar system.\r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Dwarf_planet\n\nAnswer (score=9):\n\nPluto is still considered a [dwarf planet][1]. This was because it did not meet [the full criteria for being classified as a planet][2]. Most notably it did not clear its orbit of other debris.\r\n\r\nThis is still considered controversial as many scientist do not agree with the definition of what a planet is and still consider Pluto to meet planet criteria. So it is possible that this may change especially if the criteria for what is a planet changes due to new discoveries. This is very possible with the work the [Kepler telescope][3] is producing. As we discover new planets in large numbers you can be sure they will be finding ever different planets that will force a redefinition of what a planet is.\r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Dwarf_planet\r\n  [2]: http://en.wikipedia.org/wiki/IAU_definition_of_planet\r\n  [3]: http://en.wikipedia.org/wiki/Kepler_%28spacecraft%29\n\nAnswer (score=2):\n\nAs many already said, Pluto is now considered a \"dwarf planet\"\r\n\r\nFor your second question, there is no chance that Pluto will be reclassified as a planet again.\n\nAnswer (score=1):\n\nPluto has been reclassified as a dwarf planet.\r\n\r\nIt was reclassified as such because a growing number of objects were found similar to Pluto, which exhibited at least one notable difference from the other planets.  The choice would have been to accept these other objects as planets or to develop a new class of object.\r\n\r\nThe primary features of a planet are that it has an orbital path 'clear' of other debris, orbits around the sun, and is massive enough to maintain hydrostatic equilibrium.  Pluto (as well as other dwarf planets) fail to meet the first criterion.\n\nAnswer (score=1):\n\nIf you are interested, there is an audio recording of the IAU General Assembly session on the definition of a planet <http://www.jodcast.net/archive/200608IAU/>\n\nAnswer (score=0):\n\nI don't really care what the IAU voted. Pluto will always be a planet in my book. Astronomy is full of historic inaccuracies that we perpetuate for tradition sake. Some examples that come to mind are early/late-type galaxies, Population I/II/III stars, and brown dwarfs.\n\nAnswer (score=0):\n\nMercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn and Uranus are the primary planets in the Solar System.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 5, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "planets", "definition"], "page_start": null, "page_end": null, "quality_flags": [], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 25162, "title": "What is the current status of Pluto?", "url": "https://physics.stackexchange.com/questions/25162/what-is-the-current-status-of-pluto", "share_url": "https://physics.stackexchange.com/q/25162", "content_license": null, "owner": {"display_name": "peSHIr", "user_id": 1153, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1153/peshir"}}, "answers": [{"answer_id": 25165, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/25162/what-is-the-current-status-of-pluto/25165#25165", "share_url": "https://physics.stackexchange.com/a/25165", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "dagorym", "user_id": 335, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/335/dagorym"}}, {"answer_id": 25163, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/25162/what-is-the-current-status-of-pluto/25163#25163", "share_url": "https://physics.stackexchange.com/a/25163", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "John Conde", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, {"answer_id": 25167, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/25162/what-is-the-current-status-of-pluto/25167#25167", "share_url": "https://physics.stackexchange.com/a/25167", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "archaeme", "user_id": 3825, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/3825/archaeme"}}, {"answer_id": 25164, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/25162/what-is-the-current-status-of-pluto/25164#25164", "share_url": "https://physics.stackexchange.com/a/25164", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "acmshar", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, {"answer_id": 33551, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/25162/what-is-the-current-status-of-pluto/33551#33551", "share_url": "https://physics.stackexchange.com/a/33551", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Nicholas", "user_id": 11103, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/11103/nicholas"}}, {"answer_id": 25166, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/25162/what-is-the-current-status-of-pluto/25166#25166", "share_url": "https://physics.stackexchange.com/a/25166", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Jeremy", "user_id": 775, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/775/jeremy"}}, {"answer_id": 25168, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/25162/what-is-the-current-status-of-pluto/25168#25168", "share_url": "https://physics.stackexchange.com/a/25168", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Dromaeosaur", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:26139:0000", "text": "Question: Why did the asteroid belt between Mars and Jupiter form as it did?\n\nI'm curious about why the asteroid belt wasn't pulled by Mars's or Jupiter's gravity or formed into either moons or planets. Why did it form into an asteroid belt instead?\n\nAccepted Answer:\n\nThe answers so far leave out an important consideration, which is that of the <a href=\"http://en.wikipedia.org/wiki/Nice_model\">Nice model</a> and effects it would have had, and the evidence for the <a href=\"http://en.wikipedia.org/wiki/Late_heavy_bombardment\">Late Heavy Bombardment</a>.\r\n\r\nTo start with, models predict that Jupiter would have formed rather quickly.  The mass of Jupiter - even if it was not quite where it is now in the solar system - will perturb material out to a large distance.  What this means is that if a planet were \"trying\" to form around the position of the asteroid belt today, it would not have been able to because of gravitational instabilities created by Jupiter (and to a lesser extent, Mars -- new research indicates Mars may have been the first planet to have formed, though its mass is significantly less than Jupiter's).\r\n\r\nHowever, the asteroid belt then likely had several times its current mass.  There is a fair amount of evidence for what I mentioned in the first sentence, the Late Heavy Bombardment (LHB), which was a period likely around 3.9 billion years ago that lasted for about 200 million years when there was a sharp spike in impacts in the inner solar system (you may see slightly different numbers for these, and in fact a talk I saw this week by a dynamicist suggested that it may have started 4.2 billion years ago and lasted for 400 million years).  It's during this period that the asteroid belt would have lost a lot of its material.\r\n\r\nMaking the LHB actually happen dynamically, though, stumped a lot of people until a small group of dynamicists had a lot of drinks together in Nice, France, and came up with the idea that Jupiter and Saturn do-ce-do'ed in the early history of the solar system, coming to their currently observed positions today.  The process of them moving would have pumped a lot of gravitational energy into the asteroid belt, scattering a lot of it, causing the LHB, and leaving it roughly as we observe today.\r\n\r\nThis is more than what you asked in your question, but I think it gives an important perspective and more complete picture of the situation back then.  To recap, though, the basic idea is that of what others posted:  the other planets that formed faster caused enough gravitational sheer to prevent a planet from forming in the asteroid belt.  And, the asteroid belt today is many times smaller mass-wise to ever have accumulated into a planet, even a Mercury-sized one (despite what some people claim of it being the remnants of an exploded planet).\n\nAnswer (score=4):\n\nThe original mass of the solar disk at that position in the Solar System is speculated to be about the same as Earth. Due to gravitational perturbations of Jupiter and Mars, the Asteroid Belt was too chaotic to allow a planet to fully form. Instead of relatively gentle collisions, allowing them to accrete, the impacts of planetesimals were highly energetic.\r\n\r\nIn fact, during the first few million years of formation about 99.9% of the original material was ejected, and we're left with the Asteroid Belt as it is today. Some of the asteroids were far enough out from the sun to accumulate ice, and it is thought that many bombarded the early Earth and that's how the oceans were formed.\n\nAnswer (score=1):\n\nIt's Jupiter's gravity that prevents them from doing so. Jupiter's orbit and the continual shifting of its gravity around that part of the solar system makes it a very chaotic place as far as forming planets is concerned.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 6, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astrophysics", "solar-system"], "page_start": null, "page_end": null, "quality_flags": [], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 26139, "title": "Why did the asteroid belt between Mars and Jupiter form as it did?", "url": "https://physics.stackexchange.com/questions/26139/why-did-the-asteroid-belt-between-mars-and-jupiter-form-as-it-did", "share_url": "https://physics.stackexchange.com/q/26139", "content_license": null, "owner": {"display_name": "Annika Peterson", "user_id": 4179, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/4179/annika-peterson"}}, "answers": [{"answer_id": 26142, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/26139/why-did-the-asteroid-belt-between-mars-and-jupiter-form-as-it-did/26142#26142", "share_url": "https://physics.stackexchange.com/a/26142", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Stuart Robbins", "user_id": 5902, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/5902/stuart-robbins"}}, {"answer_id": 26141, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/26139/why-did-the-asteroid-belt-between-mars-and-jupiter-form-as-it-did/26141#26141", "share_url": "https://physics.stackexchange.com/a/26141", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "ghoppe", "user_id": 2872, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/2872/ghoppe"}}, {"answer_id": 26140, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/26139/why-did-the-asteroid-belt-between-mars-and-jupiter-form-as-it-did/26140#26140", "share_url": "https://physics.stackexchange.com/a/26140", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Rogue", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:47754:0000", "text": "Question: Minimum size of an asteroid to actually impact earth\n\nFrom what I understand, an object entering the atmosphere will start to burn up from the tremendous resistance of the atmosphere.  Presumably, for asteroids under a certain size, they will burn up completely and never impact the surface of the earth.  \r\n\r\nDo we have a way of determining the minimum size needed for actual impact?  \r\n\r\nIf so, roughly what is the size and how does it compare to the average size of asteroids that pass by us regularly?\n\nAccepted Answer:\n\nAs mentioned in NotAstronaut's answer, objects smaller than 25 meters will typically burn up in the atmosphere. One can very easily see why this should be the case using [Newton's impact depth formula][1]. This is based on approximating the problem by assuming that the matter in the path of the object is being pushed at the same velocity of the object, so as soon as the object has swiped out path containing the same mass as its own mass, it will have lost all of its initial momentum. All its kinetic energy will then have dissipated there, so if this happens in the atmosphere it will have burned up before reaching the ground. \r\n\r\nThis is, of course, a gross oversimplification, but it will yield correct order of magnitude estimates. We can then calculate the critical diameter as follows. The mass of the atmosphere per unit area equals the atmospheric pressure at sea level divided by the gravitational acceleration, so this is about $10^4\\text{ kg/m}^2$. If an asteroid of diameter $D$ and density $\\rho$ is to penetrate the atmosphere, its mass of $1/6\\pi \\rho D^3$ should be larger than the mass of the atmosphere it will encounter on its way to the ground, which is $5/2 \\pi 10^3 D^2\\text{ kg/m}^2$. Therefore:\r\n\r\n$$ D > \\frac{1.5\\times 10^4}{\\rho} \\text{ kg/m}^2$$\r\n\r\nIf we take the density $\\rho$ to be that of a typical rock of $3\\times 10^3 \\text{ kg}/\\text{m}^3$, then we see that $D>5\\text{ m}$, which is reasonably close order of magnitude estimate to the correct answer.\r\n\r\n\r\n  [1]: https://en.wikipedia.org/wiki/Impact_depth\n\nAnswer (score=6):\n\nIt needs to be more than 25m or otherwise it will burn up in the atmosphere according to this Nasa article\r\n\"Space rocks smaller than about 25 meters (about 82 feet) will most likely burn up as they enter the Earth's atmosphere and cause little or no damage.\"\r\nhttps://www.nasa.gov/mission_pages/asteroids/overview/fastfacts.html\n\nAnswer (score=3):\n\nConsider a specific example. The [leonids](http://en.wikipedia.org/wiki/Leonids) arrive at the top of the atmosphere at $72$ km/s with a maximal mass of around $0.5$ g. According to the article these particles are $0.01$ m across. They reach the ground. If such a particle strikes the $30$ km high [atmosphere](http://www.albany.edu/faculty/rgk/atm101/structur.htm) at $45$ degrees it must travel around $4 \\times 10^4$ m before it hits the ground, mostly burning up. If it started at rest at the top of the atmosphere it would only accelerate to about $1$ km/s. As you can see it very much depends on azimuth, latitude, meteor composition, and speed.\n\nAnswer (score=0):\n\nBelow is quoted from \"How big does a meteor have to be to make it to the ground?\"  10 October 2000.  HowStuffWorks.com. <http://science.howstuffworks.com/question486.htm>  27 December 2012.\r\n\r\n\"So how big does a meteoroid have to be to make it to the surface of the Earth? Surprisingly, most of the meteoroids that reach the ground are especially small -- from microscopic debris to dust-particle-size pieces. ... Typically, though, a meteoroid would have to be about the size of a marble for a portion of it to reach the Earth's surface. Smaller particles burn up in the atmosphere about 50 to 75 miles (80 to 120 kilometers) above the Earth.\"", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 7, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "atmospheric-science", "earth", "drag"], "page_start": null, "page_end": null, "quality_flags": ["mixed_license"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 47754, "title": "Minimum size of an asteroid to actually impact earth", "url": "https://physics.stackexchange.com/questions/47754/minimum-size-of-an-asteroid-to-actually-impact-earth", "share_url": "https://physics.stackexchange.com/q/47754", "content_license": null, "owner": {"display_name": "Todd R", "user_id": 16785, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/16785/todd-r"}}, "answers": [{"answer_id": 327689, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/47754/minimum-size-of-an-asteroid-to-actually-impact-earth/327689#327689", "share_url": "https://physics.stackexchange.com/a/327689", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "Count Iblis", "user_id": 47511, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/47511/count-iblis"}}, {"answer_id": 327682, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/47754/minimum-size-of-an-asteroid-to-actually-impact-earth/327682#327682", "share_url": "https://physics.stackexchange.com/a/327682", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "NotAstronaut", "user_id": 153534, "user_type": "unregistered", "profile_url": "https://physics.stackexchange.com/users/153534/notastronaut"}}, {"answer_id": 47762, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/47754/minimum-size-of-an-asteroid-to-actually-impact-earth/47762#47762", "share_url": "https://physics.stackexchange.com/a/47762", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "BrianWa", "user_id": 17137, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/17137/brianwa"}}, {"answer_id": 47758, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/47754/minimum-size-of-an-asteroid-to-actually-impact-earth/47758#47758", "share_url": "https://physics.stackexchange.com/a/47758", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "raindrop", "user_id": 11833, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/11833/raindrop"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:243985:0000", "text": "Question: Are there any models for distribution of asteroid sizes in a belt?\n\nQuestion:\r\n---------\r\n\r\n**Is there any function that can describe the distribution* of asteroid diameters expected within an*' asteroid belt?**\r\n\r\n*: distribution as frequency of occurrence within the asteroid belt or absolute numbers, not as positional distribution or orbits inside the belt.\r\n\r\n*':Not THE asteroid belt. So the answer could/should fit Sol's 'main' asteroid belt, but also any rather usual one on any other star system.\r\n\r\nPossible independent variables used by the model given\r\n------------------------------------------------------\r\nThe fewer used the better, as the model will be broader, but if by introducing any the precision of the result increases by a magnitude, then you are welcome.\r\nThe variables at my disposal to find out the distribution of sizes are:\r\n\r\n - Initial mass of the belt on system formation.\r\n - Inner and outer radius of the ring.\r\n - Current age of the system.\r\n - Star(s) radius, mass, luminosity and temperature.\r\n - Planets around, and their masses and orbital characteristics. (this would not include planetesimals produced by the model, of course)\r\n\r\nYou can add any constant that you find necessary.\r\n\r\nAssumptions\r\n-----------\r\nThe more general the solution presented, the better, but any amount of them taken are acceptable to answer the question.\r\n\r\n - Considering a star system old enough to be stable.\r\n - Constant density of the bodies in it.\r\n - It's a rather standard main belt. This means that it does not need to represent oort cloud or anything further than ~100AU, neither heavily disturbed belts like Kuiper or singular through expectedly common ones like trojan 'belts'. (but will be very welcome if does!)\r\n\r\nOther considerations and previous research\r\n-----------------------------------------------------------\r\n\r\nThe model needs to fit only standard relatively usual scenarios: there happen to be the conditions that allow the formation of and asteroid belt but prevent it's accretion into a planet, (e.g., a Jovian around) but not any other strange condition (like gas planets drifting inwards in the formation period and altering significantly the distribution).\r\n\r\nTo be precise, I'd like to find out the most general of such expected distributions.\r\nIf it's relevant o known, I'll be happy to have the distinct versions for the 'rocky' zone of the system, without volatiles, and the icy part.\r\n\r\nI found an article about the distribution of asteroids in our main asteroid belt, but couldn’t get anything like what I ask from it: [http://orbit.psi.edu/~tricaric/pdf/skads.pdf][1]\r\n\r\nWikipedia also has something to say about it in the case of the Kuiper Belt: [https://en.wikipedia.org/wiki/Kuiper_belt#Mass_and_size_distribution][2]\r\nBut as there seem to be so many things that do not fit the expected models about that belt, I don't dare make generalizations from it.\r\n\r\nI understand that there won't be probably any accepted model of the kind I'm looking for, and giving an exact answer might involve doing original research, but I'd like to find out at least an acceptable and credible rule of thumb that has some decent foundation.\r\n\r\nIt's allowed that the diameter of the given asteroids is up to be enough to be considered planetoids (around 10^3 km), but if it's significantly bigger, please tell me how it does not clean it's orbit or form a 'trojan' type of ring instead of a typical more or less evenly spreaded ring, and I'll be happy to consider correct the 'magno-planetoids' :D.\r\n\r\n**Edit 1:**\r\nFormatting changes and better parametrization of what's expected to be in and what may be left out of the answer.\r\n\r\n**Edit 2:**\r\nI've found some interesting article: https://www-n.oca.eu/morby/papers/fossileSFD.pdf\r\nI'll post what I find in it as an answer if I'm able to get it from there, though it looks like it will only model our belt, and so not be enough.