text stringlengths 26 3.6k | page_title stringlengths 1 71 | source stringclasses 1
value | token_count int64 10 512 | id stringlengths 2 8 | url stringlengths 31 117 | topic stringclasses 4
values | section stringlengths 4 49 ⌀ | sublist stringclasses 9
values |
|---|---|---|---|---|---|---|---|---|
Maxwell Gap and Ringlet
The Maxwell Gap lies within the outer part of the C Ring. It also contains a dense non-circular ringlet, the Maxwell Ringlet. In many respects this ringlet is similar to the ε ring of Uranus. There are wave-like structures in the middle of both rings. While the wave in the ε ring is thought to be caused by Uranian moon Cordelia, no moon has been discovered in the Maxwell gap as of July 2008.
B Ring
The B Ring is the largest, brightest, and most massive of the rings. Its thickness is estimated as 5 to 15 m and its optical depth varies from 0.4 to greater than 5, meaning that >99% of the light passing through some parts of the B Ring is blocked. The B Ring contains a great deal of variation in its density and brightness, nearly all of it unexplained. These are concentric, appearing as narrow ringlets, though the B Ring does not contain any gaps. In places, the outer edge of the B Ring contains vertical structures deviating up to 2.5 km (1½ miles) from the main ring plane, a significant deviation from the vertical thickness of the main A, B and C rings, which is generally only about 10 meters (about 30 feet). Vertical structures can be created by unseen embedded moonlets.
A 2016 study of spiral density waves using stellar occultations indicated that the B Ring's surface density is in the range of 40 to 140 g/cm2, lower than previously believed, and that the ring's optical depth has little correlation with its mass density (a finding previously reported for the A and C rings). The total mass of the B Ring was estimated to be somewhere in the range of 7 to kg. This compares to a mass for Mimas of kg.
Spokes | Rings of Saturn | Wikipedia | 374 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
Until 1980, the structure of the rings of Saturn was explained as being caused exclusively by the action of gravitational forces. Then images from the Voyager spacecraft showed radial features in the B Ring, known as spokes, which could not be explained in this manner, as their persistence and rotation around the rings was not consistent with gravitational orbital mechanics. The spokes appear dark in backscattered light, and bright in forward-scattered light; the transition occurs at a phase angle near 60°. The leading hypothesis regarding the spokes' composition is that they consist of microscopic dust particles suspended away from the main ring by electrostatic repulsion, as they rotate almost synchronously with the magnetosphere of Saturn. The precise mechanism generating the spokes is still unknown. It has been suggested that the electrical disturbances might be caused by either lightning bolts in Saturn's atmosphere or micrometeoroid impacts on the rings. Alternatively, it is proposed that the spokes are very similar to a phenomenon known as lunar horizon glow or dust levitation, and caused by intense electric fields across the terminator of ring particles, not electrical disturbances.
The spokes were not observed again until some twenty-five years later, this time by the Cassini space probe. The spokes were not visible when Cassini arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe their formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and they were next seen in images taken on 5 September 2005.
The spokes appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter and midsummer and reappearing as Saturn comes closer to equinox. Suggestions that the spokes may be a seasonal effect, varying with Saturn's 29.7-year orbit, were supported by their gradual reappearance in the later years of the Cassini mission.
Moonlet
In 2009, during equinox, a moonlet embedded in the B ring was discovered from the shadow it cast. It is estimated to be in diameter. The moonlet was given the provisional designation S/2009 S 1.
Cassini Division | Rings of Saturn | Wikipedia | 446 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
The Cassini Division is a region in width between Saturn's A Ring and B Ring. It was discovered in 1675 by Giovanni Cassini at the Paris Observatory using a refracting telescope that had a 2.5-inch objective lens with a 20-foot-long focal length and a 90x magnification. From Earth it appears as a thin black gap in the rings. However, Voyager discovered that the gap is itself populated by ring material bearing much similarity to the C Ring. The division may appear bright in views of the unlit side of the rings, since the relatively low density of material allows more light to be transmitted through the thickness of the rings.
The inner edge of the Cassini Division is governed by a strong orbital resonance. Ring particles at this location orbit twice for every orbit of the moon Mimas. The resonance causes Mimas' pulls on these ring particles to accumulate, destabilizing their orbits and leading to a sharp cutoff in ring density. Many of the other gaps between ringlets within the Cassini Division, however, are unexplained.
Huygens Gap
Discovered in 1981 through images sent back by Voyager 2, the Huygens Gap is located at the inner edge of the Cassini Division. It contains the dense, eccentric Huygens Ringlet in the middle. This ringlet exhibits irregular azimuthal variations of geometrical width and optical depth, which may be caused by the nearby 2:1 resonance with Mimas and the influence of the eccentric outer edge of the B-ring. There is an additional narrow ringlet just outside the Huygens Ringlet.
A Ring
The A Ring is the outermost of the large, bright rings. Its inner boundary is the Cassini Division and its sharp outer boundary is close to the orbit of the small moon Atlas. The A Ring is interrupted at a location 22% of the ring width from its outer edge by the Encke Gap. A narrower gap 2% of the ring width from the outer edge is called the Keeler Gap.
The thickness of the A Ring is estimated to be 10 to 30 m, its surface density from 35 to 40 g/cm2 and its total mass as 4 to kg (just under the mass of Hyperion). Its optical depth varies from 0.4 to 0.9. | Rings of Saturn | Wikipedia | 471 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
Similarly to the B Ring, the A Ring's outer edge is maintained by orbital resonances, albeit in this case a more complicated set. It is primarily acted on by the 7:6 resonance with Janus and Epimetheus, with other contributions from the 5:3 resonance with Mimas and various resonances with Prometheus and Pandora. Other orbital resonances also excite many spiral density waves in the A Ring (and, to a lesser extent, other rings as well), which account for most of its structure. These waves are described by the same physics that describes the spiral arms of galaxies. Spiral bending waves, also present in the A Ring and also described by the same theory, are vertical corrugations in the ring rather than compression waves.
In April 2014, NASA scientists reported observing the possible formative stage of a new moon near the outer edge of the A Ring.
Encke Gap
The Encke Gap is a 325-km (200 mile) wide gap within the A ring, centered at a distance of 133,590 km (83,000 miles) from Saturn's center. It is caused by the presence of the small moon Pan, which orbits within it. Images from the Cassini probe have shown that there are at least three thin, knotted ringlets within the gap. Spiral density waves visible on both sides of it are induced by resonances with nearby moons exterior to the rings, while Pan induces an additional set of spiralling wakes.