\r\n\r\n  [1]: http://orbit.psi.edu/~tricaric/pdf/skads.pdf\r\n  [2]: https://en.wikipedia.org/wiki/Kuiper_belt#Mass_and_size_distribution\n\nAccepted Answer:\n\n[This paper][1] goes through the process of comparing a few models for size distribution of Kuiper Belt objects. The approach is much more empirical than what you seem to have in mind, but it does highlight that the size distribution is a power law, or a combination of multiple power laws.\r\n\r\nHere is [another paper][2] which is also rather empirical, but discusses some theoretical implications for their observations and points out some relevant physical processes in star forming regions. This is before the formation of an individual asteroid belt or even proto-planetary disk, but similar processes may apply to the aggregation and destruction of dust/grains/pebbles in an asteroid belt.\r\n\r\nAs always in astrophysics, the answer is \"it's (very) complicated\", but hopefully that gives you a couple of hints to go on...\r\n\r\n\r\n  [1]: https://arxiv.org/abs/1210.4827\r\n  [2]: https://arxiv.org/abs/1605.06136", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 8, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astronomy", "astrophysics", "planets"], "page_start": null, "page_end": null, "quality_flags": ["single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 243985, "title": "Are there any models for distribution of asteroid sizes in a belt?", "url": "https://physics.stackexchange.com/questions/243985/are-there-any-models-for-distribution-of-asteroid-sizes-in-a-belt", "share_url": "https://physics.stackexchange.com/q/243985", "content_license": null, "owner": {"display_name": "Oxy", "user_id": 111624, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/111624/oxy"}}, "answers": [{"answer_id": 284601, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/243985/are-there-any-models-for-distribution-of-asteroid-sizes-in-a-belt/284601#284601", "share_url": "https://physics.stackexchange.com/a/284601", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Kyle Oman", "user_id": 11053, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/11053/kyle-oman"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:25681:0000", "text": "Question: Is the Apophis asteroid a concern?\n\nI was watching the [TED][3] lecture *[How to defend Earth from asteroids ][1]*, and the speaker [Phil Plait][2] spoke of the possibility of this asteroid hitting Earth. He also said something about a [Mayan][4] legend. He says it's not a concern, is there a consensus on that? And what is the source for this Mayan legend?\r\n\r\n  [1]: http://www.youtube.com/watch?v=rjECbQ1r-k0&feature=uploademail\r\n  [2]: http://en.wikipedia.org/wiki/Phil_Plait\r\n  [3]: http://en.wikipedia.org/wiki/TED_%28conference%29\r\n  [4]: http://en.wikipedia.org/wiki/Maya_civilization\n\nAccepted Answer:\n\n[Apophis][1] is of no real concern to us as far as we can tell.  That said, it doesn't mean another asteroid doesn't have our name on it...\r\n\r\n[According to the most reliable data we have regarding this particular asteroid][2] it only has a 1 chance in about 250,000 of hitting the earth.  [University of Hawaii states][3]:\r\n\r\n>“Our new orbit solution shows that Apophis will miss Earth’s surface in 2036 by a scant 20,270 miles, give or take 125 miles,” Tholen said. “That's slightly closer to Earth than most of our communications and weather satellites.” He credits the large telescopes and superb atmospheric conditions on Mauna Kea for being able to make these determinations.\r\n\r\nConsidering how big space is, this is pretty much a hit in cosmic terms.  Keep in mind that [this information has been readily available since 2009 at the NASA website as well][4].\r\n\r\n[As for the bat guano crazy 2012 stuff][5]...  Suffice it to say that anyone that honestly buys into that is mentally feeble, and should seek psychiatric help...\r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/99942_Apophis\r\n  [2]: http://www.psi.edu/epo/faqs.htm\r\n  [3]: http://www.ifa.hawaii.edu/info/press-releases/Apophis-TholenOct09/\r\n  [4]: http://www.nasa.gov/home/hqnews/2009/oct/HQ_09-232_Apophis_Update.html\r\n  [5]: http://www.2012hoax.org/", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 9, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astronomy"], "page_start": null, "page_end": null, "quality_flags": ["single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 25681, "title": "Is the Apophis asteroid a concern?", "url": "https://physics.stackexchange.com/questions/25681/is-the-apophis-asteroid-a-concern", "share_url": "https://physics.stackexchange.com/q/25681", "content_license": null, "owner": {"display_name": "YUASK", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, "answers": [{"answer_id": 25682, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/25681/is-the-apophis-asteroid-a-concern/25682#25682", "share_url": "https://physics.stackexchange.com/a/25682", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Larian LeQuella", "user_id": 2715, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/2715/larian-lequella"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:26572:0000", "text": "Question: Are there Trojan family or Hilda family satellites locked in Earth's orbit?\n\nJupiter has many [Trojan asteroids][1] located at Lagrangian points L4 and L5 and [Hilda asteroids][2] dispersed between points L3, L4, and L5.\r\n\r\nDoes the Earth have similar satellites?  If so, how many?\r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Jupiter_Trojan\r\n  [2]: http://en.wikipedia.org/wiki/Hilda_family\n\nAccepted Answer:\n\n[Yes, but just barely (i.e. only one so far), and only for the L4 point][1].  And this was only recently discovered.  [Dr. Phil Plait (an Astronomer) covers this at his Discover Magazine blog in his usual easy to read style][2].\r\n\r\n>NASA’s Wide-field Infrared Survey Explorer (WISE) has found the very first asteroid that (more or less) shares an orbit with Earth! Called 2010 TK7, this asteroid is about 300 meters (roughly 1000 feet) across, and is the first in an up-to-now theoretical class of objects called Earth Trojans.\r\n\r\n[The University of Western Ontario astronomer Paul Weigert has a page describing this][3].\r\n\r\n[Additionally, you can get a pre-print of their paper (PDF).][4]\r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Earth_trojan_asteroid\r\n  [2]: http://blogs.discovermagazine.com/badastronomy/2011/07/27/wise-finds-the-very-first-earth-trojan-asteroid/\r\n  [3]: http://www.astro.uwo.ca/~wiegert/2010TK7/\r\n  [4]: http://www.astro.uwo.ca/~wiegert/2010TK7/2010TK7.pdf", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 10, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astrophysics", "planets", "lagrangian-formalism", "satellites"], "page_start": null, "page_end": null, "quality_flags": ["single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 26572, "title": "Are there Trojan family or Hilda family satellites locked in Earth's orbit?", "url": "https://physics.stackexchange.com/questions/26572/are-there-trojan-family-or-hilda-family-satellites-locked-in-earths-orbit", "share_url": "https://physics.stackexchange.com/q/26572", "content_license": null, "owner": {"display_name": "oosterwal", "user_id": 1465, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1465/oosterwal"}}, "answers": [{"answer_id": 26573, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/26572/are-there-trojan-family-or-hilda-family-satellites-locked-in-earths-orbit/26573#26573", "share_url": "https://physics.stackexchange.com/a/26573", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Larian LeQuella", "user_id": 2715, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/2715/larian-lequella"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:271459:0000", "text": "Question: What is the scientific basis for comets vs. main-belt comets being composed of different materials?\n\nDo we observe different events or activities on main-belt comets, or do they behave exactly the same as regular comets?  \r\n(Apart from the obvious difference in orbit size/shape.)\r\n\r\n<!--**Side note:**  \r\nPeople have mentioned the theories of collisions exposing new layers of ices in the asteroid belt.  \r\nThe answer [here][1] indicates that asteroids have a LOT of space between them - approx 2 million miles for asteroids larger than 1km.  \r\nSo the theories of collisions exposing new layers of ices, seem statically far fetched.  \r\nFurthermore, the asteroids are all orbiting at the same general speed and direction at this point, so any collision would be at relatively low speed.-->\r\n\r\nThe [Wikipedia article][2] for main-belt comets currently states the following:  \r\n\r\n - *\"The Jet Propulsion Laboratory defines a main-belt asteroid as an asteroid with a semi-major axis (average distance from the Sun) of more than 2 AU but less than 3.2 AU, and a perihelion (closest approach distance to the Sun) of no less than 1.6 AU.\"*  \r\n(So this appears to denote the main asteroid belt, not the Kuiper belt or the Oort cloud.)\r\n\r\n - *\"David Jewitt from UCLA points out that [main-belt comets] are **most likely not comets with sublimating ice, but asteroids that exhibit dust activity**, and hence he and others started calling these class of objects **active asteroids**.\"*  \r\n(The article cites [this video][3], from the official HST channel, as the source for this quote.)\r\n\r\nIf \"active asteroids\" behave exactly like comets, with no measurable difference in observation, how can he claim the two types of objects are likely composed of different materials, and operate with different mechanics?  \r\nWhat is the scientific basis for this differentiation?\r\n\r\n\r\n  [1]: http://physics.stackexchange.com/a/26717/75724\r\n  [2]: https://en.wikipedia.org/wiki/Main-belt_comet\r\n  [3]: https://www.youtube.com/watch?v=CGgRNWUFfZ0\n\nAccepted Answer:\n\nI believe it is safe to say the following:\r\n\r\n 1. If we observe two objects exhibiting identical behavior/measurements/observations, the only reasonable scientific deduction is that the objects are most likely composed of the same materials and operate with the same mechanics - at the very least until future observations can confirm/deny. This appears to be indisputable logic, and the physics community at StackExchange has collectively been unable to demonstrate otherwise as of yet, thereby indirectly validating the integrity of this logic.\r\n 2. Regarding the observational data, David Jewitt of UCLA stated, ***\"In most cases, we don't know if they are icy. In some cases we know that they are not.\"*** Therefore, because we have confirmed through observation that objects exhibiting cometary behavior are not icy, but rather rocky, with no direct observations of ice (yet); using the deduction explained in **#1** above, we can infer that most/all comets are probably not icy, and that most/all comets probably operate with mechanics that do not require ice (i.e. not sublimation).\r\n 3. If the current standard model of the solar system cannot support this scenario, but we cannot modify or discount the logic explained in **#1** above, then the standard model must be altered to conform with this interpretation until future observations can confirm/deny relevant predictions. I will not explain details of what changes would need to be made (unless requested), but generally speaking it would require a fairly comprehensive upheaval of the majority of standard model predictions and mechanics relating to the operation and formation of our solar system.\r\n 4. This is no different from the way the standard model was modified to conform with predictions in the early 1900's, such as the original prediction that comets are composed of sublimating ices, before we had sufficient technology to observe cometary objects being composed of rocky substances instead. As our observations become more accurate with newer technology, our scientific predictions change, and the standard model is expected to reshape in order to match more recent predictions, observations, and interpretations.\r\n 5. If the scientific and academic communities fail to allow the standard model to adjust to accommodate modern observations and the indisputable logic of **#1** above, then they are failing to uphold the definition of the scientific method, and by doing so are stunting the progress of science and greater knowledge for humanity.\n\nAnswer (score=9):\n\nThe key is that asteroids (including \"main-belt comets\") have *very* different orbits from true comets, and this says a lot about their history.\r\n\r\nComets are located in the outer solar system. While they can occasionally be perturbed onto an orbit that takes them close to the Sun, these orbits are necessarily highly eccentric. That is, if you find yourself very far from the Sun, even though you may be on a course for the inner solar system you must return to where you are presently unless some further interaction occurs. The only way for something that started out far away to end up on a roughly circular asteroid-like orbit is to be perturbed inward and then somehow be perturbed again in the inner solar system in just the right way.\r\n\r\nThus most asteroids have been in that $2\\mathrm{-}3\\ \\mathrm{AU}$ region for the entire history of the solar system. Conversely, an object that started out as an asteroid would have a very hard time ending up in a comet-like orbit, so comets must have formed far away.\r\n\r\nNow consider the concept of the [ice line][ice line]. In the early solar system, when there was still a disk of gas a dust swirling around the Sun, somewhere around the present-day asteroid belt was the boundary between where volatiles like water and ammonia could freeze out. Beyond this line, you can get bodies consisting of mostly ice (unless they get too big and start collecting hydrogen and helium via gravity, in which case we call them the outer planets). But just as Earth is more rock and metal than ice, so too are many asteroids.\r\n\r\nMoreover, the present-day ice line is at about $5\\ \\mathrm{AU}$. Inside this radius, exposed water will not last long (compared to the age of the solar system). If an asteroid were made of pure ice (like a comet), it would have sublimated billions of years ago.\r\n\r\nThe thinking is that when we see volatile outgassing from an asteroid, it must be a mixed ice-rock body that recently underwent a collision that exposed a new layer of ice. Now the exposed regions can outgas like a comet, but only for a relatively brief time.\r\n\r\n  [ice line]: http://en.wikipedia.org/wiki/Frost_line_(astrophysics)\n\nAnswer (score=5):\n\nI watched the video in your post, or a similiar one,  on the BBC and from what I remember Jewitt was making a distinction between comets, asteroids and asteroids with \"jets\" of outgoing matter.\n\nI am pretty sure main belt refers to the asteroid belt.\n\n[![enter image description here][1]][1]\n\nJust to clarify the location(s), Jewitt states:\n\n>The significance of the activity is that at least some appear to be a distinct, third comet reservoir in the Solar system (after the Oort cloud and the Kuiper belt). There is no established dynamical pathway between these other reservoirs and the main-belt in the modern solar system, so any ice probably has led a different history from ice in the other comets. Comparing objects from the three reservoirs allows us to sample the protoplanetary disk of the Sun at three locations (the asteroid belt - near 3 AU and formation temperature near 150 K, Oort Cloud comets - 5 to 30 AU formation temperatures 100 to about 50 K; Kuiper belt - beyond 30 AU, 50 K and colder).\n\nFrom [his website][2]:\n\n>Active Asteroids are a newly recognized class of body in the solar system. They are remarkable for having both the orbital characteristics of asteroids and the physical characteristics of comets. What this means is that they look like comets because they show comae and tails but they have orbits interior to Jupiter's (a < a_J) and Tisserand parameters substantially larger than 3, like asteroids.\n>\n>[Note: the definition is an empirical one, based on simple measurable quantities, and does not presume an origin for the comet-like activity by the sublimation of ice. In fact, as described here, there are many possible causes for mass loss from asteroids in addition to sublimation. For this reason, the preferred term for these bodies is \"Active Asteroids\". Formerly, we called them \"Main Belt Comets\" but this was misinterpreted by many people to mean that the active objects are necessarily icy. In most cases, we don't know if they are icy. In some cases we know that they are not.]\n\nHe states that some active asteroids are created by impacts, rotational bursting and other less likely effects such as thermal heating.\n\n  [1]: https://i.sstatic.net/8XdhR.jpg\n  [2]: http://www2.ess.ucla.edu/~jewitt/mbc.html", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 11, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astronomy", "astrophysics", "solar-system", "comets"], "page_start": null, "page_end": null, "quality_flags": ["mixed_license"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 271459, "title": "What is the scientific basis for comets vs. main-belt comets being composed of different materials?", "url": "https://physics.stackexchange.com/questions/271459/what-is-the-scientific-basis-for-comets-vs-main-belt-comets-being-composed-of-d", "share_url": "https://physics.stackexchange.com/q/271459", "content_license": null, "owner": {"display_name": "Giffyguy", "user_id": 75724, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/75724/giffyguy"}}, "answers": [{"answer_id": 273722, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/271459/what-is-the-scientific-basis-for-comets-vs-main-belt-comets-being-composed-of-d/273722#273722", "share_url": "https://physics.stackexchange.com/a/273722", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Giffyguy", "user_id": 75724, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/75724/giffyguy"}}, {"answer_id": 271468, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/271459/what-is-the-scientific-basis-for-comets-vs-main-belt-comets-being-composed-of-d/271468#271468", "share_url": "https://physics.stackexchange.com/a/271468", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "user10851", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, {"answer_id": 271463, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/271459/what-is-the-scientific-basis-for-comets-vs-main-belt-comets-being-composed-of-d/271463#271463", "share_url": "https://physics.stackexchange.com/a/271463", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "user108787", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:309954:0000", "text": "Question: How are gravity tractors possible?\n\nI've heard about how NASA is planning to redirect asteroids by having a satellite tug on it gravitationally using a large enough bolder, but how is that possible? \r\n\r\nI thought that changing the trajectory of something as big as a mountain would require a lot of kinetic energy and I don't see how a tiny satellite can have that much kinetic energy.\n\nAccepted Answer:\n\n> I thought that changing the trajectory of something as big as a mountain would require a lot of kinetic energy and I don't see how a tiny satellite can have that much kinetic energy.\r\n\r\nA gravity tractor would require lots and lots of time, and most likely involve lots and lots of little satellites.\r\n\r\nSuppose we knew that an asteroid would hit the Earth if we did nothing about it, and suppose that we knew that several decades in advance of the predicted collision date. With that kind of lead time, changing the asteroid's velocity by a centimeter per second or so would suffice to make the asteroid miss colliding with the Earth. An acceleration of 10<sup>-10</sup> m/s<sup>2</sup> applied continuously over the course of a decade would more than do the trick.\r\n\r\nThis tiny acceleration is well suited to vehicles with high specific impulse thrusters. A tiny amount of thrust would suffice to keep the vehicle from falling into the asteroid. Keeping the vehicle relatively close to the asteroid would pull the asteroid behind the vehicle thanks to gravitation. Should vehicle #1 run out of propellant mass prior to achieving the desired delta-V, replace it with vehicle #2. Rinse and repeat.\r\n\r\nWhat if we don't have several decades of advance notice? The answer is simple: This approach won't work. We'll need to use something more drastic.\n\nAnswer (score=1):\n\nAn alternative technique to Electro's answer, although with the same name, as described on [Gravity Tractor][1].\r\n\r\nThis is done in 4 stages: \r\n\r\n1. Guide a spaceprobe, which is fitted with a claw arrangement, to the asteroid that requires it's orbit redirected and land on it.\r\n\r\n2. Pick up the largest boulder that fits into the claw, as illustrated below.\r\n\r\n[![enter image description here][2]][2] \r\n\r\n3. Now lift off the asteroid, having  increased the mass of the spaceprobe by the mass of the attached rock, and \"hover\" over a point on the asteroid, deflecting it by continually pulling on it.\r\n\r\n4.  **Given enough time**, the tiny gravitational pull of the spaceprobe will affect the orbt of the asteroid. This is on a timescale of years.\r\n\r\n\r\n  [1]: http://www.space.com/25897-asteroid-deflection-enhanced-gravity-tractor.html\r\n  [2]: https://i.sstatic.net/YMB4o.gif", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 12, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "newtonian-mechanics", "newtonian-gravity", "rocket-science", "propulsion"], "page_start": null, "page_end": null, "quality_flags": [], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 309954, "title": "How are gravity tractors possible?", "url": "https://physics.stackexchange.com/questions/309954/how-are-gravity-tractors-possible", "share_url": "https://physics.stackexchange.com/q/309954", "content_license": null, "owner": {"display_name": "Noah", "user_id": 144488, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/144488/noah"}}, "answers": [{"answer_id": 309983, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/309954/how-are-gravity-tractors-possible/309983#309983", "share_url": "https://physics.stackexchange.com/a/309983", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "David Hammen", "user_id": 52112, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/52112/david-hammen"}}, {"answer_id": 309966, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/309954/how-are-gravity-tractors-possible/309966#309966", "share_url": "https://physics.stackexchange.com/a/309966", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "user140606", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:77871:0000", "text": "Question: Can an asteroid have a magnetic field?\n\nSome M-type asteroids are [believed][1] to be metallic, even possessing Nickel-Iron.\r\n\r\nGiven \r\n\r\n - the proximity of the asteroid belt to Jupiter's magnetosphere, \r\n - the regularity of motion of any given M asteroid relative to Jupiter\r\n - the ease with which Nickel/Iron may be magnetized, and \r\n - the strength of Jupiter's magnetic field\r\n\r\nIs it reasonable to expect an M-type asteroid that fulfills such conditions to exhibit a magnetic field?\r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Iron_meteorite\n\nAccepted Answer:\n\nIn a general sense, yes, asteroids can have magnetic fields - for example, in the article [\"Magnetic Field Signatures Near Galileo's closest approach to Gaspra\"][1] (Kivelson et al. 1993), Gaspra, an S-type asteroid with an unusually metal and olivine abundance exhibited what has been suggested by the article as a measurable magnetic field.\r\n\r\nIn the article [\"Observations of the Magnetic Fields Inside and Outside the\r\nSolar System: From Meteorites (~ 10 attoparsecs),\r\nAsteroids, Planets, Stars, Pulsars, Masers, To Protostellar\r\nCloudlets (< 1 parsec)\"][2] (Vallee, 1998) (section 2.1.3 on page 6-7), further information is provided about the asteroid Gaspra and also mentions another S-type asteroid 243 Ida as having a measurable (likely to be remanent) magnetic field.\r\n\r\nAccording to the article [\"Possible evidence for partial differentiation of asteroid Lutetia from Rosetta\"][3] (Weiss et al. 2012), the M-type asteroid 21 Lutetia also had a measurable magnetic field.  However, \r\n\r\n> whether Lutetia has substantial fine-scale remanent\r\nmagnetization like that expected from an internal core\r\ndynamo (Weiss et al., 2008) or externally generated fields in the\r\nearly solar system (Weiss et al., 2010).\r\n\r\n\r\nAdditional resources:\r\n\r\n> Weiss, B.P., Berdahl, S., Elkins-Tanton, L.T., Stanley, S., Lima, E.A., Carporzen, L.,\r\n2008. Magnetism on the angrite parent body and the early differentiation of\r\nplanetesimals. Science 322, 713–716.\r\n\r\n> Weiss, B.P., Gattacceca, J., Stanley, S., Rochette, P., Christensen, U.R., 2010.\r\nPaleomagnetic records of meteorites and early planetesimal differentiation.\r\nSpace Science Reviews 152, 341–390.\r\n\r\n  [1]: http://www.igpp.ucla.edu/people/mkivelson/Publications/146-93Sci261331.pdf\r\n  [2]: http://ned.ipac.caltech.edu/level5/March03/Vallee2/paper.pdf\r\n  [3]: http://scripts.mit.edu/~paleomag/articles/Weiss_2012_PSS.pdf", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 13, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astronomy", "magnetic-fields"], "page_start": null, "page_end": null, "quality_flags": ["single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 77871, "title": "Can an asteroid have a magnetic field?", "url": "https://physics.stackexchange.com/questions/77871/can-an-asteroid-have-a-magnetic-field", "share_url": "https://physics.stackexchange.com/q/77871", "content_license": null, "owner": {"display_name": "Everyone", "user_id": 5265, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/5265/everyone"}}, "answers": [{"answer_id": 77873, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/77871/can-an-asteroid-have-a-magnetic-field/77873#77873", "share_url": "https://physics.stackexchange.com/a/77873", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "user29350", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:502859:0000", "text": "Question: How to estimate the mass of an incoming asteroid?\n\nA second galactic visitor has just been observed: see this [link][1].\r\n\r\nFor a mass which is small enough to have no noticable effect on other masses, is there any other way to infer the mass? Its own motion is a function of other masses. Obviously a sufficiently resolved photograph giving the volume, and a wild guess as to the density would work, but that would be a WAG, and in any case we don't have a real picture of this thing really.\r\n\r\nIf we could get a probe to it, we could put something in orbit around it. Is there any other way?\r\n\r\n\r\n  [1]: https://www.forbes.com/sites/jonathanocallaghan/2019/09/11/a-second-interstellar-object-has-almost-certainly-been-found-in-our-solar-system/?fbclid=IwAR1hSUVxstqSE_VAMgl-Y0MLURrV4WGdXxKJTYtwAikB_aA4mQImWnrDnZg#271654467b99\n\nAccepted Answer:\n\nIf it is small enough and observations of its trajectory are accurate enough then one can use the fact that the relative sizes of radiation pressure (from the Sun) and gravity depend on the density and geometry.\r\n\r\ne.g., Suppose the thing was a black sphere (NB adding an albedo just changes the arithmetic rather than the principle) of density $\\rho$ and radius $a$ and was at a distance $r$ from the Sun.\r\n\r\nThe gravitational acceleration would be \r\n$${\\bf g} = -\\frac{GM_\\odot}{r^2}e_r\\ ,$$\r\nand is independent of the mass of the object.\r\n\r\nThe acceleration due to radiation pressure would be\r\n$$ {\\bf g}_{\\rm rad} = \\left(\\frac{L_\\odot}{4\\pi r^2}\\right)\\left(\\frac{\\pi a^2}{4\\pi a^3\\rho/3}\\right)e_r = \\left(\\frac{3L_\\odot}{16\\pi a\\rho r^2}\\right)e_r\\ .$$\r\nWhich depends on some combination of the size and density of the object. In this case, we are not interested in the density and we can assume the size can be measured in some way, so $\\rho = 3m/4\\pi a^3$ and the gravitational acceleration due to radiation pressure is\r\n$$ {\\bf g}_{\\rm rad} =  \\left(\\frac{L_\\odot a^2}{4 m r^2}\\right) e_r\\ .$$\r\n\r\nThus the trajectory  will be determined by an acceleration that is slightly smaller than the gravity due to the Sun by an amount that depends on the ratio the area it presents to the Sun divided by its mass.\r\n\r\nThis could well be a tiny perturbation - though obviously gets bigger the more like a \"solar sail\" the object is (i.e. a large $a^2/m$) - but of course the effects are integrated over the course of its passage through the Solar System.\r\n\r\nIt was exactly this type of argument that lead [Bialy & Loeb (2018][1]) to suggest that \"Oumuamua\" (the first example of an identified interstellar rock) had a non-gravitational acceleration that could be explained if it had a high area to mass ratio, such that it might be less than a cm thick and weigh only of order 1000 kg.\r\n\r\nThe problem with using this technique is that you have to eliminate or separate it from other sources of non-gravitational acceleration - like outgassing from cometary material.\r\n\r\n\r\n  [1]: https://arxiv.org/abs/1810.11490", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 14, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-4.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "mass", "astronomy", "measurements"], "page_start": null, "page_end": null, "quality_flags": ["single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 502859, "title": "How to estimate the mass of an incoming asteroid?", "url": "https://physics.stackexchange.com/questions/502859/how-to-estimate-the-mass-of-an-incoming-asteroid", "share_url": "https://physics.stackexchange.com/q/502859", "content_license": null, "owner": {"display_name": "Dov", "user_id": 2653, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/2653/dov"}}, "answers": [{"answer_id": 748926, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/502859/how-to-estimate-the-mass-of-an-incoming-asteroid/748926#748926", "share_url": "https://physics.stackexchange.com/a/748926", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "ProfRob", "user_id": 43351, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/43351/profrob"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:26476:0000", "text": "Question: Can a minimum ultimate tensile strength (UTS) for an asteroid be established based on its diameter and rotation?\n\nI found it fascinating that many asteroids [rotate with a period of just seconds][1]. For this fast of a rotation on this size of object, I thought that would actually cause significant acceleration on the \"equator\" of the rotation. This is not the case for planets where gravitation still causes a net downward force on its equator. An entity on the surface of such as asteroid will have to hold on to the surface in order to keep from flying off.\r\n\r\nHere is what I calculate for the surface acceleration (again, *off of the surface*) for the notable examples in the above link:\r\n\r\n    Asteroid    Period (s)  Radius (m)  v^2/r (m/s)\r\n    -----------------------------------------------\r\n    2010 JL88     24.5       15          0.987\r\n    2010 WA       31          3          0.123\r\n    2008 HJ       42.7       24          0.520\r\n    2000 DO8      78         30          0.195\r\n    2003 DW10    100         20          0.079\r\n    2003 EM1     111.6       33          0.105\r\n\r\nI consider these accelerations to be quite considerable. If the surface was in any way sandy, for instance, the top layer would just fly off. Obviously some sort of cohesion is necessary.\r\n\r\n<h1>My Question</h1>\r\n\r\nAre the fastest rotation periods of asteroids limited by asteroid material, or is there no astrophysical way for them to be spun fast enough to matter in the first place? Given what we know about asteroids, what is the surface acceleration at which we expect them to fall apart at, and how does that compare to what we observe?\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Largest_asteroids#Fastest_rotating_objects\n\nAccepted Answer:\n\nThese objects have been called \"superfast rotators\" lately.  A google search for this term found [a very nice paper][1] which talks about tensile strength and theoretical explanations for high rotation rates.\r\n\r\n\r\n  [1]: http://www.amostech.com/TechnicalPapers/2009/Astronomy/Ryan.pdf\n\nAnswer (score=1):\n\nI inadvertently stumbled upon a relevant quote about this.\r\n\r\nhttp://books.google.com/books?id=JwHTyO6IHh8C&lpg=PA471&ots=AI73Vfs-4N&dq=asteroid%20internal%20pressure&pg=PA471#v=onepage&q=asteroid%20internal%20pressure&f=false\r\n\r\n> While no asteroid larger than ~150m shows evidence for global cohesion, almost all asteroids smaller than this must be cohesive.  One might suppose that every larger asteroid is a gravitational aggregate of smaller pieces, whereas every small asteroid is a fast-rotating collisional shard.  But the term \"monolith\" for these smallest asteroids is misleading.  Consider a spherical object of uniform density $\\rho$ rotating with a frequency $\\omega$; the mean stress accross its equator is ~$R^2 \\rho \\omega^2$.  For the well-studied fast rotator 1998 KY26 (*Ostro et al., 1999*), self-gravity is not capable of holding it together; however, its ~11-min period and ~30-m diameter requires only a tensile strength of ~$300 dyn/cm^2$ (presuming $\\rho \\approx 1.3 g/cm^3$ for this C-type), order of magnitude weaker than the tensile strength of snow.\r\n\r\nThe question was somewhat misguided.  Of course you can't determine the strength of asteroids from their rotation, largely because the rotational stress isn't very restrictive.\r\n\r\nI find the $R^2$ term from this reference somewhat misleading, because the important to observe about such a arbitrary spherical rotating body is that the maximum period of rotation without the equator exceeding escape velocity is irrelevant of $R$.  The centripetal acceleration is directly proportional to $R$ and so is gravity.\r\n\r\nFor the examples in the OP, however, there is a valid point that the necessary material strength just isn't very important due to the $R^2$ dependence.  To apply the reference's logic to the first one:\r\n\r\n$$\\omega = \\frac{2 \\pi}{T} = \\frac{2 \\pi}{24.5 s} $$\r\n\r\n$$ \\sigma \\approx R^2 \\rho \\omega^2 = (15 m) \\left(1.3 \\frac{g}{cm^3} \\right) \\left( \\frac{2 \\pi}{24.5 s} \\right)^2 = 19 kPa  $$\r\n\r\nThe snow reference comes to $30 Pa$.  Even so, a few kPa isn't much at all.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 15, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "material-science", "rotation"], "page_start": null, "page_end": null, "quality_flags": ["html_content_needs_postprocessing"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 26476, "title": "Can a minimum ultimate tensile strength (UTS) for an asteroid be established based on its diameter and rotation?", "url": "https://physics.stackexchange.com/questions/26476/can-a-minimum-ultimate-tensile-strength-uts-for-an-asteroid-be-established-bas", "share_url": "https://physics.stackexchange.com/q/26476", "content_license": null, "owner": {"display_name": "Alan Rominger", "user_id": 1255, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1255/alan-rominger"}}, "answers": [{"answer_id": 26477, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/26476/can-a-minimum-ultimate-tensile-strength-uts-for-an-asteroid-be-established-bas/26477#26477", "share_url": "https://physics.stackexchange.com/a/26477", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Sonia", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, {"answer_id": 56381, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/26476/can-a-minimum-ultimate-tensile-strength-uts-for-an-asteroid-be-established-bas/56381#56381", "share_url": "https://physics.stackexchange.com/a/56381", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Alan Rominger", "user_id": 1255, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1255/alan-rominger"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:26080:0000", "text": "Question: How bright will an asteroid be?\n\n[In the news][1], it has been stated that there will be a fairly large asteroid passing fairly close to Earth soon. I've been trying to find a good observation guide, including determining how bright it will be, with bad luck. Specifically, I want to find out if this is a naked eye visible object, and if so, how to find it. Thanks!\r\n\r\n\r\n  [1]: http://lightyears.blogs.cnn.com/2011/11/03/asteroid-to-pass-closer-to-earth-than-the-moon/?hpt=hp_t2\n\nAccepted Answer:\n\nI think you're referring to asteroid 2005 YU55 which is making an approach on November 8.\r\n\r\n[This article on NASA JPL's Asteroid Watch site][1] gives some details including:\r\n\r\n> The asteroid's surface is darker than charcoal at optical wavelengths.\r\n> Amateur astronomers who want to get a glimpse at YU55 will need a\r\n> telescope with an aperture of 6 inches (15 centimeters) or larger.\r\n\r\nwhich to me implies that it would not be visible with the _naked eye_.  It's estimated size is 1,300 feet (400 meters).\r\n\r\n<br>\r\nJPL has also posted an [hour-long video here][2] discussing the asteroid.  \r\n<br>\r\n\r\nAnother NASA site, the [Near Earth Object Program][3], has better technical information which may help locate the asteroid.  Some excerpts:\r\n\r\n> ...the object will reach a visual brightness of 11th magnitude and\r\n> should be easily visible to observers in the northern and southern\r\n> hemispheres. The closest approach to Earth and the Moon will be\r\n> respectively 0.00217 AU and 0.00160 AU on 2011 November 8 at 23:28 and\r\n> November 9 at 07:13 UT.\r\n> \r\n> ...\r\n>\r\n> The best time for new ground-based optical and infrared observations\r\n> will be late in the day on November 8, after 21:00 hours UT from the\r\n> eastern Atlantic and western Africa zone.\r\n\r\n<br>\r\nTo get the exact coordinates, you can try NASA's Solar System Dynamics site.  [The page for asteroid 2005 YU55][4] has orbital elements with a [link to generate ephemeris][5].\r\n\r\n\r\n  [1]: http://www.jpl.nasa.gov/asteroidwatch/newsfeatures.cfm?release=2011-332\r\n  [2]: http://www.ustream.tv/recorded/18250783\r\n  [3]: http://neo.jpl.nasa.gov/news/news171.html\r\n  [4]: http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2005%20YU55;orb=1\r\n  [5]: http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=sb&sstr=2005%20YU55\n\nAnswer (score=3):\n\nDon't bother checking exact locations until a few hours before closest passage (on Tuesday evening in North America) because the orbit will have last minute changes.  The asteroid is predicted to be around 10th magnitude, but from personal experience with several earlier asteroid passages you will need at least a 10-inch aperture to see it. What has worked for me is to find a small star pattern somewhere ahead of the asteroids position on its track, concentrate on that and wait for the asteroid to move into view. In most cases, the asteroids have been ahead of their predicted positions by about 5 minutes, probably because of acceleration by the Earth's gravity. Good luck!", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 16, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astronomy"], "page_start": null, "page_end": null, "quality_flags": [], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 26080, "title": "How bright will an asteroid be?", "url": "https://physics.stackexchange.com/questions/26080/how-bright-will-an-asteroid-be", "share_url": "https://physics.stackexchange.com/q/26080", "content_license": null, "owner": {"display_name": "PearsonArtPhoto", "user_id": 871, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/871/pearsonartphoto"}}, "answers": [{"answer_id": 26081, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/26080/how-bright-will-an-asteroid-be/26081#26081", "share_url": "https://physics.stackexchange.com/a/26081", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "user921", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, {"answer_id": 26082, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/26080/how-bright-will-an-asteroid-be/26082#26082", "share_url": "https://physics.stackexchange.com/a/26082", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Geoff Gaherty", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:575034:0000", "text": "Question: How does one calculate the chance that an asteroid hits the Earth and what does this chance mean?\n\nI read in [this][1] article (in \"Independent\"):\r\n\r\n> An asteroid that is projected to come close to Earth later this year has a 0.41 percent chance of hitting the planet, according to Nasa data.\r\nThe Center for Near-Earth Object Studies (CNEOS), from Nasa’s Jet Propulsion Laboratory, said the celestial object, known as 2018VP1, is predicted to pass near Earth one day before the US presidential election on 2 November.\r\n\r\nAs usual, I started wondering. How did the people involved at NASA calculated this 0,41% chance of an impact on the earth? What does it even mean to say this? I had the impression that the trajectory of an object thought to come close to earth within a few months could be calculated exactly.  \r\nSo how did they calculate this uncertainty? Which variables are involved and why they possess uncertainty?