Johann Encke himself did not observe this gap; it was named in honour of his ring observations. The gap itself was discovered by James Edward Keeler in 1888. The second major gap in the A ring, discovered by Voyager, was named the Keeler Gap in his honor.
The Encke Gap is a gap because it is entirely within the A Ring. There was some ambiguity between the terms gap and division until the IAU clarified the definitions in 2008; before that, the separation was sometimes called the "Encke Division".
Keeler Gap | Rings of Saturn | Wikipedia | 411 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
The Keeler Gap is a 42-km (26 mile) wide gap in the A ring, approximately 250 km (150 miles) from the ring's outer edge. The small moon Daphnis, discovered 1 May 2005, orbits within it, keeping it clear. The moon's passage induces waves in the edges of the gap (this is also influenced by its slight orbital eccentricity). Because the orbit of Daphnis is slightly inclined to the ring plane, the waves have a component that is perpendicular to the ring plane, reaching a distance of 1500 m "above" the plane.
The Keeler gap was discovered by Voyager, and named in honor of the astronomer James Edward Keeler. Keeler had in turn discovered and named the Encke Gap in honor of Johann Encke.
Propeller moonlets
In 2006, four tiny "moonlets" were found in Cassini images of the A Ring. The moonlets themselves are only about a hundred meters in diameter, too small to be seen directly; what Cassini sees are the "propeller"-shaped disturbances the moonlets create, which are several km (miles) across. It is estimated that the A Ring contains thousands of such objects. In 2007, the discovery of eight more moonlets revealed that they are largely confined to a 3,000 km (2000 mile) belt, about 130,000 km (80,000 miles) from Saturn's center, and by 2008 over 150 propeller moonlets had been detected. One that has been tracked for several years has been nicknamed Bleriot.
Roche Division
The separation between the A ring and the F Ring has been named the Roche Division in honor of the French physicist Édouard Roche. The Roche Division should not be confused with the Roche limit which is the distance at which a large object is so close to a planet (such as Saturn) that the planet's tidal forces will pull it apart. Lying at the outer edge of the main ring system, the Roche Division is in fact close to Saturn's Roche limit, which is why the rings have been unable to accrete into a moon. | Rings of Saturn | Wikipedia | 427 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
Like the Cassini Division, the Roche Division is not empty but contains a sheet of material. The character of this material is similar to the tenuous and dusty D, E, and G Rings. Two locations in the Roche Division have a higher concentration of dust than the rest of the region. These were discovered by the Cassini probe imaging team and were given temporary designations: R/2004 S 1, which lies along the orbit of the moon Atlas; and R/2004 S 2, centered at 138,900 km (86,300 miles) from Saturn's center, inward of the orbit of Prometheus.
F Ring
The F Ring is the outermost discrete ring of Saturn and perhaps the most active ring in the Solar System, with features changing on a timescale of hours. It is located 3,000 km (2000 miles) beyond the outer edge of the A ring. The ring was discovered in 1979 by the Pioneer 11 imaging team. It is very thin, just a few hundred km (miles) in radial extent. While the traditional view has been that it is held together by two shepherd moons, Prometheus and Pandora, which orbit inside and outside it, recent studies indicate that only Prometheus contributes to the confinement. Numerical simulations suggest the ring was formed when Prometheus and Pandora collided with each other and were partially disrupted.
More recent closeup images from the Cassini probe show that the F Ring consists of one core ring and a spiral strand around it. They also show that when Prometheus encounters the ring at its apoapsis, its gravitational attraction creates kinks and knots in the F Ring as the moon 'steals' material from it, leaving a dark channel in the inner part of the ring. Since Prometheus orbits Saturn more rapidly than the material in the F ring, each new channel is carved about 3.2 degrees in front of the previous one.
In 2008, further dynamism was detected, suggesting that small unseen moons orbiting within the F Ring are continually passing through its narrow core because of perturbations from Prometheus. One of the small moons was tentatively identified as S/2004 S 6. | Rings of Saturn | Wikipedia | 438 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
As of 2023, the clumpy structure of the ring "is thought to be caused by the presence of thousands of small parent bodies (1.0 to 0.1 km in size) that collide and produce dense strands of micrometer- to centimeter-sized particles that re-accrete over a few months onto the parent bodies in a steady-state regime."
Outer rings
Janus/Epimetheus Ring
A faint dust ring is present around the region occupied by the orbits of Janus and Epimetheus, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006. The ring has a radial extent of about 5,000 km (3000 miles). Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.
G Ring
The G Ring is a very thin, faint ring about halfway between the F Ring and the beginning of the E Ring, with its inner edge about 15,000 km (10,000 miles) inside the orbit of Mimas. It contains a single distinctly brighter arc near its inner edge (similar to the arcs in the rings of Neptune) that extends about one-sixth of its circumference, centered on the half-km (500 yard) diameter moonlet Aegaeon, which is held in place by a 7:6 orbital resonance with Mimas. The arc is believed to be composed of icy particles up to a few m in diameter, with the rest of the G Ring consisting of dust released from within the arc. The radial width of the arc is about 250 km (150 miles), compared to a width of 9,000 km (6000 miles) for the G Ring as a whole. The arc is thought to contain matter equivalent to a small icy moonlet about a hundred m in diameter. Dust released from Aegaeon and other source bodies within the arc by micrometeoroid impacts drifts outward from the arc because of interaction with Saturn's magnetosphere (whose plasma corotates with Saturn's magnetic field, which rotates much more rapidly than the orbital motion of the G Ring). These tiny particles are steadily eroded away by further impacts and dispersed by plasma drag. Over the course of thousands of years the ring gradually loses mass, which is replenished by further impacts on Aegaeon. | Rings of Saturn | Wikipedia | 484 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
Methone Ring Arc
A faint ring arc, first detected in September 2006, covering a longitudinal extent of about 10 degrees is associated with the moon Methone. The material in the arc is believed to represent dust ejected from Methone by micrometeoroid impacts. The confinement of the dust within the arc is attributable to a 14:15 resonance with Mimas (similar to the mechanism of confinement of the arc within the G ring). Under the influence of the same resonance, Methone librates back and forth in its orbit with an amplitude of 5° of longitude.
Anthe Ring Arc
A faint ring arc, first detected in June 2007, covering a longitudinal extent of about 20 degrees is associated with the moon Anthe. The material in the arc is believed to represent dust knocked off Anthe by micrometeoroid impacts. The confinement of the dust within the arc is attributable to a 10:11 resonance with Mimas. Under the influence of the same resonance, Anthe drifts back and forth in its orbit over 14° of longitude.