\r\n\r\n\r\n  \r\n\r\n\r\n \r\n\r\n\r\n  [1]: https://mashupmd.com/asteroid-heading-towards-earth-has-0-41-per-cent-chance-of-hitting-planet/?utm_source=related_article_headline&utm_medium=Asteroid%20heading%20towards%20Earth%20%E2%80%98has%200.41%20per%20cent%20chance%20of%20hitting%20planet%E2%80%99-clk_0&utm_campaign=mashupmd_related_articles\n\nAccepted Answer:\n\nAn object discovered recently (2018, I would assume from the name) will have some uncertainty in its position and 3D velocity. That uncertainty will translate into a bigger uncertainty in its position at some time in the future.\r\n\r\nIf you take a simple example of an object at the origin, measured to be moving along the x-axis. To keep things simple, assume there are no forces acting upon it, but there is an uncertainty in its three velocity components. i.e. it has velocity components $v_x \\pm \\Delta v_x$, $0 \\pm \\Delta v_y$ and $0\\pm \\Delta v_z$.\r\n\r\nNow you have a disc, at position $x_0$ along the x-axis, oriented so the flat part is in the $yz$ plane, and with some radius $r$ and you want to work out whether your object will hit it. Obviously if the velocity has its centrally estimated value it will hit it bang on. But after a time $t= x_0/v_x$, if the velocity components in the $y$ and $z$ directions did have a non-zero value, as suggested by their error bars, then the uncertainty in the y and z coordinates at time $t$ would be $\\Delta v_y t$ and $\\Delta v_z t$. The probability of hitting the disc would be 1 minus the integral of the probability distribution of the y,z position from a radius $r$ out to infinity. You can imagine this like a cone of possible positions that grows with time, and you are calculating what fraction of the cone cross-sectional area is intercepted by the disc.\r\n\r\nThe task is also complicated for small asteroids because they can be accelerated by non gravitational effects. These include outgassing and mass loss (more important for comets) and the Yarkovsky effect (anisotropic re-radiation of solar flux) and Poynting-Robertson drag (e.g. [Broz et al. 2005][1]), leading to rather unpredictable, gradual changes in the orbit.\r\n\r\nWith this particular object I think it is just that the orbital parameters are rather imprecise. The closest approach is projected to be about 400,000 km, but with a 3-sigma uncertainty of about 4 million km (and the time of arrival is uncertain by a few days). Only a small fraction of that probability distribution ends up with an impact. Since the object is also about 1-m in size, it is unlikely to much damage and would break up in the atmosphere. Because it is so faint, I doubt the parameters will be improved until a few weeks before the fly-by.\r\n\r\n  [1]: https://sirrah.troja.mff.cuni.cz/~mira/fyzika_malych_teles/iaus229_nongravs_review_broz.pdf\n\nAnswer (score=2):\n\nEvery measurement, and every digital computation, has limited accuracy: it might not be spot on the true value. It will have an \"error bound\", a range of values within which the true value will (almost) certainly be found. The further a value is from the measured value, the less likely it is to be the true value.\r\n\r\nIn a sequence of measurements and calculations, the error bound will grow, the uncertainty increase, fewer possibilities can be ruled out and any given value will have less chance of being the true value.\r\n\r\nThe likelihood of a given outcome being the true value is then expressed as a probability, say 0.41% or 1 in 240.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 17, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-4.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "newtonian-mechanics", "orbital-motion", "probability", "estimation"], "page_start": null, "page_end": null, "quality_flags": [], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 575034, "title": "How does one calculate the chance that an asteroid hits the Earth and what does this chance mean?", "url": "https://physics.stackexchange.com/questions/575034/how-does-one-calculate-the-chance-that-an-asteroid-hits-the-earth-and-what-does", "share_url": "https://physics.stackexchange.com/q/575034", "content_license": null, "owner": {"display_name": "Deschele Schilder", "user_id": 98822, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/98822/deschele-schilder"}}, "answers": [{"answer_id": 575048, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/575034/how-does-one-calculate-the-chance-that-an-asteroid-hits-the-earth-and-what-does/575048#575048", "share_url": "https://physics.stackexchange.com/a/575048", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "ProfRob", "user_id": 43351, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/43351/profrob"}}, {"answer_id": 575061, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/575034/how-does-one-calculate-the-chance-that-an-asteroid-hits-the-earth-and-what-does/575061#575061", "share_url": "https://physics.stackexchange.com/a/575061", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "Guy Inchbald", "user_id": 219654, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/219654/guy-inchbald"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:439977:0000", "text": "Question: Why is ʻOumuamua traveling so slowly?\n\nImagine a space rock that starts at rest at the Lagrange point between the sun and our nearest stellar neighbor, Alpha Centauri.  That would put it at about 2 light years away from the sun.\r\n\r\nNow, it gets nudged slightly toward the sun.  How fast will it be going when it nears the sun?\r\n\r\nI used the formula for instantaneous velocity of a falling object that has traveled distance over a large fall distance, found on this Wikipedia page:\r\n\r\nhttps://en.wikipedia.org/wiki/Equations_for_a_falling_body\r\n\r\n\r\nUsing the following values:\r\n\r\nG = 6.674 × 10^−11 N·m^2/kg^2\r\n\r\nM = 1.989 × 10^30 kg\r\n\r\nr = 695,700,000 m\r\n\r\nd = 2 light years = 9.4607 × 10^15 m \r\n\r\n\r\nI calculate that by the time the space rock reaches the sun, it is traveling at 617,752 m/s.  I would think that this would be the *minimum* speed of any interstellar object that reaches us because, presumably, it would already have some nonzero speed when it enters the sun's gravitational influence.\r\n\r\nHowever, news reports say that ʻOumuamua is only traveling at 26,330 m/s.  Why am I off by a factor of 23?\n\nAccepted Answer:\n\nIt is travelling at that speed *now* after being decelerated by the Sun's gravitational field as it climbs out of the Sun's gravitational potential.\r\n\r\n\r\nThe maximum speed at perihelion was about 87 km/s.\r\n\r\nThe discrepancy between this and your 617 km/s figure is just that perihelion (closest approach to the Sun) was around 0.25 au. If its trajectory had taken it much closer to the Sun, its speed would have approached the figure you calculated.\r\n\r\nRoughly: for a body with zero kinetic energy at infinity, it will travel with a speed $\\sqrt{2GM_{\\odot}/R}$ when at a distance $R$ from the Sun (just conservation of energy).\r\n\r\nFor $R=0.25$ au, we get a speed of 84.3 km/s. Because Oumuamua started with a speed of about 26 km/s at infinity, its maximum speed was a touch greater.\r\n\r\nThe main misunderstanding here is just applying conservation of energy and assuming the asteroid can get arbitrarily close to the Sun. This isn't the case because angular momentum must also be conserved and this limited the closest approach to 0.25 au.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 18, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-4.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "newtonian-gravity", "solar-system", "celestial-mechanics", "free-fall"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 439977, "title": "Why is ʻOumuamua traveling so slowly?", "url": "https://physics.stackexchange.com/questions/439977/why-is-%ca%bboumuamua-traveling-so-slowly", "share_url": "https://physics.stackexchange.com/q/439977", "content_license": null, "owner": {"display_name": "SlowMagic", "user_id": 134671, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/134671/slowmagic"}}, "answers": [{"answer_id": 439979, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/439977/why-is-%ca%bboumuamua-traveling-so-slowly/439979#439979", "share_url": "https://physics.stackexchange.com/a/439979", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "ProfRob", "user_id": 43351, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/43351/profrob"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:200085:0000", "text": "Question: Asteroid as a \"free slingshot?\"\n\nThe question [Halley's Comet as a \"Free Taxi\"][1] had an interesting thought, even though it didn't work out. I was wondering about a variation on the idea. Use a cable, as illustrated below. Could this be made anything like practical given reasonable assumptions? \r\n\r\n[![enter image description here][2]][2]\r\n\r\nThe idea is \r\n\r\n* Wait for an asteroid to fly close to Earth. \r\n* Put a probe, long cable, and net in its path.\r\n* When the probe has swung through a half circle, it lets go of the cable. \r\n\r\nThoughts\r\n\r\n* Military planes have picked up downed pilots with a system something like this. \r\n* An asteroid of a few tons would be small enough to grab, and large enough to throw a probe. \r\n* It would take relatively little energy to reach an asteroid that passes near Earth, especially if you don't have to match its speed. \r\n* The strength of the cable would be a limit on how much speed you could gain. A long cable reduces acceleration, but has more mass. \r\n* The asteroid might be moving fast. To avoid breaking the cable, you might have to use a rocket to partially catch up. \r\n* How would you make the net? \r\n* In the illustration, the probe is slung in the direction the asteroid travels. This might not be an interesting direction. The probe could let go early to choose another direction. But this would reduce the final speed. \r\n* The probe would acquire spin. You might be able to avoid that by attaching the cable to the probe with a hinge aligned with the probe's center of mass.\r\n\r\n---\r\nLate thought - Add a small rocket to the net. Accelerate the free end of the cable to match velocity with the asteroid while leaving the probe still. Now you don't smash the net, and you still haven't used all that much energy. \r\n\r\nIt still may not be a reasonable approach, but it is sounding more possible now. \r\n\r\n  [1]: https://physics.stackexchange.com/q/199899/37364\r\n  [2]: https://i.sstatic.net/nMNVk.png\n\nAccepted Answer:\n\nThink of shooting a shell from a battleship at an instrument, and designing a device that allows it to hold on and continue working while it is transported by the shell.  That's probably easier than doing the same with an asteroid.\r\n\r\nIf you take a look at [this table][1] of near-earth asteroid approaches, you'll see that the relative velocity column has units of km/s.  We simply have no materials that can cope with such a difference without breaking or crushing.  \r\n\r\nRight now, we tend to limit accelerations on launch vehicles to around $4g$ or so.  Lets say we were dedicated and built some instruments that could withstand a very aggressive $40g$.  Now we want it to grab on to an asteroid with a relative velocity of $6 km/s$.  it would require a minimum of $15s$ for the acceleration.  In that time, the asteroid would have moved over $90km$.  We'd need to have some mechanism that is far stronger than anything we have today, and it would have to be able to interact with the object at distances on the order of $100km$.  \r\n\r\n\r\n  [1]: http://neo.jpl.nasa.gov/ca/", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 19, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "rocket-science", "solar-system-exploration"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 200085, "title": "Asteroid as a \"free slingshot?\"", "url": "https://physics.stackexchange.com/questions/200085/asteroid-as-a-free-slingshot", "share_url": "https://physics.stackexchange.com/q/200085", "content_license": null, "owner": {"display_name": "mmesser314", "user_id": 37364, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/37364/mmesser314"}}, "answers": [{"answer_id": 200092, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/200085/asteroid-as-a-free-slingshot/200092#200092", "share_url": "https://physics.stackexchange.com/a/200092", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "BowlOfRed", "user_id": 55662, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/55662/bowlofred"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:70444:0000", "text": "Question: How early might a moon-sized asteroid be detected?\n\nIf an enormous asteroid, approximately the size of our moon (~2000-mile diameter), was passing close to Earth, how early might we detect something that large? Or alternatively, how close could something that large get without detection?\r\n\r\n[edited to change term from meteor to asteroid]\n\nAccepted Answer:\n\nYou want the time. Simply put, for that the minimum requirement is position (and hence the distance) and velocity. To know the position you need to detect it. Once you detect it, you can calculate the trajectory and thus the time you have to settle your issues (assuming it is on a collision course).\r\n\r\nI got this from CNN:\r\n>The B612 Foundation is building the Sentinel Space Telescope, the world's most powerful asteroid detection and tracking system, to see the millions of asteroids we can't see today and could pose threats to our planet.\r\n\r\nAlso, NEOSSat, the Near Earth Object Surveillance Satellite, is a micro-satellite launched in February 2013 by the Canadian Space Agency (CSA) that will hunt for NEOs in space.\r\n\r\nTracking systems are recording asteroids even as large as 140 meters. Any asteroid with a radius more than 300 meters means an assured global catastrophe. [Check this out](http://pan-starrs.ifa.hawaii.edu/public/asteroid-threat/asteroid_threat.html). The size that you are asking about is so big that it will create noticeable gravitational effects (like perturbation in orbit) and so we will know about it.\n\nAnswer (score=2):\n\nSince Pluto is about the size of our moon, clearly we can detect an object of that size at least at the distance of Pluto.  We can probably spot something like that some distance into the Kuiper belt with current technology.\r\n\r\nOf course the ability to detect something and actually detecting it are two different things.  We have to be looking in the right place at the right time under the right conditions to detect it.\n\nAnswer (score=1):\n\nTo give an indication, comet ISON was seen first by two amateur astronomers when it was at magnitude 18.8, so a large body approaching from a distance might be found at about the same magnitude. Depending on albido, a moon size object could reach that magnitude at about twice the distance of pluto. The actual distance would also depend on how fast it was moving because it needs to move against background stars to be spotted by comet hunters. \r\n\r\nA large professional telescope could see it at a much larger distance but that would require incredible luck.\r\n\r\nIf luck was against us it could come a little closer before being spotted. If it came from behind the Sun it might be missed until the Earth moved round but it is hard to imagine that it could come within the orbits of the gas giants before being seen unless it was coming in very fast.\r\n\r\nOf course this is a highly unlikely event. we have never seen an object anything like that size come into the solar system and if it did then the chances of it coming close to Earth are miniscule. The real danger is from much smaller bodies.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 20, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astronomy"], "page_start": null, "page_end": null, "quality_flags": ["low_score"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 70444, "title": "How early might a moon-sized asteroid be detected?", "url": "https://physics.stackexchange.com/questions/70444/how-early-might-a-moon-sized-asteroid-be-detected", "share_url": "https://physics.stackexchange.com/q/70444", "content_license": null, "owner": {"display_name": "Tanya", "user_id": 26836, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/26836/tanya"}}, "answers": [{"answer_id": 70563, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/70444/how-early-might-a-moon-sized-asteroid-be-detected/70563#70563", "share_url": "https://physics.stackexchange.com/a/70563", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Nix", "user_id": 24705, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/24705/nix"}}, {"answer_id": 89393, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/70444/how-early-might-a-moon-sized-asteroid-be-detected/89393#89393", "share_url": "https://physics.stackexchange.com/a/89393", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Olin Lathrop", "user_id": 20848, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/20848/olin-lathrop"}}, {"answer_id": 89396, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/70444/how-early-might-a-moon-sized-asteroid-be-detected/89396#89396", "share_url": "https://physics.stackexchange.com/a/89396", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Philip Gibbs - inactive", "user_id": 1341, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1341/philip-gibbs-inactive"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:53177:0000", "text": "Question: What are the odds of 2012 DA14 hitting a satellite or the moon?\n\nAsteroid [2012 DA14](http://en.wikipedia.org/wiki/2012_DA14) will pass by close to Earth. *Very* close. So close, in fact, that it's inside the orbit of the moon and even inside the orbit of geostationary satellites, as shown by this illustration:\r\n\r\n![2012 DA14 approach to Earth][1]\r\n*Source: [NASA](http://neo.jpl.nasa.gov/news/news174.html)*.\r\n\r\nWhat the figure doesn't show is the angle of its path relative to the orbital plane of the moon and the orbital plane of geostationary satellites. What is this angle? What is the risk that it hits either the moon, or a geostationary satellite?\r\n\r\nI guess it's quite small, to quote Douglas Adams:\r\n\r\n> \"Space,\" it says, \"is big. Really big. You just won't believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space, listen...\"\r\n\r\nbut I'd be interested in a slightly more quantitative analysis.\r\n\r\n<sup>\r\n(Googling this event is quite amusing, with one source [godlikepredictions](http://www.godlikeproductions.com/forum1/message2075927/pg1) claiming that *Asteroid 2012 DA14 will pass earth within .09 AU, missing Earth, but what is located at 1 AU from Earth???? Our moon!!!!!!* (SIC).</sup>\r\n\r\n  [1]: https://i.sstatic.net/XoFFZ.jpg\n\nAccepted Answer:\n\nSee the [JPL Java applet][1] to see how the orbit is tilted wrt the Earth. From playing about with the applet I'd say it's orbit is tilted by something between 10 and 15 degrees relative to us. That means it isn't in the same plane as the Moon or geostationary satellites and therefore has zero chance of hitting either.\r\n\r\nIt could hit any satellites orbiting at the distance of closest approach, 27,700 km. However I don't think there are many satellites at that distance. The GPS satellites get closest, but I think the highest of those are around 26,000 km, so the chance of hitting a satellite is zero as well.\r\n\r\nBoring really :-)\r\n\r\n**Response to comment**:\r\n\r\nBecause the orbit of 2012-DA14 is tilted wrt to the Earth's orbit it only intersects the plane of the Earth's orbit at two points. Only one of these is relevant to this discussion, and it's the one at the closest approach of 27,700 km. So it can only intersect the orbit of a satellite if the altitude of that satellite is 27,700 km. This makes it impossible to hit the moon or and geostationary satellites i.e. the chance of collision is zero not \"very small\".\r\n\r\nThe chance of hitting a GPS satellite is also zero because the highest of them are at about 26,000 km.\r\n\r\nI don't know if there are any satellites at an altitude of 27,700 km (though I don't think so), and if there are satellites at this altitude the chance would be just \"virtually zero\".\r\n\r\n  [1]: http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2012%20DA14;orb=1", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 21, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "space"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 53177, "title": "What are the odds of 2012 DA14 hitting a satellite or the moon?", "url": "https://physics.stackexchange.com/questions/53177/what-are-the-odds-of-2012-da14-hitting-a-satellite-or-the-moon", "share_url": "https://physics.stackexchange.com/q/53177", "content_license": null, "owner": {"display_name": "gerrit", "user_id": 6319, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/6319/gerrit"}}, "answers": [{"answer_id": 53210, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/53177/what-are-the-odds-of-2012-da14-hitting-a-satellite-or-the-moon/53210#53210", "share_url": "https://physics.stackexchange.com/a/53210", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "John Rennie", "user_id": 1325, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1325/john-rennie"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:503910:0000", "text": "Question: What effect would this most recent asteroid fly-by have had if it impacted?