Pallene Ring
A faint dust ring shares Pallene's orbit, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006. The ring has a radial extent of about 2,500 km (1500 miles). Its source is particles blasted off Pallene's surface by meteoroid impacts, which then form a diffuse ring around its orbital path.
E Ring | Rings of Saturn | Wikipedia | 294 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
Although not confirmed until 1980, the existence of the E ring was a subject of debate among astronomers at least as far back as 1908. In a narrative timeline of Saturn observations, Arthur Francis O'Donel Alexander attributes the first observation of what would come to be called the E Ring to Georges Fournier, who on 5 September 1907 at Mont Revard observed a "luminous zone" "surrounding the outer bright ring." The next year, on 7 October 1908, E. Schaer independently observed "a new dusky ring...surrounding the bright rings of Saturn" at the Geneva Observatory. Following up on Schaer's discovery, W. Boyer, T. Lewis, and Arthur Eddington found signs of a discontinuous ring matching Schaer's description, but described their observations as "uncertain." After Edward Barnard, using the what was at the time the world's best telescope, failed to find signs of a ring. E. M. Antoniadi argued for the ring's existence in a 1909 publication, recalling a observations by William Wray on 26 December 1861 of a "very faint light...so as to give the impression that it was the dusky ring," but after Barnard's negative result most astronomers became skeptical of the E Ring's existence. | Rings of Saturn | Wikipedia | 267 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
Unlike the A, B, and C rings, the E Ring's small optical depth and large vertical extent mean it is best viewed edge-on, which is only possible once every 14–15 years, so perhaps for this reason, it was not until the 1960's that the E Ring was again the subject of observations. Although some sources credit Walter Feibelman with the E Ring's discovery in 1966, his paper published the following year announcing the observations begins by acknowledging the existing controversy and the long record of observations both supporting and disputing the ring's existence, and carefully stresses his interpretation of the data as a new ring as "tentative only." A reanalysis of Feibelman's original observations, conducted in anticipation of the coming Saturn flyby by Pioneer 11, once again called the evidence for this outer ring "shaky." Even polarimetric observations by Pioneer 11 failed to conclusively identify E Ring during its 1979 flyby, though "its existence was inferred from [particle, radiation, and magnetic field measurements]." Only after a digital reanalysis of the 1966 observations as well as several independent observations using ground- and space-based telescopes existence was finally confirmed in a 1980 paper by Feibelman and Klinglesmith.
The E Ring is the second outermost ring and is extremely wide; it consists of many tiny (micron and sub-micron) particles of water ice with silicates, carbon dioxide and ammonia. The E Ring is distributed between the orbits of Mimas and Titan. Unlike the other rings, it is composed of microscopic particles rather than macroscopic ice chunks. In 2005, the source of the E Ring's material was determined to be cryovolcanic plumes emanating from the "tiger stripes" of the south polar region of the moon Enceladus. Unlike the main rings, the E Ring is more than 2,000 km (1000 miles) thick and increases with its distance from Enceladus. Tendril-like structures observed within the E Ring can be related to the emissions of the most active south polar jets of Enceladus. | Rings of Saturn | Wikipedia | 438 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
Particles of the E Ring tend to accumulate on moons that orbit within it. The equator of the leading hemisphere of Tethys is tinted slightly blue due to infalling material. The trojan moons Telesto, Calypso, Helene and Polydeuces are particularly affected as their orbits move up and down the ring plane. This results in their surfaces being coated with bright material that smooths out features.
Phoebe ring
In October 2009, the discovery of a tenuous disk of material just interior to the orbit of Phoebe was reported. The disk was aligned edge-on to Earth at the time of discovery. This disk can be loosely described as another ring. Although very large (as seen from Earth, the apparent size of two full moons), the ring is virtually invisible. It was discovered using NASA's infrared Spitzer Space Telescope, and was seen over the entire range of the observations, which extended from 128 to 207 times the radius of Saturn, with calculations indicating that it may extend outward up to 300 Saturn radii and inward to the orbit of Iapetus at 59 Saturn radii. The ring was subsequently studied using the WISE, Herschel and Cassini spacecraft; WISE observations show that it extends from at least between 50 and 100 to 270 Saturn radii (the inner edge is lost in the planet's glare). Data obtained with WISE indicate the ring particles are small; those with radii greater than 10 cm comprise 10% or less of the cross-sectional area.
Phoebe orbits the planet at a distance ranging from 180 to 250 radii. The ring has a thickness of about 40 radii. Because the ring's particles are presumed to have originated from impacts (micrometeoroid and larger) on Phoebe, they should share its retrograde orbit, which is opposite to the orbital motion of the next inner moon, Iapetus. This ring lies in the plane of Saturn's orbit, or roughly the ecliptic, and thus is tilted 27 degrees from Saturn's equatorial plane and the other rings. Phoebe is inclined by 5° with respect to Saturn's orbit plane (often written as 175°, due to Phoebe's retrograde orbital motion), and its resulting vertical excursions above and below the ring plane agree closely with the ring's observed thickness of 40 Saturn radii. | Rings of Saturn | Wikipedia | 478 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
The existence of the ring was proposed in the 1970s by Steven Soter. The discovery was made by Anne J. Verbiscer and Michael F. Skrutskie (of the University of Virginia) and Douglas P. Hamilton (of the University of Maryland, College Park). The three had studied together at Cornell University as graduate students.
Ring material migrates inward due to reemission of solar radiation, with a speed inversely proportional to particle size; a 3 cm particle would migrate from the vicinity of Phoebe to that of Iapetus over the age of the Solar System. The material would thus strike the leading hemisphere of Iapetus. Infall of this material causes a slight darkening and reddening of the leading hemisphere of Iapetus (similar to what is seen on the Uranian moons Oberon and Titania) but does not directly create the dramatic two-tone coloration of that moon. Rather, the infalling material initiates a positive feedback thermal self-segregation process of ice sublimation from warmer regions, followed by vapor condensation onto cooler regions. This leaves a dark residue of "lag" material covering most of the equatorial region of Iapetus's leading hemisphere, which contrasts with the bright ice deposits covering the polar regions and most of the trailing hemisphere.