\n\nAccording to [this article][1], an asteroid came dangerously close to the Earth.  The asteroid was roughly the size of a football field.\r\n\r\nWhat would the result of this asteroid be if it had impacted square on the Earth's surface?  If negligible, at what mass does an asteroid impact become critical?\r\n\r\n\r\n  [1]: https://www.buzzfeednews.com/article/danvergano/nasa-emails-asteroid-2019-ok\n\nAccepted Answer:\n\nAsteroid 2019 OK is about the size of a football field, and was travelling about 54,379 miles per hour, with closest approach of about 44,300 miles. If it would have hit Earth it would have been a city killer, with about 30 times the energy of the Hiroshima bomb.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 22, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-4.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 503910, "title": "What effect would this most recent asteroid fly-by have had if it impacted?", "url": "https://physics.stackexchange.com/questions/503910/what-effect-would-this-most-recent-asteroid-fly-by-have-had-if-it-impacted", "share_url": "https://physics.stackexchange.com/q/503910", "content_license": null, "owner": {"display_name": "Jason P Sallinger", "user_id": 85056, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/85056/jason-p-sallinger"}}, "answers": [{"answer_id": 503923, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/503910/what-effect-would-this-most-recent-asteroid-fly-by-have-had-if-it-impacted/503923#503923", "share_url": "https://physics.stackexchange.com/a/503923", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "Adrian Howard", "user_id": 233597, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/233597/adrian-howard"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:34170:0000", "text": "Question: Impact of Apophis\n\nI read about the near-earth asteroid [99942 Apophis](http://en.wikipedia.org/wiki/99942_Apophis). It is in the first place of Potentially Hazardous Asteroids (visiting earth at an altitude of about 36,000km on 2028). But, scientists have calculated that the asteroid would impact earth on 2036. What is the fact behind this? I mean, Why is this difference in period of 8 years for a close-encounter and then impact? Is Apophis, a part of the asteroid belt between Mars and Jupiter or is it a part of the solar system? Would we actually get an \"Armageddon\"? Even if it doesn't impact, it could cause some damages either to our atmosphere or even our environment, Nah?\n\nAccepted Answer:\n\nThe orbital period of Apophis is about 324 days, but the orbits of asteroids tend to be chaotic. Because their orbits are eccentric they may make close approaches to other bodies like the Earth or Venus, and this will change the orbit in ways that are difficult to calculate. Well, difficult to calculate far into the future anyway. Technically the orbits of all Solar System bodies, including the Earth, are chaotic but since the Earth doesn't make any close approaches to other heavy objects it's orbit changes only slowly and within narrow bands.\r\n\r\nAnyhow, the variable orbit of Apophis is why it's hard to be precise about whether it will hit the Earth. The close approach in 2029 will change it's orbit in such a way that 7 years (i.e. 8 orbits) later it could collide wth Earth. But then we don't know exactly how close it will come in 2029 so we can't predict exactly how close it will come in 2036.\r\n\r\nI'm not sure that anyone knows where Apophis came from. A quick Google shed no light on the subject. I would guess it originated in the Asteroid belt because if it came from much farther out it's orbital period would be longer.\r\n\r\nAs to the impact, there are so many variables it's difficult to say what damage there would be. Apophis is a lot smaller than the asteroid that killed off the dinosaurs, so we're not looking at the end of all life on Earth. However if it hit a big city there would be little if anything left of the inhabitants!", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 23, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "earth"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 34170, "title": "Impact of Apophis", "url": "https://physics.stackexchange.com/questions/34170/impact-of-apophis", "share_url": "https://physics.stackexchange.com/q/34170", "content_license": null, "owner": {"display_name": "anon", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, "answers": [{"answer_id": 34176, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/34170/impact-of-apophis/34176#34176", "share_url": "https://physics.stackexchange.com/a/34176", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "John Rennie", "user_id": 1325, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1325/john-rennie"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:517185:0000", "text": "Question: An asteroid strikes Earth. I’m skydiving. Do I die?\n\nLet’s say I’m skydiving at a high altitude and, while falling, an asteroid strikes Earth. Is there a certain mass / momentum of the asteroid below which I would have a chance of surviving? Or would any reasonably sized body smashing into Earth have an impact strong enough to create vibrations through air that would kill me, or something to that effect?\n\nEdit: I’m not certain on how big an object should be to be considered an asteroid. Let’s say it’s wider than 500 meters.\n\nAccepted Answer:\n\nThe question is badly posed: survival depends not just on the asteroid mass and velocity, but also distance. If you are far away most effects attenuate a lot - the skydiving has only minor effects. \r\n\r\nThe danger from asteroid impacts come from several sources: heat emissions, the overpressure wave, ejecta, and long-term effects. The fact that you are up in the air protects you from seismic effects (falling buildings and tsunamis). Collins, Melosh and Marcus have [an excellent paper][1] describing their estimates of the effect sizes which is used in [this web app for simulating impact effects][2]. In general the formulas are not very neat.\r\n\r\nA big meteor impact releases heat not unlike a nuclear fireball. If you are too close you will be burned. If we assume a 500 m rock asteroid moving at 17 km/s and impacting at 45 degrees hitting 70 km away, you will get third degree burns over much of your body (but your clothing or parachute will not ignite) - since you are skydiving there will also not be any blocking objects and there is less protection from the curvature of the Earth. The [overpressure][3] is 45 kPa, which means fatal lung damage is possible but not guaranteed. On the positive side, at this distance the ejecta will mostly be fine dust rather than rocks. A 500 meter impactor also is too small to cause global agricultural failure, so it is not a civilization-killer. Just bad news even up in the air. \r\n\r\nOne can work out inverses of the equations in the paper to get a \"deadly radius\" depending on mass and velocity, but it would be a rather sketchy estimate.\r\n\r\n  [1]: https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/effects.pdf\r\n  [2]: https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/\r\n  [3]: https://en.wikipedia.org/wiki/Overpressure", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 24, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-4.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "collision", "earth", "estimation", "explosions"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 517185, "title": "An asteroid strikes Earth. I’m skydiving. Do I die?", "url": "https://physics.stackexchange.com/questions/517185/an-asteroid-strikes-earth-i-m-skydiving-do-i-die", "share_url": "https://physics.stackexchange.com/q/517185", "content_license": null, "owner": {"display_name": "Descartes Before the Horse", "user_id": 206197, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/206197/descartes-before-the-horse"}}, "answers": [{"answer_id": 517189, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/517185/an-asteroid-strikes-earth-i-m-skydiving-do-i-die/517189#517189", "share_url": "https://physics.stackexchange.com/a/517189", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "Anders Sandberg", "user_id": 165299, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/165299/anders-sandberg"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:353636:0000", "text": "Question: When using radar in space to find asteroids is it helpful to know the distance? Or do you just need the direction?\n\nCouldn't they just send out the largest sounding pulse in a given direction that they could safely generate, and if there is anything in that direction no matter if it's by Jupiter or pluto as long as the receiver is pointing that way it'd pick it up right? Or am I missing something about focusing the outgoing beam?\n\nAccepted Answer:\n\nFocus is not the right term or concept. It is forming a beam, whose 3dB beam-width is usually called the beam-width. It'll keep covering a greater area as it expands out, but the angular spread is the same at all distances. Of course, if too far out you get too little power you can not  see it or measure it too well. \r\n\r\nSo as you search space with radar you want some angular resolution that defines the direction uncertainty.\r\n\r\nBut you asked about radial distance. Yes, if it's really close to earth it could be dangerous, if further less so. Also when you detect asteroid you typically want to track their path, and you need some initial conditions and then updates, and position and velocity is what you might get: angular direction and range, and Doppler change for radial velocity. \r\n\r\nHowever, you typically do better of course with optics, much greater wide angle angular resolution (just the diffraction limit). To locate in distance you triangulate. \r\n\r\nBack to radar, keep in mind radar range (i.e., how far you can detect) depends on the power emitted (your 'largest sounding pulse'), the integration time (pulse width or Doppler integration time), your RF noise figure (how much internal RF noise), the size (really radar cross section) of the object, and most importantly proportional to 1/$R^4$ (two $R^2$'s, one each way). That means it dies pretty rapidly with distance. You can bounce radar off Mars or some of the other planets, but harder to detect off smaller bodies like asteroids.\r\n\r\nSo ,yes, as long as pointing right you'll get a return signal, but it may be way below your noise threshold. \r\n\r\nBecause narrower beam widths concentrate the power more, you see further, but cover less area searching a certain amount of time. Wider beams allow faster searches but you get a weaker return. So they write the equations and trade off on what they want to do. In terrestrial ground to air radar search radars usually have a wider beam width (or multiple narrower beam and multiple transmitter/receivers (and greater costs), while tracking radars are narrower beam widths, while for astrophysics for eg passive Radio and microwave receivers the sky is so big you typically use narrower beam widths and move the antennas around to search. \r\n\r\nIt's all a big trade off.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 25, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astronomy", "radar"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 353636, "title": "When using radar in space to find asteroids is it helpful to know the distance? Or do you just need the direction?", "url": "https://physics.stackexchange.com/questions/353636/when-using-radar-in-space-to-find-asteroids-is-it-helpful-to-know-the-distance", "share_url": "https://physics.stackexchange.com/q/353636", "content_license": null, "owner": {"display_name": "user273872", "user_id": 99567, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/99567/user273872"}}, "answers": [{"answer_id": 353791, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/353636/when-using-radar-in-space-to-find-asteroids-is-it-helpful-to-know-the-distance/353791#353791", "share_url": "https://physics.stackexchange.com/a/353791", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Bob Bee", "user_id": 108333, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/108333/bob-bee"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:49985:0000", "text": "Question: How a spacecraft travelling near light speed avoid asteroids?\n\nHow would a spacecraft traveling near light speed avoid a (relatively dense group of) asteroids?  Or suppose such spacecraft is designed, how would the physics work for steering it inside such a \"cloud\" of asteroids?\r\n\r\nIf it simply by designing the spacecraft to have huge acceleration, wouldn't that mean lots of constraints on the material the spacecraft can be made of?\n\nAccepted Answer:\n\nThe [speed of protons in the LHC][1] is at 99.9999991%  of the speed of light.\r\n\r\nYou can see what happens when a proton hits a proton travelling in the opposite direction at those speeds.\r\n\r\n![higgs candidate][2]\r\n\r\n>A candidate event in the search for the Higgs boson, showing two electrons and two muons (Image: CMS/CERN)\r\n\r\nNow the spacecraft in question presumably is traveling at a similar fraction of the speed of light with respect with the asteroid cluster. 8 TeV (the energy of the proton in the above interaction) is still enormous energy at contact. What will happen is that the individual protons of the spacecraft will react as elementary particles creating an enormous number of secondaries similar to  the image above. Immediate destruction of spacecraft.\r\n\r\nAvoidance can only happen by detecting the presence of the cluster some fraction of a light year away from it and plot course accordingly. \r\n\r\nIMO the main danger to such a fast traveling spacecraft will come from the low density ions in space, which will produce such interactions inimical to life. A strong magnetic field might throw away ions, but there are also neutral particles in the cosmic dust which cannot be stopped easily. A very strong and very massive shielding system will be necessary.\r\n\r\nIt's important to also note asteroid fields are not very dense at all. It's not like the objects in Saturn's rings or the fictional asteroid field in Star Wars.  The asteroids are incredibly far in between in most cases so your chances of hitting an asteroid aren't really much greater at a higher velocity, it will just cause much more damage if it does hit you. So the biggest problem as stated above would be the particles in the interstellar medium. Of course if you have the technology to travel in such a manner in the first place you would have access to technologies we haven't yet conceived and it could be that at that level of technology this wouldn't be an issue.\r\n\r\n  [1]: http://arxiv.org/abs/1001.1330\r\n  [2]: https://i.sstatic.net/x5wBQ.png\n\nAnswer (score=1):\n\nYes, you'd need large acceleration if you want it to avoid things in very small time, t.\r\nYOu could also have explosives to blow up asteroids.\r\nYou wouldn't want to collide with the asteroids since if the asteroid is much more massive than you you would either blow up , or if you had elastic armor you'd just bounce away.\n\nAnswer (score=0):\n\nAs far as I know, 20 or 25 km/s is the maximum velocity achieved by practical spacecrafts. If such a Sci-fi craft exists (as in Wall-E - where the craft cruises through the debris), we have a lot of choices. Physics is not necessarily *needed*. Sci-Fi is very good.\r\n\r\nSpacecrafts (I mean, those used in space-travel) of the past and present are mostly guided by a bunch of *astro* guys on ground. If they were able to [design the crafts and guide them through the asteroid belt](http://curious.astro.cornell.edu/question.php?number=588) (like the Voyager twins) by using some transmitter-receiver mechanism as in RADAR or SONAR or a Computer-aided [Guidance system](http://en.wikipedia.org/wiki/Inertial_navigation_system), then we could think of the future easily.\r\n\r\nFirst, we'll be in an *age of space-travel*, where we'll have the farthest seeing telescopes (best ever ones than Hubble or James), and we'll have all exact locations and events (that may happen during travel, like future impacts on our space ship, etc.). So, guiding the craft wouldn't be so difficult like making some *Brownian motions* like the molecules do, or even land in several asteroids resting some time, like that.\r\n\r\n> If it simply by designing the spacecraft to have huge acceleration, wouldn't that mean lots of constraints on the material the spacecraft can be made of?\r\n\r\nNot only the material because, the material is required only to provide resistance to the impact-*able* asteroids or when you're planning to dash through all the objects on your path. If the craft is planned to make sudden turns on its way, it should also have opposite thrusters with more or less the same power. If I'm allowed to talk still, I'll go into Sci-Fi.\n\nAnswer (score=-1):\n\nIts not possible under our current technologies. Even a small rock can blow out the entire ship because its mass would be very high in that relativistic frame.\r\n\r\nAs the question is fictional in nature, I can provide one solution from sci-fi world: [Deflector Shields](http://en.memory-alpha.org/wiki/Deflector_shield). Force field based deflector shields aren't completely fictional. There have been attempts to create real ones. So, if you're going to make a spaceship which could travel near $c$, try inventing perfect deflector shields first.\n\nAnswer (score=-2):\n\ne=mc2 tells us that, as an object gains velocity it becomes more \"massive\" (not larger but carries more mass) when approaching near light speeds, you could hit a planet, and it should be like driving through a mm thick wall of water", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 26, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "special-relativity", "kinematics", "space-travel"], "page_start": null, "page_end": null, "quality_flags": ["low_score"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 49985, "title": "How a spacecraft travelling near light speed avoid asteroids?", "url": "https://physics.stackexchange.com/questions/49985/how-a-spacecraft-travelling-near-light-speed-avoid-asteroids", "share_url": "https://physics.stackexchange.com/q/49985", "content_license": null, "owner": {"display_name": "qazwsx", "user_id": 4836, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/4836/qazwsx"}}, "answers": [{"answer_id": 50002, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/49985/how-a-spacecraft-travelling-near-light-speed-avoid-asteroids/50002#50002", "share_url": "https://physics.stackexchange.com/a/50002", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "anna v", "user_id": 1492, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1492/anna-v"}}, {"answer_id": 49991, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/49985/how-a-spacecraft-travelling-near-light-speed-avoid-asteroids/49991#49991", "share_url": "https://physics.stackexchange.com/a/49991", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "raindrop", "user_id": 11833, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/11833/raindrop"}}, {"answer_id": 50000, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/49985/how-a-spacecraft-travelling-near-light-speed-avoid-asteroids/50000#50000", "share_url": "https://physics.stackexchange.com/a/50000", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "anon", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, {"answer_id": 50001, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/49985/how-a-spacecraft-travelling-near-light-speed-avoid-asteroids/50001#50001", "share_url": "https://physics.stackexchange.com/a/50001", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Earth is a Spoon", "user_id": 2170, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/2170/earth-is-a-spoon"}}, {"answer_id": 65042, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/49985/how-a-spacecraft-travelling-near-light-speed-avoid-asteroids/65042#65042", "share_url": "https://physics.stackexchange.com/a/65042", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Mouldy", "user_id": 24647, "user_type": "unregistered", "profile_url": "https://physics.stackexchange.com/users/24647/mouldy"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:759291:0000", "text": "Question: Deriving the destruction energies of asteroids\n\nI wanted to check the validity of an expression for the energy required to destroy an asteroid.\r\n\r\nAssuming that the asteroid is spherical, the gravitational binding energy can be given as \r\n\r\n$U_{GBE} = 3/5 * GM^2/ R$.\r\n\r\nHowever, this is only the energy to remove aggregates away from each other. There is a required energy to destroy the asteroid in the first place where I've assumed that by breaking a certain number of bonds, the destruction energy can be given as\r\n\r\n$U_D = E (kJ/mol) * N * n (mol)$\r\n\r\nwhere $E$ is the enthalpy of the bond (or energy required to break an atom-atom bond), $N$ is the number of atom-atom bonds and $n$ is the number of moles. Would the addition of these two energies therefore be a good way to approximate the energy needed to destory an asteroid?\n\nAccepted Answer:\n\nThese are good thoughts, but of course everything is more complicated in practice. \r\n\r\nThe gravitational binding energy is right. After you break it up into separate atoms, that is the energy needed to move each atom an infinite distance from all the others. \r\n\r\nSuppose you break it up into small rocks. The energy needed to separate each rock an infinite distance from the others isn't much different. You can see that by calculating the binding energy of a rock and multiplying by the number of rocks.\r\n\r\nThe energy needed to break bonds is a reasonable thought. But some asteroids are snowballs. Some are loose conglomerations of rubble. Some are solid nickel-iron where the composition is like the core of a planet. Some are solid rocks like the crust of a planet. \r\n\r\nIf you think of energy per bond and how many bonds, you are implicitly thinking about a material like a perfect crystal, where all the bonds are the same. Even in a crystal, imperfections determine the strength. A perfect crystal of iron is much, much stronger than iron we use every day. \r\n\r\nAnother problem is how much of the energy you bring to the asteroid will go into breaking bonds and separating pieces? \r\n\r\nIf you attach a rocket to each piece, you have to consider the efficiency of rockets. If you make a big explosion inside the asteroid, you might model that as suddenly converting some explosive into hot gas. How much energy goes into heat? How efficient is gas at pushing pieces? \r\n\r\nFor breaking bonds, suppose you swing a sledge hammer at a rock. How much of the kinetic energy goes into breaking bonds? If you hit soft clay, bonds are broken and rearranged, but you still have one piece. If you hit glass, the result is more what you want, but it still isn't straightforward to figure out efficiency. \r\n\r\nSome of these things can be handled by measurement. Hit a rock and measure how much energy it takes to break it. Set off an explosive and measure the kinetic energy of flying pieces. It might be a challenge to figure out how to make these measurements. But once you do, you begin to have realistic data.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 27, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-4.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "thermodynamics", "energy", "astrophysics", "binding-energy"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 759291, "title": "Deriving the destruction energies of asteroids", "url": "https://physics.stackexchange.com/questions/759291/deriving-the-destruction-energies-of-asteroids", "share_url": "https://physics.stackexchange.com/q/759291", "content_license": null, "owner": {"display_name": "John", "user_id": 352519, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/352519/john"}}, "answers": [{"answer_id": 759306, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/759291/deriving-the-destruction-energies-of-asteroids/759306#759306", "share_url": "https://physics.stackexchange.com/a/759306", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "mmesser314", "user_id": 37364, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/37364/mmesser314"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:400309:0000", "text": "Question: Suitability of FLUKA for Simulation of Backscattered Radiation off an Asteroid\n\nThis one's pretty simple.  I'm looking at trying to simulate the backscattered radiation coming off of a S-type asteroid shape model due to impinging GCRs and solar particles.  Ideally my simulation would include dynamic factors like the relative motion and attitude of a spacecraft in orbit around the asteroid (like the Rosetta spacecraft for instance), the orientation of the asteroid with respect to solar influx, and internal density non-homogeneities within the asteroid model to look at their effect on surface neutron emissions etc.  \r\n\r\nI know this is a decidedly complex simulation, and I'm curious regarding your opinions on whether FLUKA (specifically using Flair GUI) is up to it?  I think GEANT4 or GRAS might be able to do so, but not so sure about FLUKA/Flair.  \r\n\r\nOther information: I've got access to a high power computing array in the lab (so I'm not overly worried about crashing the program with an enormous task), and have the latest versions of both FLUKA and GEANT4.\r\n\r\nThanks!\r\n-D. Hodge\n\nAccepted Answer:\n\nA bit meta to answer my own question, but here are some salient points which I've uncovered.  Figured that some one may find this useful so here goes:\r\n\r\n - FLUKA doesn't do multi-stage scattering very easily.  The sim/physics is fine but the extrication of the data is somewhat nightmarish.  You have to build a multi-stage run in FORTRAN, pump primary GCRs into the asteroid model, then use those as the input to a second sim with any detectors you're interested in.  This is, again, to the best of my knowledge (which at this point extends to having successfully run a very basic sim along the above lines). \r\n - Frankly, the documentation for GEANT4 is vastly superior to FLUKA's, it's *not* written in FORTRAN (which I hope won't step on any toes, but GEANT4's C++ is much more contemporary to my thinking), and there are fairly lively user forums for GEANT4 with more than FLUKA's apparent ~5-10 people in the entire world who can successfully answer questions and suggest debug options. \r\n\r\nI'd like to emphasize that the second point is subjective, and based upon my personal travails with FLUKA, and my less painful experience with GEANT4.  It is worth noting that the Flair GUI for FLUKA is quite excellent and can do most run of the mill sims quickly and without having to dive into the FORTRAN guts of FLUKA.  Complex and customized sims, however, I'd definitely go with GEANT4. \r\n\r\n-Cheers", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 28, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "radiation", "simulations", "backscattering"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "mixed_license", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 400309, "title": "Suitability of FLUKA for Simulation of Backscattered Radiation off an Asteroid", "url": "https://physics.stackexchange.com/questions/400309/suitability-of-fluka-for-simulation-of-backscattered-radiation-off-an-asteroid", "share_url": "https://physics.stackexchange.com/q/400309", "content_license": null, "owner": {"display_name": "D. Hodge", "user_id": 189553, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/189553/d-hodge"}}, "answers": [{"answer_id": 407248, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/400309/suitability-of-fluka-for-simulation-of-backscattered-radiation-off-an-asteroid/407248#407248", "share_url": "https://physics.stackexchange.com/a/407248", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "D. Hodge", "user_id": 189553, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/189553/d-hodge"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:89067:0000", "text": "Question: Asteroid collision debris calculation\n\nI wonder how to determine the directions, in which the collision debris is launched when 2 asteroids collide.\r\nI am aware of: m1*v1 + m2*v2 = m*v = m3*v3 + m4*v4 + m5*v5 + ...\r\nand this works just fine for the masses and valocity, however I find it difficult to determine the boundaries of the directions and under what circumstances shatter be produced or the asteroids will just \"merge\". \r\n\r\nAll info is appreciated :)\n\nAccepted Answer:\n\nI think that this problem doesn't have an exact answer. Some time ago, I talked about this with the astrophysicist Paolicchi (this is [the asteroid][1] named after him) who works on the field. The conclusion is that debris are produced at random and you can only impose some (\"few\") constraints globally, say on big branches of the asteroids belt or of planetary ring. There debris \"termalize\" after a big number of collision and remain at rest with respect to each others. In the case of just a collision the physics is complicated... I list just some points:\r\n\r\n 1. the asteroids are typically non-[self-gravitating objects][2], that is they're mainly taken together by (local) electric forces. Hence, they are not round and, typically, they doesn't merge, since matter globally is electrically neutral. The same is true for artificial asteroids;\r\n 2. since they spin and they have complex shapes their collisions are very difficult to modelize. You can apply the conservation of momenta but you have no boundaries on the velocities of each single fragment;\r\n 3. for some purposes, it could be useful to approximate the production of debris as proportional to the energy  in the center of mass of the two colliding asteroids: $E_{\\text{cm}}\\propto n$ of fragments. Then assume a $n$-body decay, each with the same mass. Even in the case of $n\\gtrsim 3$, you can only have a [phase-space][3] for these debris and some probability density functions for their production angles.\r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Paolicchi\r\n  [2]: http://astronomy.swin.edu.au/cosmos/S/Self-gravitation\r\n  [3]: http://en.wikipedia.org/wiki/Phase_space", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 29, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "particle-physics", "collision"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 89067, "title": "Asteroid collision debris calculation", "url": "https://physics.stackexchange.com/questions/89067/asteroid-collision-debris-calculation", "share_url": "https://physics.stackexchange.com/q/89067", "content_license": null, "owner": {"display_name": "MerlinBG", "user_id": 35284, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/35284/merlinbg"}}, "answers": [{"answer_id": 89083, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/89067/asteroid-collision-debris-calculation/89083#89083", "share_url": "https://physics.stackexchange.com/a/89083", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "AstoundingJB", "user_id": 19872, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/19872/astoundingjb"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:704178:0000", "text": "Question: Destroy an asteroid by EM waves by making its crystalline meshes enter in resonance?\n\nCould we destroy an asteroid made of crystalline materials heading towards the Earth by bombarding it with electromagnetic rays, making its crystalline meshes enter in vibration resonance and decompose it little by little? I estimate at first that the asteroid does not turn on itself.\r\n\r\nIf you need an example, imagine a ball of 1km radius made entirely of iron heading towards the Earth with a slightly curved trajectory at any variable speed.\n\nAccepted Answer:\n\nYou **can,** of course, use EM radiation to break the bonds of a crystal and thus cause damage to the asteroid. In a sense, this _is_ a form of \"resonance\" because the bonds will react to photons of some wavelengths more than of others - that's why materials have \"color\", after all. The trick is that you won't be able to _completely destroy_ it without supplying energy approaching the order of the energy required to vaporize the asteroid, since that's pretty much what \"vaporize\" _means_ - it means to completely dissolve the crystal lattice, so that every atom is separated from every other. **Conservation of energy cannot be cheated!**\r\n\r\nHowever, a much better strategy is to **not** \"completely destroy\" the asteroid, but to only vaporize a little bit, so as to turn it into an exhaust plume that then produces thrust which pushes the asteroid out of its collision trajectory. Given the huge distances and times involved in a Solar orbit, even 0.1 m/s of dV could do a lot - over a year, or ~32 Ms, that's about 3200 km of deflection. If aimed right, that should convert an impact to at least a grazing pass within a few hundred km of Earth's surface for most impact cases.\r\n\r\nAnd yes, a laser (EM radiation) could be an effective means and, indeed, taken literally and in light of what I just said, it pretty much _is_ exactly what you are claiming - the laser photons \"resonate\" some of the crystal to the breaking point, sending atoms flying into space, and it happens a little at a time, but you'd have to first get one out to the asteroid, and you'd need it to keep running and aimed in the same spot for long enough to deliver the desired momentum change. There is nothing at all physically impossible about this - the challenge is all in engineering.\r\n\r\nHeck, an arguably even simpler solution is to just focus the Sun's light directly, instead of having the laser middleman - though then you lose the advantage of being able to tune the frequency, but conversion to laser energy has inefficiency issues that likely outweigh any gains from such as being able to hit specific resonance modes maximizing surface absorption. That said, your lense or parabolic mirror will still need to be very large, so once more, engineering is going to determine what the best method is.\n\nAnswer (score=2):\n\nThis line of thought is similar to Nikola Tesla's, when he made his mechanical oscillator\n\nhttps://en.m.wikipedia.org/wiki/Tesla%27s_oscillator\n\nIdea goes like this: any object should have some resonant frequency, and if you guess this frequency correctly, you can keep pumping energy into this vibration till object is destroyed due to accumulated energy\n\nSame line of reasoning went into many other Tesla's works, including his wireless energy transfer system for example\n\nSo, if everything is as simple as that, why dont we have any noticeable tech done this way, why is resonance restricted to carefully designed systems, and very rarely every day, natural objects?\n\nReason is that other than resonant frequency objects have a measure of how much energy they dissipate per oscillation. You can intuitively understand it as time of ringing. A glass will ring for quite some time, and chunk of rubber will not ring as long. It is not quite precise description, but a good start\n\nhttps://en.m.wikipedia.org/wiki/Q_factor\n\nThis is why Tesla's building destroying oscillator did not work - building is simply not a good oscillator. As you pump more power, building dissipates all of that power in heat in just one oscillation, and no accumulation of energy occurs even at very low movement.\n\nThere are cases when skyscrapers vibrate in a strong wind, with some accumulation of the energy between periods, but it is mostly due to how wind makes vortexes, very large amount of energy is used, and because skyscrapers are still not as bad of an oscillator as ordinary buildings or dirt would be.\n\nSame for wireless energy transfer - most energy is lost unless very controlled environemnt and very good oscillators are in use.\n\nAnd the same for your asteroid case. It is just a pile of rocks and sand bunched together. Even if there is a frequency on which it oscillates the best, it is still a very bad oscillator, and it will not accumulate energy in its vibration for more than a few oscillations.\n\nWe can evaporate parts of an asteroid with lasers, and evapotated parts provide a little bit of thrust. But because ateroids fly around at speeds on the order of 10 km/s it is hard to concentrate the laser on an asteroid for a long time. It is just too far away.\n\nI would expect asteroid to resonate best on rotational frequency - about once per hour, on acoustic frequency - about once per few milliseconds, and plasmons in sand particles - about visible light. None of them are what people think of when talk about radio\n\nP.S. a chunk of iron doesnt change much. It would oscillate better at acousting frequency, but it would also be much stronger mechanically. And chances to destroy it by resonance remotely are as low. Especially how hard it is to concentrate radio waves at audio frequencies. And even if you will manage using very strong lazer and jini explosions, and non-natural, single crystal asteroid, explode from resonance, it will split into a few large chunks. for planet defence it is not enough for example.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 30, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-4.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "electromagnetism", "resonance"], "page_start": null, "page_end": null, "quality_flags": ["low_score"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 704178, "title": "Destroy an asteroid by EM waves by making its crystalline meshes enter in resonance?", "url": "https://physics.stackexchange.com/questions/704178/destroy-an-asteroid-by-em-waves-by-making-its-crystalline-meshes-enter-in-resona", "share_url": "https://physics.stackexchange.com/q/704178", "content_license": null, "owner": {"display_name": "Emile Couzin", "user_id": 333176, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/333176/emile-couzin"}}, "answers": [{"answer_id": 704219, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/704178/destroy-an-asteroid-by-em-waves-by-making-its-crystalline-meshes-enter-in-resona/704219#704219", "share_url": "https://physics.stackexchange.com/a/704219", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "The_Sympathizer", "user_id": 14275, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/14275/the-sympathizer"}}, {"answer_id": 704196, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/704178/destroy-an-asteroid-by-em-waves-by-making-its-crystalline-meshes-enter-in-resona/704196#704196", "share_url": "https://physics.stackexchange.com/a/704196", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "Surprised Seagull", "user_id": 282084, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/282084/surprised-seagull"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:104542:0000", "text": "Question: Properties of an object whose collision with Earth would completely disintegrate Earth\n\n[This](http://fbcomics.com/home/tag/Planet) picture got me thinking, whether such collision would be possible, and would it really look like that from the moon.\r\n\r\nWhat would be the properties (mass, size, velocity etc.) of an object, whose collision with Earth would completely destroy it in the manner of that picture? That is, after the collision, a large portion (if not most) of Earth would continue straight along the same path as that of the colliding object, but from the opposite side of the planet.\r\n\r\nIf such an event were to happen, would the sight in that picture be an accurate depiction when observed from the surface of the moon?\n\nAccepted Answer:\n\nEarth is really, really big (in comparison to that projectile). In order for an object to completely penetrate it, it would need to have enough force to go through 12,742 kilometers of solid and liquid. It would need either extreme mass or extreme speed.\r\n\r\nin the case of extreme mass, at a certain point, the object wouldn't go straight through earth as much as tear it apart from a distance due to gravitational forces and the roche limit. This happens at the approach side, not at the exit side.\r\n\r\nIn the case of extreme speed, at a certain point, an object would be so fast that the collision turns the object and a large part of earth into plasma. The object itself would not survive as shown in the image, unless it's also extremely resistant to the massive forces involved in this event.\r\n\r\nSo an object which would be capable of this would need to be both extremely massive and extremely fast. As luck would have it, Randall Munroe actually made [an educated guess on the consequences of a 100 meter wide diamond meteor striking earth at relativistic speeds][1]. His conclusion was that the meteor would have an effect far worse than that of the image above. The entire planet would desintegrate into a massive cloud of plasma, which would destroy most of the moon as well, sweep away most of Mars and Venus and have a sizeable effect on the Sun.