Possible ring system around Rhea | Rings of Saturn | Wikipedia | 268 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
Saturn's second largest moon Rhea has been hypothesized to have a tenuous ring system of its own consisting of three narrow bands embedded in a disk of solid particles. These putative rings have not been imaged, but their existence has been inferred from Cassini observations in November 2005 of a depletion of energetic electrons in Saturn's magnetosphere near Rhea. The Magnetospheric Imaging Instrument (MIMI) observed a gentle gradient punctuated by three sharp drops in plasma flow on each side of the moon in a nearly symmetric pattern. This could be explained if they were absorbed by solid material in the form of an equatorial disk containing denser rings or arcs, with particles perhaps several decimeters to approximately a meter in diameter. A more recent piece of evidence consistent with the presence of Rhean rings is a set of small ultraviolet-bright spots distributed in a line that extends three quarters of the way around the moon's circumference, within 2 degrees of the equator. The spots have been interpreted as the impact points of deorbiting ring material. However, targeted observations by Cassini of the putative ring plane from several angles have turned up nothing, suggesting that another explanation for these enigmatic features is needed. | Rings of Saturn | Wikipedia | 253 | 977592 | https://en.wikipedia.org/wiki/Rings%20of%20Saturn | Physical sciences | Solar System | Astronomy |
A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a sharp snapping sound.
When used in materials science, it is generally applied to materials that fail when there is little or no plastic deformation before failure. One proof is to match the broken halves, which should fit exactly since no plastic deformation has occurred.
Brittleness in different materials
Polymers
Mechanical characteristics of polymers can be sensitive to temperature changes near room temperatures. For example, poly(methyl methacrylate) is extremely brittle at temperature 4˚C, but experiences increased ductility with increased temperature.
Amorphous polymers are polymers that can behave differently at different temperatures. They may behave like a glass at low temperatures (the glassy region), a rubbery solid at intermediate temperatures (the leathery or glass transition region), and a viscous liquid at higher temperatures (the rubbery flow and viscous flow region). This behavior is known as viscoelastic behavior. In the glassy region, the amorphous polymer will be rigid and brittle. With increasing temperature, the polymer will become less brittle.
Metals
Some metals show brittle characteristics due to their slip systems. The more slip systems a metal has, the less brittle it is, because plastic deformation can occur along many of these slip systems. Conversely, with fewer slip systems, less plastic deformation can occur, and the metal will be more brittle. For example, HCP (hexagonal close packed) metals have few active slip systems, and are typically brittle.
Ceramics
Ceramics are generally brittle due to the difficulty of dislocation motion, or slip. There are few slip systems in crystalline ceramics that a dislocation is able to move along, which makes deformation difficult and makes the ceramic more brittle.
Ceramic materials generally exhibit ionic bonding. Because of the ions’ electric charge and their repulsion of like-charged ions, slip is further restricted.
Changing brittle materials
Materials can be changed to become more brittle or less brittle.
Toughening | Brittleness | Wikipedia | 424 | 978913 | https://en.wikipedia.org/wiki/Brittleness | Physical sciences | Solid mechanics | Physics |
When a material has reached the limit of its strength, it usually has the option of either deformation or fracture. A naturally malleable metal can be made stronger by impeding the mechanisms of plastic deformation (reducing grain size, precipitation hardening, work hardening, etc.), but if this is taken to an extreme, fracture becomes the more likely outcome, and the material can become brittle. Improving material toughness is, therefore, a balancing act.
Naturally brittle materials, such as glass, are not difficult to toughen effectively. Most such techniques involve one of two mechanisms: to deflect or absorb the tip of a propagating crack or to create carefully controlled residual stresses so that cracks from certain predictable sources will be forced closed. The first principle is used in laminated glass where two sheets of glass are separated by an interlayer of polyvinyl butyral. The polyvinyl butyral, as a viscoelastic polymer, absorbs the growing crack. The second method is used in toughened glass and pre-stressed concrete. A demonstration of glass toughening is provided by Prince Rupert's Drop. Brittle polymers can be toughened by using metal particles to initiate crazes when a sample is stressed, a good example being high-impact polystyrene or HIPS. The least brittle structural ceramics are silicon carbide (mainly by virtue of its high strength) and transformation-toughened zirconia.
A different philosophy is used in composite materials, where brittle glass fibers, for example, are embedded in a ductile matrix such as polyester resin. When strained, cracks are formed at the glass–matrix interface, but so many are formed that much energy is absorbed and the material is thereby toughened. The same principle is used in creating metal matrix composites.
Effect of pressure
Generally, the brittle strength of a material can be increased by pressure. This happens as an example in the brittle–ductile transition zone at an approximate depth of in the Earth's crust, at which rock becomes less likely to fracture, and more likely to deform ductilely (see rheid).
Crack growth
Supersonic fracture is crack motion faster than the speed of sound in a brittle material. This phenomenon was first discovered by scientists from the Max Planck Institute for Metals Research in Stuttgart (Markus J. Buehler and Huajian Gao) and IBM Almaden Research Center in San Jose, California (Farid F. Abraham). | Brittleness | Wikipedia | 501 | 978913 | https://en.wikipedia.org/wiki/Brittleness | Physical sciences | Solid mechanics | Physics |
The rings of Jupiter are a system of faint planetary rings. The Jovian rings were the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. The main ring was discovered in 1979 by the Voyager 1 space probe and the system was more thoroughly investigated in the 1990s by the Galileo orbiter. The main ring has also been observed by the Hubble Space Telescope and from Earth for several years. Ground-based observation of the rings requires the largest available telescopes.
The Jovian ring system is faint and consists mainly of dust. It has four main components: a thick inner torus of particles known as the "halo ring"; a relatively bright, exceptionally thin "main ring"; and two wide, thick and faint outer "gossamer rings", named for the moons of whose material they are composed: Amalthea and Thebe.
The main and halo rings consist of dust ejected from the moons Metis, Adrastea and perhaps smaller, unobserved bodies as the result of high-velocity impacts. High-resolution images obtained in February and March 2007 by the New Horizons spacecraft revealed a rich fine structure in the main ring.
In visible and near-infrared light, the rings have a reddish color, except the halo ring, which is neutral or blue in color. The size of the dust in the rings varies, but the cross-sectional area is greatest for nonspherical particles of radius about 15 μm in all rings except the halo. The halo ring is probably dominated by submicrometre dust. The total mass of the ring system (including unresolved parent bodies) is poorly constrained, but is probably in the range of 1011 to 1016 kg. The age of the ring system is also not known, but it is possible that it has existed since the formation of Jupiter.
A ring or ring arc appears to exist close to the moon Himalia's orbit. One explanation is that a small moon recently crashed into Himalia and the force of the impact ejected the material that forms the ring. | Rings of Jupiter | Wikipedia | 416 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
Discovery and structure
Jupiter's ring system was the third to be discovered in the Solar System, after those of Saturn and Uranus. It was first observed on 4 March 1979 by the Voyager 1 space probe. It is composed of four main components: a thick inner torus of particles known as the "halo ring"; a relatively bright, exceptionally thin "main ring"; and two wide, thick and faint outer "gossamer rings", named after the moons of whose material they are composed: Amalthea and Thebe. The principal attributes of the known Jovian Rings are listed in the table.
In 2022, dynamical simulations suggested that the relative meagreness of Jupiter's ring system, compared to that of the smaller Saturn, is due to destabilising resonances created by the Galilean satellites.
Main ring
Appearance and structure
The narrow and relatively thin main ring is the brightest part of Jupiter's ring system. Its outer edge is located at a radius of about (; = equatorial radius of Jupiter or ) and coincides with the orbit of Jupiter's smallest inner satellite, Adrastea. Its inner edge is not marked by any satellite and is located at about ().
Thus the width of the main ring is around . The appearance of the main ring depends on the viewing geometry. In forward-scattered light the brightness of the main ring begins to decrease steeply at (just inward of the Adrastean orbit) and reaches the background level at —just outward of the Adrastean orbit. Therefore, Adrastea at clearly shepherds the ring. The brightness continues to increase in the direction of Jupiter and has a maximum near the ring's center at , although there is a pronounced gap (notch) near the Metidian orbit at . The inner boundary of the main ring, in contrast, appears to fade off slowly from to , merging into the halo ring. In forward-scattered light all Jovian rings are especially bright. | Rings of Jupiter | Wikipedia | 404 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
In back-scattered light the situation is different. The outer boundary of the main ring, located at , or slightly beyond the orbit of Adrastea, is very steep. The orbit of the moon is marked by a gap in the ring so there is a thin ringlet just outside its orbit. There is another ringlet just inside Adrastean orbit followed by a gap of unknown origin located at about . The third ringlet is found inward of the central gap, outside the orbit of Metis. The ring's brightness drops sharply just outward of the Metidian orbit, forming the Metis notch. Inward of the orbit of Metis, the brightness of the ring rises much less than in forward-scattered light. So in the back-scattered geometry the main ring appears to consist of two different parts: a narrow outer part extending from to , which itself includes three narrow ringlets separated by notches, and a fainter inner part from to , which lacks any visible structure like in the forward-scattering geometry. The Metis notch serves as their boundary. The fine structure of the main ring was discovered in data from the Galileo orbiter and is clearly visible in back-scattered images obtained from New Horizons in February–March 2007. The early observations by Hubble Space Telescope (HST), Keck and the Cassini spacecraft failed to detect it, probably due to insufficient spatial resolution. However the fine structure was observed by the Keck telescope using adaptive optics in 2002–2003.
Observed in back-scattered light the main ring appears to be razor thin, extending in the vertical direction no more than 30 km. In the side scatter geometry the ring thickness is 80–160 km, increasing somewhat in the direction of Jupiter. The ring appears to be much thicker in the forward-scattered light—about 300 km. One of the discoveries of the Galileo orbiter was the bloom of the main ring—a faint, relatively thick (about 600 km) cloud of material which surrounds its inner part. The bloom grows in thickness towards the inner boundary of the main ring, where it transitions into the halo.
Detailed analysis of the Galileo images revealed longitudinal variations of the main ring's brightness unconnected with the viewing geometry. The Galileo images also showed some patchiness in the ring on the scales 500–1000 km. | Rings of Jupiter | Wikipedia | 464 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
In February–March 2007 New Horizons spacecraft conducted a deep search for new small moons inside the main ring. While no satellites larger than 0.5 km were found, the cameras of the spacecraft detected seven small clumps of ring particles. They orbit just inside the orbit of Adrastea inside a dense ringlet. The conclusion, that they are clumps and not small moons, is based on their azimuthally extended appearance. They subtend 0.1–0.3° along the ring, which correspond to –. The clumps are divided into two groups of five and two members, respectively. The nature of the clumps is not clear, but their orbits are close to 115:116 and 114:115 resonances with Metis. They may be wavelike structures excited by this interaction.
Spectra and particle size distribution
Spectra of the main ring obtained by the HST, Keck, Galileo and Cassini have shown that particles forming it are red, i.e. their albedo is higher at longer wavelengths. The existing spectra span the range 0.5–2.5 μm. No spectral features have been found so far which can be attributed to particular chemical compounds, although the Cassini observations yielded evidence for absorption bands near 0.8 μm and 2.2 μm. The spectra of the main ring are very similar to Adrastea and Amalthea.
The properties of the main ring can be explained by the hypothesis that it contains significant amounts of dust with 0.1–10 μm particle sizes. This explains the stronger forward-scattering of light as compared to back-scattering. However, larger bodies are required to explain the strong back-scattering and fine structure in the bright outer part of the main ring.
Analysis of available phase and spectral data leads to a conclusion that the size distribution of small particles in the main ring obeys a power law
where n(r) dr is a number of particles with radii between r and r + dr and is a normalizing parameter chosen to match the known total light flux from the ring. The parameter q is 2.0 ± 0.2 for particles with r < 15 ± 0.3 μm and q = 5 ± 1 for those with r > 15 ± 0.3 μm. The distribution of large bodies in the mm–km size range is undetermined presently. The light scattering in this model is dominated by particles with r around 15 μm. | Rings of Jupiter | Wikipedia | 501 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
The power law mentioned above allows estimation of the optical depth of the main ring: for the large bodies and for the dust. This optical depth means that the total cross section of all particles inside the ring is about 5000 km². The particles in the main ring are expected to have aspherical shapes. The total mass of the dust is estimated to be 107−109 kg. The mass of large bodies, excluding Metis and Adrastea, is 1011−1016 kg. It depends on their maximum size— the upper value corresponds to about 1 km maximum diameter. These masses can be compared with masses of Adrastea, which is about 2 kg, Amalthea, about 2 kg, and Earth's Moon, 7.4 kg.
The presence of two populations of particles in the main ring explains why its appearance depends on the viewing geometry. The dust scatters light preferably in the forward direction and forms a relatively thick homogenous ring bounded by the orbit of Adrastea. In contrast, large particles, which scatter in the back direction, are confined in a number of ringlets between the Metidian and Adrastean orbits.