\r\n\r\n\r\n  [1]: https://what-if.xkcd.com/20/\n\nAnswer (score=0):\n\nIMHO a dust-particle-sized black hole, roughly of the mass of Pluto, moving at few hundreds km/s, could produce effects similar to those are pictured, namely:\r\n\r\n* Ejecta above the entry point (destruction of the crust by the hole’s gravity);\r\n* Except vicinity of aforementioned point, an almost undamaged leading hemisphere (tidal forces become too weak to break the crust severely, whereas too few time elapsed for sound waves to propagate across the planet);\r\n* Grave damage and incandescent plume at the tailing hemisphere (the hole, on its descent, should drag a significant part of the Earth’s interior that will then penetrate the mantle and core at supersonic speeds, generating shock waves);\r\n* A cloud of plasma around the object (for few seconds, or maybe less, the hole should hold a globular cloud of matter around it, before it partially dissipates to outer space, partially falls inside, and possibly transforms to accretion disc).\r\n\r\nIt really should appear “out of nowhere”, as the explanatory text says (note to @miikkas: Ī changed the URL since the one at memecdn.com now gives 404). Modern Solar System astronomy does not deploy instruments to detect such objects purposely, whereas without impacts it will be completely black to human eye and telescopes.\r\n\r\nBlack holes of smaller masses, moreover, will be virtually traceless because probability to significantly perturb a noticeable body will be minor.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 31, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "earth", "collision"], "page_start": null, "page_end": null, "quality_flags": ["low_score"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 104542, "title": "Properties of an object whose collision with Earth would completely disintegrate Earth", "url": "https://physics.stackexchange.com/questions/104542/properties-of-an-object-whose-collision-with-earth-would-completely-disintegrate", "share_url": "https://physics.stackexchange.com/q/104542", "content_license": null, "owner": {"display_name": "miikkas", "user_id": 29599, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/29599/miikkas"}}, "answers": [{"answer_id": 104545, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/104542/properties-of-an-object-whose-collision-with-earth-would-completely-disintegrate/104545#104545", "share_url": "https://physics.stackexchange.com/a/104545", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Nzall", "user_id": 40357, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/40357/nzall"}}, {"answer_id": 142151, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/104542/properties-of-an-object-whose-collision-with-earth-would-completely-disintegrate/142151#142151", "share_url": "https://physics.stackexchange.com/a/142151", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Incnis Mrsi", "user_id": 56960, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/56960/incnis-mrsi"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:518066:0000", "text": "Question: Is it possible for meteors and asteriods to become natural satellites of Earth?\n\nWe know that the moon isn't falling on the earth because it's in orbit. If we make an asteriod or meteor spin is it possible for it to become a natural satellite of Earth?\n\nAccepted Answer:\n\nIf your question is asking whether it's possible for an asteroid to be *naturally* captured and put into orbit around the earth, the answer is \"yes\".  But it requires a very specific set of conditions.\r\n\r\nThe incoming asteroid would need to be deflected by the Moon into a direction and speed that corresponds to an orbital trajectory.  That means that its speed, the distance by which it misses the moon, and the angle of its approach relative to the Moon's direction of motion, would all have to fit within specific ranges. The vast majority of approach trajectories would result in the asteroid just flying by with its final trajectory altered by interactions with the Earth and Moon. \r\n\r\nMost likely a captured asteroid would be thrown into a highly elongated elliptical orbit around the Earth, but then it would interact with the Moon every time it looped around the Earth -- so the orbit would change.  It could simply be tossed away into an escape trajectory, or into a trajectory that hits the Earth.  For it to end up in a relatively *stable* orbit, it would need to survive multiple interactions with the Moon and end up in that stable orbit.  So, requiring a *stable* orbit further narrows down the range of possible approach trajectories.\n\nAnswer (score=0):\n\nPlease clarify what you mean by spinning ( i.e., the revolve\r\n about the earth or rotate about it's own axis). But for the rest of the question yes anything (artificial or natural) can be made a satellite of earth as long as it has a velocity $v = \\sqrt {G \\frac {M_{earth}}{R_{orbit}}}$. ( Also it's direction of motion should be such that it doesn't crash into the earth (🌍).\n\nAnswer (score=-2):\n\nThe moon itself was an asteroid that became natural satellite of Earth. \r\n\r\nAs for 'spin' in the case of the moon, the periods of spinning around the moon's axis and spinning around the earth are the same as one month. That is why we only see one side of the moon. The dark side of the moon is usually concealed from the public (unless you are Pink Floyd), just like DT's tax return record.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 32, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-4.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "gravity", "orbital-motion", "earth", "meteors"], "page_start": null, "page_end": null, "quality_flags": ["low_score"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 518066, "title": "Is it possible for meteors and asteriods to become natural satellites of Earth?", "url": "https://physics.stackexchange.com/questions/518066/is-it-possible-for-meteors-and-asteriods-to-become-natural-satellites-of-earth", "share_url": "https://physics.stackexchange.com/q/518066", "content_license": null, "owner": {"display_name": "O S", "user_id": 139956, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/139956/o-s"}}, "answers": [{"answer_id": 518107, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/518066/is-it-possible-for-meteors-and-asteriods-to-become-natural-satellites-of-earth/518107#518107", "share_url": "https://physics.stackexchange.com/a/518107", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "S. McGrew", "user_id": 183212, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/183212/s-mcgrew"}}, {"answer_id": 518093, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/518066/is-it-possible-for-meteors-and-asteriods-to-become-natural-satellites-of-earth/518093#518093", "share_url": "https://physics.stackexchange.com/a/518093", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "user238497", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, {"answer_id": 518095, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/518066/is-it-possible-for-meteors-and-asteriods-to-become-natural-satellites-of-earth/518095#518095", "share_url": "https://physics.stackexchange.com/a/518095", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "MadMax", "user_id": 190414, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/190414/madmax"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:383331:0000", "text": "Question: How fast would the destruction caused by an asteroid impact travel on earth?\n\nI know it depends on mass and speed, so take for example the asteroid that caused dinosaurs mass extinction. I read that it was 6km (10 miles) wide. How fast did the destruction - trees, rocks, dirt, debris - traveled? Was it the same speed as the shockwave?\n\nAccepted Answer:\n\nI think you multiplied instead of dividing for km/miles. Either way, that is a big rock going very fast. Won't the atmosphere slow it down? Not really. Its diameter is as much as the thicker part of the atmosphere. There is a second or less from hitting the whips of atmosphere and hitting the surface.\r\n\r\nSuper-heated compressed air will be pushed at hypersonic (or orbital) speeds - fast enough and hot enough to cut through rock- and contact with the ground will squirt material out at those speeds as well. Molten white hot material mixed with cooler material pushed by it and it will travel at hypersonic speeds for great distances with some going into space and coming down anywhere on Earth. The atmospheric shockwave will be supersonic for a long ways then become the speed of sound. Note the speed of sound is determined by temperature, not density. Compression by the shock wave will have some strange heating effects.\r\n\r\nAll in all, a very bad day but quite something to see from a safe distance. By the way, it would looke rather slow. Hollywood explosions of this sort move faster than light if you do thumbnail calculations.\n\nAnswer (score=0):\n\nYou’re close, but more along the lines of 5 minutes, but still would agree that is relatively slow compared to movies where I’ve matched out close to 60k mph in some instances, like in Greenland (I think is the name), the shockwave moves from north eastern Florida coast and it travels nearly 600 miles in 33 seconds. And this is actually over 65k mph haha", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 33, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "momentum", "mass-energy", "explosions"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "mixed_license"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 383331, "title": "How fast would the destruction caused by an asteroid impact travel on earth?", "url": "https://physics.stackexchange.com/questions/383331/how-fast-would-the-destruction-caused-by-an-asteroid-impact-travel-on-earth", "share_url": "https://physics.stackexchange.com/q/383331", "content_license": null, "owner": {"display_name": "RealAnyOne", "user_id": 165069, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/165069/realanyone"}}, "answers": [{"answer_id": 383364, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/383331/how-fast-would-the-destruction-caused-by-an-asteroid-impact-travel-on-earth/383364#383364", "share_url": "https://physics.stackexchange.com/a/383364", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "C. Towne Springer", "user_id": 37182, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/37182/c-towne-springer"}}, {"answer_id": 826285, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/383331/how-fast-would-the-destruction-caused-by-an-asteroid-impact-travel-on-earth/826285#826285", "share_url": "https://physics.stackexchange.com/a/826285", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "chris wright", "user_id": 425639, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/425639/chris-wright"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:601136:0000", "text": "Question: How to estimate the minimum radius of asteroid the rocky portion of which melted due to radioactive decay?\n\nOne of the mechanisms for the heating of asteroids in the early history of the Solar System is believed to be decay of the isotope ${}^{26}\\mathrm{Al}$. This was created by the supernova that produced the dust cloud from which the asteroids formed. For example [in this paper][1] the astrophysicist [G. Jeffrey Taylor][2] wrote:\r\n\r\n>${}^{26}\\mathrm{Al}$ was present when meteorites were forming (see PSRD article [Using Aluminum-26 as a Clock for Early Solar System Events][3]). It is a radioactive isotope with a half-life of only 700 thousand years, so its presence means that the solar system formed within a few half-lives of the formation of ${}^{26}\\mathrm{Al}$ in an exploding star. It decayed by emitting a beta particle (an electron), creating ${}^{26}\\mathrm{Mg}$ (magnesium-26) and releasing energy. The energy released is considerable. If ${}^{26}\\mathrm{Al}$ made up only $5 \\times 10^{-5}$ ($0.005\\%$) of all the aluminum in a chondrite (most is aluminum-27, which is not radioactive), it would release enough energy to melt asteroids **a few kilometers across and larger**. Lower amounts of ${}^{26}\\mathrm{Al}$ cause less melting.\r\n\r\n(my emphasis)\r\n\r\nI have been trying to estimate the minimum radius that the asteroid would need to have for it to melted due to radioactive decay but I do not know how this calculation is done. Can anyone describe how this calculation is done and how Taylor arrived at the result *a few kilometers across and larger*?\r\n\r\n\r\n  [1]: http://www.psrd.hawaii.edu/April04/asteroidHeating.html\r\n  [2]: https://www.higp.hawaii.edu/index.php/people/g-jeffrey-taylor/\r\n  [3]: http://www.psrd.hawaii.edu/Sept02/Al26clock.html\n\nAccepted Answer:\n\nPlanetary science is rife is order-of-magnitude estimates, and this is a classic problem for that kind of solution.\r\n\r\nStep one is somewhat contradictory: start with a spherical asteroid. Make it homogenous, too. It has:\r\n\r\nA radius: $R$\r\n\r\nA melting point: $T_0$\r\n\r\nA density: $\\rho$\r\n\r\nAn $^{26}$Al content per unit mass: $n$\r\n\r\nand a surface temperature: $T$\r\n\r\nand maybe some other parameters (emissivity, mass, and so on)\r\n\r\nIn the zeroth order approximation, the temperature is uniform. Power is generated according to the total number of Al-26 decays:\r\n\r\n$$ P_{in} \\propto (\\frac 4 3 \\pi R^3)(n\\rho)\\frac{E_{Al}}{t_{\\frac 1 2}}$$\r\n\r\nwhich most importantly has a volume term, which goes as $R^3$.\r\n\r\nIn thermal equilibrium, that power is radiated from the surface though blackbody radiation. The Stefan-Boltzmann Law relates the total power radiated per unit area:\r\n\r\n$$ j = \\sigma T^4 $$\r\n\r\nwhich we can sum over our spherical asteroid:\r\n\r\n\r\n$$ P_{out} = (4\\pi R^2)j $$\r\n\r\nand the equilibrium condition:\r\n\r\n$$P_{out} = P_{in} $$\r\n\r\nhas:\r\n\r\n$$ T^4 R^2 \\propto R^3 $$\r\n\r\nand you can solve for the $R$ that yields $T=T_0$.\r\n\r\nDeeper estimates might include surface emissivity, or a temperature gradient inside the asteroid. The former is a minor correction, the latter is an involved analysis, and if this estimate fails miserably (e.g., comes out at 50 km), may be required, as it points to an interior that is hotter than the surface.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 34, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-4.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "radiation", "solar-system", "radioactivity"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 601136, "title": "How to estimate the minimum radius of asteroid the rocky portion of which melted due to radioactive decay?", "url": "https://physics.stackexchange.com/questions/601136/how-to-estimate-the-minimum-radius-of-asteroid-the-rocky-portion-of-which-melted", "share_url": "https://physics.stackexchange.com/q/601136", "content_license": null, "owner": {"display_name": "Jokerp", "user_id": 257200, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/257200/jokerp"}}, "answers": [{"answer_id": 601312, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/601136/how-to-estimate-the-minimum-radius-of-asteroid-the-rocky-portion-of-which-melted/601312#601312", "share_url": "https://physics.stackexchange.com/a/601312", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "JEB", "user_id": 82339, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/82339/jeb"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:158553:0000", "text": "Question: Can asteroids fall on earth and hit the surface at 90 degree angle?\n\nI always Saw in movies ,cartoons every where that asteroids hit the earth at an angle \r\n    (not 90 degree).\r\n**But Why ?**\r\n\r\n    We are living in 3-dimensional world(probably more....) .So is there an every chance that \r\n    an asteroid on one day would slam into earth at 90 degrees or is it happening at times\r\n**Or Is there any physics phenomenon that would prevent this from happening ?**\n\nAccepted Answer:\n\nThere is nothing that prevents the asteroid coming in at 90 degrees, but it is very improbable. To the extent that the asteroid velocity is large enough that the Earth's gravity doesn't change it, there is little solid angle around 90 degrees altitude.  It is like a [Rayleigh distribution][1] where there is little area near the origin.  To the extent that Earth's gravity changes the asteroid velocity, angular momentum is conserved and you need it to be (close to) zero to come in at 90 degrees.\r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Rayleigh_distribution\n\nAnswer (score=0):\n\nAsteroids and the Earth are both traveling in orbits around the Sun.\r\n\r\nThe Asteroids that cross the Earth's path are in highly eliptical\r\norbits, while the Earth is in a near circular orbit.\r\n\r\nIf you draw the two orbits on a piece of paper, they\r\nintersect at two points, but notice that the two\r\norbits are not perpendicular to one another, and \r\nthe two points of intersections are also not perpendicular to\r\none another.\r\n\r\nAlso the speeds of the Earth and the Asteroids are not the same,\r\nso the colission if and or when it occurs will be very\r\nenergetic. Energy = 1/2 Mass x Velocity Squared.\r\n\r\nThe probability that the Earth, and the Asteroid will be exactly\r\non the same plane of the ecliptic is very very small,\r\nand the probability that it will collide exactly at 90 degrees,\r\nwhile being exactly on the same plane of the ecliptic is\r\nvanishingly small.\r\n\r\nMikeClark\r\nGolden, Colorado, Usa\n\nAnswer (score=-1):\n\nI will assume that the relative speed of the asteroid irt the Earth (centre) is between 0 and 2*30 km/s (I will not check the values ). The fall to Earth centre future position started a while ago from a distant position. The in fall will take a considerable time (how much? between __ and ___, put the numbers if you wish) .\r\nAdmit that the Earth is only 1km radius: The angle between the surface and the asteroid is 90º, sure ?. But the Earth radius is much larger and the contact time is a little bit earlier than in the previous situation, and I will assume that the angle is near 90º unless the numbers __ to __ are 'on the contrary' . \r\nI admit that the above can be wrong (I used only my imagination), an the distribution of speeds and angles of impact deserve a detailed analysis. \r\nBut the next sentence is valid, imo, and deviates the angle of observation very much from 90º.  \r\nThe observer on the Earth surface, due to its rotation, is in motion between 0 km/s at the poles to 30 km/s (+-) at the equator, and in motion irt the asteroid trajectory. Due to this fact only at high latitudes the asteroid can be 'seen' as falling from 90º. \r\nEven if the asteroid had to be seen by an observer as falling from 90º then it will be perceived as an immobile star for a brief moment at zenit, and the brilliant trajectory that we observe in the meteorite fall could not be seen by that observer.       \r\nSee also the [Atmospheric_entry][1] problem. \r\n\r\n\r\n  [1]: http://en.wikipedia.org/wiki/Atmospheric_entry\n\nAnswer (score=-1):\n\nI may be totally wrong, but I doubt that a 90 degree angle is ever possible simply due to the fact that the earth is always in motion. No matter what comes at it, it will have to hit it at some sort of an angle due to earths rotation. The only exception to this, I would presume, is if the object were headed straight towards either pole, where the rotation is static. If the object is headed perpendicularly towards the pole, it would maintain the same trajectory as the earth's spin will not alter the placement of the impact. Maybe???\n\nAnswer (score=-1):\n\nLet's try agreeing to a percentage.  If 10,000, or even 100,000 \"rocks\" impact the earth, what would the estimated percentage of them striking at perfect orthogonal angles.  Further, what would be the odds of 2 out of 2, 3 out of 3, 10 out of 10, or even 20 out of 20 rocks striking orthogonally?  Given a larger asteroid, an impact that dead-on-center would probably be felt all over the planet (if it struck a land formation).  Striking at an angle would increase the collision time, and significantly lower the force of impact.  Lets say a 1 kilometer-wide asteroid has an orthogonal impact right over the Mariana trench in the Philippine Sea; is that part of the ocean \"deep enough\" to slow down the collision event?  -- The ensuing tidal wave would obviously be felt by the entire planet (eventually--because all Pacific coastal manufacturing and shipping facilities would be gone), but given an impact of that magnitude, would the flood waters completely blow past the Panama Canal and spread to the Atlantic?", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 35, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "gravity", "space"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "mixed_license"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 158553, "title": "Can asteroids fall on earth and hit the surface at 90 degree angle?", "url": "https://physics.stackexchange.com/questions/158553/can-asteroids-fall-on-earth-and-hit-the-surface-at-90-degree-angle", "share_url": "https://physics.stackexchange.com/q/158553", "content_license": null, "owner": {"display_name": "Ein2012", "user_id": 60892, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/60892/ein2012"}}, "answers": [{"answer_id": 158554, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/158553/can-asteroids-fall-on-earth-and-hit-the-surface-at-90-degree-angle/158554#158554", "share_url": "https://physics.stackexchange.com/a/158554", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Ross Millikan", "user_id": 3257, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/3257/ross-millikan"}}, {"answer_id": 158562, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/158553/can-asteroids-fall-on-earth-and-hit-the-surface-at-90-degree-angle/158562#158562", "share_url": "https://physics.stackexchange.com/a/158562", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "user68820", "user_id": 68820, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/68820/user68820"}}, {"answer_id": 158559, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/158553/can-asteroids-fall-on-earth-and-hit-the-surface-at-90-degree-angle/158559#158559", "share_url": "https://physics.stackexchange.com/a/158559", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Helder Velez", "user_id": 1257, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/1257/helder-velez"}}, {"answer_id": 404361, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/158553/can-asteroids-fall-on-earth-and-hit-the-surface-at-90-degree-angle/404361#404361", "share_url": "https://physics.stackexchange.com/a/404361", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "Marvelously Curious", "user_id": 194846, "user_type": "unregistered", "profile_url": "https://physics.stackexchange.com/users/194846/marvelously-curious"}}, {"answer_id": 412756, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/158553/can-asteroids-fall-on-earth-and-hit-the-surface-at-90-degree-angle/412756#412756", "share_url": "https://physics.stackexchange.com/a/412756", "content_license": "CC BY-SA 4.0", "owner": {"display_name": "pfm_swtx", "user_id": 198784, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/198784/pfm-swtx"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:59802:0000", "text": "Question: Ways to spot a comet or asteroid?\n\nHow do we spot a comet or asteroid if it does not emit light or pass infront of an object which emits light?  \r\n\r\nAnd if such an object would be on a crash course with our planet how early would we be able to spot it?\n\nAccepted Answer:\n\nYeah.. You're right that those objects don't emit light. But, they can scatter electromagnetic waves. So, we can study almost many properties like size, shape, average diameter, etc. through [RADAR](http://en.wikipedia.org/wiki/Radar_astronomy#Asteroids_and_comets)\r\n\r\nRadar is the first step of the [Cosmic distance ladder](http://en.wikipedia.org/wiki/Cosmic_distance_ladder) which can be used to locate such objects and study about their physical properties. I think it's limited to our solar system. That's why we go into Parallax measurement for browsing *deeper* into our galaxy...\n\nAnswer (score=0):\n\nOne of the coolest things you can do to find \"dark\" objects like those is to use the [Goldstone Solar System Radar][1]. It's like a conventional radar system, but used on earth pointing into space, which is double awesome!\r\n\r\nNASA webpages published images of asteroids which will pass earth around 2030 (hello, [Aphophis][2]) somewhere in the early 2000's, so I guess ten years in advance seems a reasonable timeframe, given the object is big enough.\r\n\r\n\r\n  [1]: http://deepspace.jpl.nasa.gov/technology/95_20/gold.htm\r\n  [2]: http://www.jpl.nasa.gov/news/news.php?release=2013-017", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 36, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "comets", "meteorites"], "page_start": null, "page_end": null, "quality_flags": ["low_score"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 59802, "title": "Ways to spot a comet or asteroid?", "url": "https://physics.stackexchange.com/questions/59802/ways-to-spot-a-comet-or-asteroid", "share_url": "https://physics.stackexchange.com/q/59802", "content_license": null, "owner": {"display_name": "user17615", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, "answers": [{"answer_id": 59807, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/59802/ways-to-spot-a-comet-or-asteroid/59807#59807", "share_url": "https://physics.stackexchange.com/a/59807", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "anon", "user_id": null, "user_type": "does_not_exist", "profile_url": null}}, {"answer_id": 59804, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/59802/ways-to-spot-a-comet-or-asteroid/59804#59804", "share_url": "https://physics.stackexchange.com/a/59804", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Wojciech Morawiec", "user_id": 19979, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/19979/wojciech-morawiec"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:269617:0000", "text": "Question: How an asteroid enters the Earth make a completely different outcome?\n\nIn this scenario there are different ways an asteroid could enter the Earth's crust but does the aftermath differ? Think of the surface of the Earth as thin sheet of ice and the magma the water underneath much like an egg. Would an asteroid, the size of a small city 10 miles or more wide necessarily wipe out the Earth? Would most of the force go strait through the crust and the impact be absorbed by the magma? Would the shape make a difference like hitting flat like a belly flop compared a smooth entry like a Olympic diver with no splash? Would an asteroid hitting the Earth at steep angle cause the Earth's rotation to slow in half or more? \r\n\r\nThere are many questions on this but the main answer I am asking is with the right angle of impact, shape and direction of impact make a difference if the same asteroid would equal extinction or not?\n\nAccepted Answer:\n\nThis is an interesting question and one that probably needs detailed simulation to settle. But one can make the following broad prediction: the shape of the meteorite would have minimal effect on the outcome, for the following reasons:\r\n\r\n1. At the kinds energies let slip in the moments of impact and the kinds of pressures and temperatures that prevail, all kinds of matter behave in ways pretty near to those of an ideal gas. The forces between molecules that give rise to the everyday \"solidness\", \"hardness\" and \"sloshiness\" of solids and liquids are minuscule compared with those arising from the impact. The gas approximation is made, very successfully it would seem, in the modelling of the extreme environments met in the center of explosive blasts, particularly in the modelling of the detonation of thermonuclear weapons. The main mechanism slowing the impactor down is a rocket-like thrust: as the impactor lets slip enormous energy, vaporizing the Earth's crust, the backthrust from the swiftly expanding gasses allows the momentum to be transferred to the Earth;\r\n\r\n2. In many \"penetration\" type scenarios, a kind of negative feedback where increased penetration speeds and energies beget increased resistive forces means that penetration depth is only very weakly dependent on impact speed or impactor shape. Newton was well aware of this kind of mechanism and indeed proposed the law that, for impactors of similar density to that of the impacted body, the penetration depth is independent of the impact speed and equal to the length of the impactor (measured along the direction of relative impact velocity). This surprising law is discussed in [tpg2114's answer to the question \"Platform Diving: How deep does one go into the water\"](https://physics.stackexchange.com/a/146013/26076) in some detail. Apparently the surprising rule has pretty solid experimental backup and indeed if one makes detailed fluid-dynamical calculations using ram pressure drag, as [I did here in answer](https://physics.stackexchange.com/a/146016/26076) to the same question, this behavior does come out of the mathematics. Ram pressure drag is probably a good model for this kind of problem.\r\n\r\nThere are many groups around the world who have studied known impactor events in detail through computer simulation. See, for example, [this study at Princeton of the Chixulub impactor](https://www.princeton.edu/main/news/archive/S31/90/32S94/).", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 37, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "forces", "astrophysics", "collision", "earth"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 269617, "title": "How an asteroid enters the Earth make a completely different outcome?", "url": "https://physics.stackexchange.com/questions/269617/how-an-asteroid-enters-the-earth-make-a-completely-different-outcome", "share_url": "https://physics.stackexchange.com/q/269617", "content_license": null, "owner": {"display_name": "Muze", "user_id": 148704, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/148704/muze"}}, "answers": [{"answer_id": 269636, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/269617/how-an-asteroid-enters-the-earth-make-a-completely-different-outcome/269636#269636", "share_url": "https://physics.stackexchange.com/a/269636", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Selena Ballerina", "user_id": 26076, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/26076/selena-ballerina"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:375862:0000", "text": "Question: Constant thrusting force vs explosions for re-directing a giant asteroid's course\n\n**Background:**\r\n\r\n*SPOILERS*\r\n\r\nIn episode 11 of the Inuyashiki anime, Mr. Inuyashiki, a super-powerful 'robot' made from the technology of an advanced alien race, flies to space in order to stop a \"giant\" (no exact specification on it's size) asteroid from hitting the Earth. However, his chosen method is shooting missiles at it which, given the size of the asteroid, did not prove very effective. Hiro- another 'robot' that is the same as Mr. Inuyashiki- then joins him and self-destructs in an attempt to redirect the path of the asteroid. From the beggining of the anime we know that the technology they posses in their 'bodies' is enough to \"destroy the Earth\" as the aliens said. Yet it took both Hiro and Mr. Inuyashiki to self destruct in order to redirect the asteroid enough.\r\n\r\nIn previous episodes these characters exhibited enough thrusting force (from the thrusters in their back) to safely land a large travelling plane filled with people that was plummeting down without control, but it is not mentioned if that is the full extent of their strength. These characters are completely humanoid in appearance but with a completely mechanized interior.\r\n\r\n**Question:**\r\n\r\n**Under the assumption that we have thrusters that are as strong in thrust as physically and scientifically possible, created with hypothetical alien technology, but with the limitation of fitting on an average person's back would it be possible for two of such individuals to alter the course significantly enough for the asteroid to not hit Earth? Or would it be more feasible to accomplish the same mission with strong enough explosions?** \r\n\r\nFor simplicity's sake assume that the asteroid is heading straight to the center of our planet and it's size and mass is just enough to theoretically destroy all life on earth and the time that the characters have at their hands is 48 hours. Speed of the asteroid is unknown. \r\n\r\nIf impossible for the scenario in question, how much thrusting force (and/or over which period of time) would we have to apply to re-direct an asteroid that's large enough to destroy humanity? Would it make more sense to use explosions even though the force from an explosion would be spread in all directions? Or would an entirely different maneuver be more plausible?\n\nAccepted Answer:\n\nI am not an expert on asteroids, but I know that a lot of them are not solid objects. They are more conglomerates of smaller objects rather loosely held. An explosion may just fragment the asteroid into a lot of these pieces. That could amount to turning a problem of one asteroid as a bullet into a shot gun bird shot problem.\r\n\r\nThrust or force is more advised. If you have enough time you do not really need to strap on a rocket. There is about $W~=~1250w/m^2$ of solar radiation and UV accounts for about $250w/m^2$ of that. If you then have an asteroid of radius $R$ here is cross section of $2\\pi R^2$. We might then think of putting reflecting foil on the asteroid. The photon pressure from reflecting photons off is $P~=~2W/c$ and with the cosine effect we integrate over the area to get the force or thrust\r\n$$\r\nF~=~2\\sqrt{2}\\pi WR^2/c.\r\n$$\r\nFor a $100m$ radius asteroid the force is then $.37N$. A possible mass for such an asteroid is then about $2\\times 10^{10}kg$. This means the acceleration will be a very small $a~\\simeq~2\\times 10^{-8}m/s^2$. Now using the elementary $d~=~\\frac{1}{2}at^2$ and assuming solar radiation gently pushes this mass for a year this asteroid can be displaced $d~=~9\\times 10^6m$ or nearly $10,000$ kilometers. This would be enough to avoid a collision with the Earth. \r\n\r\nI mentioned ultraviolet because one could concentrate UV onto an asteroid so as to charge the dust on its surface. This would then by electrostatic repulsion be directed away. Newton's third law could then be used to move this body. \r\n\r\nAnother approach would be to pull a mass with an ion drive near the asteroid that would then thrust away at a very small velocity. The mass of this $100m$ asteroid is about $2\\times 10^{10}kg$. Now consider this ion drive tethered to a mass $m$. The gravitational attraction between this mass hovering near the surface of the asteroid and the asteroid would then be an acceleration\r\n$$\r\na_g~=~\\frac{GM}{r^2}~=~1.3\\times 10^{-4}m/s^2,\r\n$$\r\nThis means the ion propulsion system would need to provide $F~=~ma_g$ force to then gravitationally pull the asteroid away. This would require more sophisticated technology than wrapping an asteroid with foil, but this would clearly be effective.\n\nAnswer (score=0):\n\nTo deflect an asteroid, you need thrust.  In general, this means you should use thrusters, because thrusters are good at creating thrust.\r\n\r\nThere are two issues with using explosions to deflect an asteroid.  The first is that explosions generally do not do a good job of imparting momentum to an object.  They often send particles flying in all directions, rather than just the direction you actually want them to go.  They also do not couple well with the asteroid.  Thrusters are typically designed to get as much momentum per unit energy as they can.  They're good at it.\r\n\r\nThe second issue is that your materials choices for explosions are limited.  Generally speaking, explosives have a very low specific energy (energy stored per unit mass).  C4 is good for about 6.7 MJ/kg.  Contrast that with gasoline at 46.4MJ/kg, or even body fat at 37MJ/kg.  Explosives tend to have lower specific energies because they trade away some of that maximum energy density for the rate at which explosives release that energy on the order of milliseconds.\r\n\r\nBetween these effects, what you see is that explosives have a low [Specific Impulse][1] (Isp), which is a measure of how much thrust can be created per unit mass.  Isp is the real variable you should care about.  Thrusters tend to have a much higher Isp.\r\n\r\nThe one exception is when dealing with energy that is hard to unleash slowly.  Nuclear energy is a great example.  Generally speaking, its hard to make a nuclear thruster.  The physics of doing so is simply difficult.  This means that it's hard to get access to your nuclear energy slowly.  In such a case, it may be better to rely on rapid emission of energy (i.e. nuclear explosions) with all the disadvantages associated with explosions than to simply not have access to that energy at all.  I'd definitely want to exhaust all of my thruster fuel first.\r\n\r\nThere is a theoretical craft known as a [Nuclear Pulse Rocket][2]...\r\n\r\n\r\n  [1]: https://en.wikipedia.org/wiki/Specific_impulse\r\n  [2]: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 38, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "homework-and-exercises", "newtonian-mechanics", "estimation", "rocket-science"], "page_start": null, "page_end": null, "quality_flags": ["low_score"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 375862, "title": "Constant thrusting force vs explosions for re-directing a giant asteroid's course", "url": "https://physics.stackexchange.com/questions/375862/constant-thrusting-force-vs-explosions-for-re-directing-a-giant-asteroids-cours", "share_url": "https://physics.stackexchange.com/q/375862", "content_license": null, "owner": {"display_name": "Chooba", "user_id": 179425, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/179425/chooba"}}, "answers": [{"answer_id": 375876, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/375862/constant-thrusting-force-vs-explosions-for-re-directing-a-giant-asteroids-cours/375876#375876", "share_url": "https://physics.stackexchange.com/a/375876", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Lawrence B. Crowell", "user_id": 117719, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/117719/lawrence-b-crowell"}}, {"answer_id": 375868, "is_accepted": false, "url": "https://physics.stackexchange.com/questions/375862/constant-thrusting-force-vs-explosions-for-re-directing-a-giant-asteroids-cours/375868#375868", "share_url": "https://physics.stackexchange.com/a/375868", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Cort Ammon", "user_id": 47472, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/47472/cort-ammon"}}]}}}
{"unit_id": "stackexchange:physics.stackexchange:asteroids:323091:0000", "text": "Question: Hypothetical Question: Would a person be able to stop an asteroid from colliding with earth?\n\nThis is somewhat of a shower thought. Let's say there is a person with the same capabilities as superman. Would that person actually be able to stop an Asteroid from hitting earth? I can only image that a person with superhuman strength and speed would directly fly through the asteroid, not really stopping it but drilling a hole into it.\r\n\r\nEdit: I guess too many think of this as a superhero question. Think of the person trying to stop the asteroid as a super hard, human shaped object hitting the asteroid at a high speed.\n\nAccepted Answer:\n\ntrue, if the surface area of contact is tiny (which is true considering the asteroid is of radius ~5km and a human can only have a contact area of a metre at max) the pressure would be large and would indeed drill a hole.\r\n\r\nif the human has some method by which he can apply same force but say on a balloon which doesn't burst and can transfer the force onto the asteroid, he might be able to do it.", "source": "stackexchange", "source_doc_id": "physics.stackexchange", "source_title": "Physics Stack Exchange", "domain": "physics", "subdomain": "qa_community", "level": "mixed", "order_index": 39, "metadata": {"module_id": "asteroids", "source_format": "stackexchange_api_v2.3", "extraction_method": "stackexchange_api_v2_3_extractor_v1", "license": "CC-BY-SA-3.0", "language": "en", "hierarchy_path": ["Physics Stack Exchange", "asteroids", "astrophysics"], "page_start": null, "page_end": null, "quality_flags": ["low_score", "single_answer"], "ocr_confidence": null, "layout_confidence": null, "math_confidence": null, "stackexchange_attribution": {"network": "Stack Exchange", "site_name": "Physics Stack Exchange", "site_url": "https://physics.stackexchange.com", "api_site": "physics", "api_terms_url": "https://stackoverflow.com/legal/api-terms-of-use", "license_help_url": "https://physics.stackexchange.com/help/licensing", "question": {"question_id": 323091, "title": "Hypothetical Question: Would a person be able to stop an asteroid from colliding with earth?", "url": "https://physics.stackexchange.com/questions/323091/hypothetical-question-would-a-person-be-able-to-stop-an-asteroid-from-colliding", "share_url": "https://physics.stackexchange.com/q/323091", "content_license": null, "owner": {"display_name": "Redbeard", "user_id": 150717, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/150717/redbeard"}}, "answers": [{"answer_id": 323109, "is_accepted": true, "url": "https://physics.stackexchange.com/questions/323091/hypothetical-question-would-a-person-be-able-to-stop-an-asteroid-from-colliding/323109#323109", "share_url": "https://physics.stackexchange.com/a/323109", "content_license": "CC BY-SA 3.0", "owner": {"display_name": "Sakazuki Akainu", "user_id": 148748, "user_type": "registered", "profile_url": "https://physics.stackexchange.com/users/148748/sakazuki-akainu"}}]}}}