Origin and age
The dust is constantly being removed from the main ring by a combination of Poynting–Robertson drag and electromagnetic forces from the Jovian magnetosphere. Volatile materials such as ices, for example, evaporate quickly. The lifetime of dust particles in the ring is from 100 to , so the dust must be continuously replenished in the collisions between large bodies with sizes from 1 cm to 0.5 km and between the same large bodies and high velocity particles coming from outside the Jovian system. This parent body population is confined to the narrow—about —and bright outer part of the main ring, and includes Metis and Adrastea. The largest parent bodies must be less than 0.5 km in size. The upper limit on their size was obtained by New Horizons spacecraft. The previous upper limit, obtained from HST and Cassini observations, was near 4 km. The dust produced in collisions retains approximately the same orbital elements as the parent bodies and slowly spirals in the direction of Jupiter forming the faint (in back-scattered light) innermost part of the main ring and halo ring. The age of the main ring is currently unknown, but it may be the last remnant of a past population of small bodies near Jupiter. | Rings of Jupiter | Wikipedia | 489 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
Vertical corrugations
Images from the Galileo and New Horizons space probes show the presence of two sets of spiraling vertical corrugations in the main ring. These waves became more tightly wound over time at the rate expected for differential nodal regression in Jupiter's gravity field. Extrapolating backwards, the more prominent of the two sets of waves appears to have been excited in 1995, around the time of the impact of Comet Shoemaker-Levy 9 with Jupiter, while the smaller set appears to date to the first half of 1990. Galileo'''s November 1996 observations are consistent with wavelengths of and , and vertical amplitudes of and , for the larger and smaller sets of waves, respectively. The formation of the larger set of waves can be explained if the ring was impacted by a cloud of particles released by the comet with a total mass on the order of 2–5 × 1012 kg, which would have tilted the ring out of the equatorial plane by 2 km. A similar spiraling wave pattern that tightens over time has been observed by Cassini in Saturns's C and D rings.
Halo ring
Appearance and structure
The halo ring is the innermost and the vertically thickest Jovian ring. Its outer edge coincides with the inner boundary of the main ring approximately at the radius (). From this radius the ring becomes rapidly thicker towards Jupiter. The true vertical extent of the halo is not known but the presence of its material was detected as high as over the ring plane. The inner boundary of the halo is relatively sharp and located at the radius (), but some material is present further inward to approximately . Thus the width of the halo ring is about . Its shape resembles a thick torus without clear internal structure. In contrast to the main ring, the halo's appearance depends only slightly on the viewing geometry. | Rings of Jupiter | Wikipedia | 373 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
The halo ring appears brightest in forward-scattered light, in which it was extensively imaged by Galileo. While its surface brightness is much less than that of the main ring, its vertically (perpendicular to the ring plane) integrated photon flux is comparable due to its much larger thickness. Despite a claimed vertical extent of more than , the halo's brightness is strongly concentrated towards the ring plane and follows a power law of the form z−0.6 to z−1.5, where z is altitude over the ring plane. The halo's appearance in the back-scattered light, as observed by Keck and HST, is the same. However its total photon flux is several times lower than that of the main ring and is more strongly concentrated near the ring plane than in the forward-scattered light.
The spectral properties of the halo ring are different from the main ring. The flux distribution in the range 0.5–2.5 μm is flatter than in the main ring; the halo is not red and may even be blue.
Origin of the halo ring
The optical properties of the halo ring can be explained by the hypothesis that it comprises only dust with particle sizes less than 15 μm. Parts of the halo located far from the ring plane may consist of submicrometre dust. This dusty composition explains the much stronger forward-scattering, bluer colors and lack of visible structure in the halo. The dust probably originates in the main ring, a claim supported by the fact that the halo's optical depth is comparable with that of the dust in the main ring. The large thickness of the halo can be attributed to the excitation of orbital inclinations and eccentricities of dust particles by the electromagnetic forces in the Jovian magnetosphere. The outer boundary of the halo ring coincides with location of a strong 3:2 Lorentz resonance. As Poynting–Robertson drag causes particles to slowly drift towards Jupiter, their orbital inclinations are excited while passing through it. The bloom of the main ring may be a beginning of the halo. The halo ring's inner boundary is not far from the strongest 2:1 Lorentz resonance. In this resonance the excitation is probably very significant, forcing particles to plunge into the Jovian atmosphere thus defining a sharp inner boundary. Being derived from the main ring, the halo has the same age.
Gossamer rings
Amalthea gossamer ring | Rings of Jupiter | Wikipedia | 489 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
The Amalthea gossamer ring is a very faint structure with a rectangular cross section, stretching from the orbit of Amalthea at (2.54 RJ) to about (). Its inner boundary is not clearly defined because of the presence of the much brighter main ring and halo. The thickness of the ring is approximately 2300 km near the orbit of Amalthea and slightly decreases in the direction of Jupiter. The Amalthea gossamer ring is actually the brightest near its top and bottom edges and becomes gradually brighter towards Jupiter; one of the edges is often brighter than another. The outer boundary of the ring is relatively steep; the ring's brightness drops abruptly just inward of the orbit of Amalthea, although it may have a small extension beyond the orbit of the satellite ending near 4:3 resonance with Thebe. In forward-scattered light the ring appears to be about 30 times fainter than the main ring. In back-scattered light it has been detected only by the Keck telescope and the ACS (Advanced Camera for Surveys) on HST. Back-scattering images show additional structure in the ring: a peak in the brightness just inside the Amalthean orbit and confined to the top or bottom edge of the ring.
In 2002–2003 Galileo spacecraft had two passes through the gossamer rings. During them its dust counter detected dust particles in the size range 0.2–5 μm. In addition, the Galileo spacecraft's star scanner detected small, discrete bodies (< 1 km) near Amalthea. These may represent collisional debris generated from impacts with this satellite.
The detection of the Amalthea gossamer ring from the ground, in Galileo images and the direct dust measurements have allowed the determination of the particle size distribution, which appears to follow the same power law as the dust in the main ring with q=2 ± 0.5. The optical depth of this ring is about 10−7, which is an order of magnitude lower than that of the main ring, but the total mass of the dust (107–109 kg) is comparable. | Rings of Jupiter | Wikipedia | 428 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
Thebe gossamer ring
The Thebe gossamer ring is the faintest Jovian ring. It appears as a very faint structure with a rectangular cross section, stretching from the Thebean orbit at () to about (;). Its inner boundary is not clearly defined because of the presence of the much brighter main ring and halo. The thickness of the ring is approximately 8400 km near the orbit of Thebe and slightly decreases in the direction of the planet. The Thebe gossamer ring is brightest near its top and bottom edges and gradually becomes brighter towards Jupiter—much like the Amalthea ring. The outer boundary of the ring is not especially steep, stretching over . There is a barely visible continuation of the ring beyond the orbit of Thebe, extending up to () and called the Thebe Extension. In forward-scattered light the ring appears to be about 3 times fainter than the Amalthea gossamer ring. In back-scattered light it has been detected only by the Keck telescope. Back-scattering images show a peak of brightness just inside the orbit of Thebe. In 2002–2003 the dust counter of the Galileo spacecraft detected dust particles in the size range 0.2–5 μm—similar to those in the Amalthea ring—and confirmed the results obtained from imaging.
The optical depth of the Thebe gossamer ring is about 3, which is three times lower than the Amalthea gossamer ring, but the total mass of the dust is the same—about 107–109 kg. However the particle size distribution of the dust is somewhat shallower than in the Amalthea ring. It follows a power law with q < 2. In the Thebe extension the parameter q may be even smaller. | Rings of Jupiter | Wikipedia | 360 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
Origin of the gossamer rings
The dust in the gossamer rings originates in essentially the same way as that in the main ring and halo. Its sources are the inner Jovian moons Amalthea and Thebe respectively. High velocity impacts by projectiles coming from outside the Jovian system eject dust particles from their surfaces. These particles initially retain the same orbits as their moons but then gradually spiral inward by Poynting–Robertson drag. The thickness of the gossamer rings is determined by vertical excursions of the moons due to their nonzero orbital inclinations. This hypothesis naturally explains almost all observable properties of the rings: rectangular cross-section, decrease of thickness in the direction of Jupiter and brightening of the top and bottom edges of the rings.
However some properties have so far gone unexplained, like the Thebe Extension, which may be due to unseen bodies outside Thebe's orbit, and structures visible in the back-scattered light. One possible explanation of the Thebe Extension is influence of the electromagnetic forces from the Jovian magnetosphere. When the dust enters the shadow behind Jupiter, it loses its electrical charge fairly quickly. Since the small dust particles partially corotate with the planet, they will move outward during the shadow pass creating an outward extension of the Thebe gossamer ring. The same forces can explain a dip in the particle distribution and ring's brightness, which occurs between the orbits of Amalthea and Thebe.
The peak in the brightness just inside of the Amalthea's orbit and, therefore, the vertical asymmetry the Amalthea gossamer ring may be due to the dust particles trapped at the leading (L4) and trailing (L5) Lagrange points of this moon. The particles may also follow horseshoe orbits between the Lagrangian points. The dust may be present at the leading and trailing Lagrange points of Thebe as well. This discovery implies that there are two particle populations in the gossamer rings: one slowly drifts in the direction of Jupiter as described above, while another remains near a source moon trapped in 1:1 resonance with it.
Himalia ring | Rings of Jupiter | Wikipedia | 446 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
In September 2006, as NASA's New Horizons mission to Pluto approached Jupiter for a gravity assist, it photographed what appeared to be a faint, previously unknown planetary ring or ring arc, parallel with and slightly inside the orbit of the irregular satellite Himalia. The amount of material in the part of the ring or arc imaged by New Horizons was at least 0.04 km3, assuming it had the same albedo as Himalia. If the ring (arc) is debris from Himalia, it must have formed quite recently, given the century-scale precession of the Himalian orbit. It is possible that the ring could be debris from the impact of a very small undiscovered moon into Himalia, suggesting that Jupiter might continue to gain and lose small moons through collisions.
Exploration
The existence of the Jovian rings was inferred from observations of the planetary radiation belts by Pioneer 11 spacecraft in 1975. In 1979 the Voyager 1 spacecraft obtained a single overexposed image of the ring system. More extensive imaging was conducted by Voyager 2 in the same year, which allowed rough determination of the ring's structure. The superior quality of the images obtained by the Galileo orbiter between 1995 and 2003 greatly extended the existing knowledge about the Jovian rings. Ground-based observation of the rings by the Keck telescope in 1997 and 2002 and the HST in 1999 revealed the rich structure visible in back-scattered light. Images transmitted by the New Horizons spacecraft in February–March 2007 allowed observation of the fine structure in the main ring for the first time. In 2000, the Cassini'' spacecraft en route to Saturn conducted extensive observations of the Jovian ring system. Future missions to the Jovian system will provide additional information about the rings.
Gallery | Rings of Jupiter | Wikipedia | 355 | 979227 | https://en.wikipedia.org/wiki/Rings%20of%20Jupiter | Physical sciences | Solar System | Astronomy |
The rings of Uranus consists of 13 planetary rings. They are intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink. William Herschel had also reported observing rings in 1789; modern astronomers are divided on whether he could have seen them, as they are very dark and faint.
By 1977, nine distinct rings were identified. Two additional rings were discovered in 1986 in images taken by the Voyager 2 spacecraft, and two outer rings were found in 2003–2005 in Hubble Space Telescope photos. In the order of increasing distance from the planet the 13 known rings are designated 1986U2R/ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν and μ. Their radii range from about 38,000 km for the 1986U2R/ζ ring to about 98,000 km for the μ ring. Additional faint dust bands and incomplete arcs may exist between the main rings. The rings are extremely dark—the Bond albedo of the rings' particles does not exceed 2%. They are probably composed of water ice with the addition of some dark radiation-processed organics.
The majority of Uranus' rings are opaque and only a few kilometres wide. The ring system contains little dust overall; it consists mostly of large bodies 20 cm to 20 m in diameter. Some rings are optically thin: the broad and faint 1986U2R/ζ, μ and ν rings are made of small dust particles, while the narrow and faint λ ring also contains larger bodies. The relative lack of dust in the ring system may be due to aerodynamic drag from the extended Uranian exosphere.
The rings of Uranus are thought to be relatively young, and not more than 600 million years old. The Uranian ring system probably originated from the collisional fragmentation of several moons that once existed around the planet. After colliding, the moons probably broke up into many particles, which survived as narrow and optically dense rings only in strictly confined zones of maximum stability. | Rings of Uranus | Wikipedia | 443 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
The mechanism that confines the narrow rings is not well understood. Initially it was assumed that every narrow ring had a pair of nearby shepherd moons corralling it into shape. In 1986 Voyager 2 discovered only one such shepherd pair (Cordelia and Ophelia) around the brightest ring (ε), though the faint ν would later be discovered shepherded between Portia and Rosalind.
Discovery
The first mention of a Uranian ring system comes from William Herschel's notes detailing his observations of Uranus in the 18th century, which include the following passage: "February 22, 1789: A ring was suspected". Herschel drew a small diagram of the ring and noted that it was "a little inclined to the red". The Keck Telescope in Hawaii has since confirmed this to be the case, at least for the ν (nu) ring. Herschel's notes were published in a Royal Society journal in 1797. In the two centuries between 1797 and 1977 the rings are rarely mentioned, if at all. This casts serious doubt on whether Herschel could have seen anything of the sort while hundreds of other astronomers saw nothing. It has been claimed that Herschel gave accurate descriptions of the ε ring's size relative to Uranus, its changes as Uranus travelled around the Sun, and its color. | Rings of Uranus | Wikipedia | 264 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
The definitive discovery of the Uranian rings was made by astronomers James L. Elliot, Edward W. Dunham, and Jessica Mink on March 10, 1977, using the Kuiper Airborne Observatory, and was serendipitous. They planned to use the occultation of the star SAO 158687 by Uranus to study the planet's atmosphere. When their observations were analysed, they found that the star disappeared briefly from view five times both before and after it was eclipsed by the planet. They deduced that a system of narrow rings was present. The five occultation events they observed were denoted by the Greek letters α, β, γ, δ and ε in their papers. These designations have been used as the rings' names since then. Later they found four additional rings: one between the β and γ rings and three inside the α ring. The former was named the η ring. The latter were dubbed rings 4, 5 and 6—according to the numbering of the occultation events in one paper. Uranus' ring system was the second to be discovered in the Solar System, after that of Saturn. In 1982, on the fifth anniversary of the rings' discovery, Uranus along with the eight other planets recognized at the time (i.e. including Pluto) aligned on the same side of the Sun.
The rings were directly imaged when the Voyager 2 spacecraft flew through the Uranian system in 1986. Two more faint rings were revealed, bringing the total to eleven. The Hubble Space Telescope detected an additional pair of previously unseen rings in 2003–2005, bringing the total number known to 13. The discovery of these outer rings doubled the known radius of the ring system. Hubble also imaged two small satellites for the first time, one of which, Mab, shares its orbit with the outermost newly discovered μ ring.
General properties | Rings of Uranus | Wikipedia | 380 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
As currently understood, the ring system of Uranus comprises thirteen distinct rings. In order of increasing distance from the planet they are: 1986U2R/ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν, μ rings. They can be divided into three groups: nine narrow main rings (6, 5, 4, α, β, η, γ, δ, ε), two dusty rings (1986U2R/ζ, λ) and two outer rings (ν, μ). The rings of Uranus consist mainly of macroscopic particles and little dust, although dust is known to be present in 1986U2R/ζ, η, δ, λ, ν and μ rings. In addition to these well-known rings, there may be numerous optically thin dust bands and faint rings between them. These faint rings and dust bands may exist only temporarily or consist of a number of separate arcs, which are sometimes detected during occultations. Some of them became visible during a series of ring plane-crossing events in 2007. A number of dust bands between the rings were observed in forward-scattering geometry by Voyager 2. All rings of Uranus show azimuthal brightness variations.
The rings are made of an extremely dark material. The geometric albedo of the ring particles does not exceed 5–6%, while the Bond albedo is even lower—about 2%. The rings particles demonstrate a steep opposition surge—an increase of the albedo when the phase angle is close to zero. This means that their albedo is much lower when they are observed slightly off the opposition. The rings are slightly red in the ultraviolet and visible parts of the spectrum and grey in near-infrared. They exhibit no identifiable spectral features. The chemical composition of the ring particles is not known. They cannot be made of pure water ice like the rings of Saturn because they are too dark, darker than the inner moons of Uranus. This indicates that they are probably composed of a mixture of the ice and a dark material. The nature of this material is not clear, but it may be organic compounds considerably darkened by the charged particle irradiation from the Uranian magnetosphere. The rings' particles may consist of a heavily processed material which was initially similar to that of the inner moons. | Rings of Uranus | Wikipedia | 473 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
As a whole, the ring system of Uranus is unlike either the faint dusty rings of Jupiter or the broad and complex rings of Saturn, some of which are composed of very bright material—water ice. There are similarities with some parts of the latter ring system; the Saturnian F ring and the Uranian ε ring are both narrow, relatively dark and are shepherded by a pair of moons. The newly discovered outer ν and μ rings of Uranus are similar to the outer G and E rings of Saturn. Narrow ringlets existing in the broad Saturnian rings also resemble the narrow rings of Uranus. In addition, dust bands observed between the main rings of Uranus may be similar to the rings of Jupiter. In contrast, the Neptunian ring system is quite similar to that of Uranus, although it is less complex, darker and contains more dust; the Neptunian rings are also positioned further from the planet.
Narrow main rings
ε (epsilon) ring
The ε ring is the brightest and densest part of the Uranian ring system, and is responsible for about two-thirds of the light reflected by the rings. While it is the most eccentric of the Uranian rings, it has negligible orbital inclination. The ring's eccentricity causes its brightness to vary over the course of its orbit. The radially integrated brightness of the ε ring is highest near apoapsis and lowest near periapsis. The maximum/minimum brightness ratio is about 2.5–3.0. These variations are connected with the variations of the ring width, which is 19.7 km at the periapsis and 96.4 km at the apoapsis. As the ring becomes wider, the amount of shadowing between particles decreases and more of them come into view, leading to higher integrated brightness. The width variations were measured directly from Voyager 2 images, as the ε ring was one of only two rings resolved by Voyager's cameras. Such behavior indicates that the ring is not optically thin. Indeed, occultation observations conducted from the ground and the spacecraft showed that its normal optical depth varies between 0.5 and 2.5, being highest near the periapsis. The equivalent depth of the ε ring is around 47 km and is invariant around the orbit. | Rings of Uranus | Wikipedia | 462 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
The geometric thickness of the ε ring is not precisely known, although the ring is certainly very thin—by some estimates as thin as 150 m. Despite such infinitesimal thickness, it consists of several layers of particles. The ε ring is a rather crowded place with a filling factor near the apoapsis estimated by different sources at from 0.008 to 0.06. The mean size of the ring particles is 0.2–20.0 m, and the mean separation is around 4.5 times their radius. The ring is almost devoid of dust, possibly due to the aerodynamic drag from Uranus' extended atmospheric corona. Due to its razor-thin nature the ε ring is invisible when viewed edge-on. This happened in 2007 when a ring plane-crossing was observed. The temperature of the ε ring was measured by ALMA to be .
The Voyager 2 spacecraft observed a strange signal from the ε ring during the radio occultation experiment. The signal looked like a strong enhancement of the forward-scattering at the wavelength 3.6 cm near ring's apoapsis. Such strong scattering requires the existence of a coherent structure. That the ε ring does have such a fine structure has been confirmed by many occultation observations. The ε ring seems to consist of a number of narrow and optically dense ringlets, some of which may have incomplete arcs.
The ε ring is known to have interior and exterior shepherd moons—Cordelia and Ophelia, respectively. The inner edge of the ring is in 24:25 resonance with Cordelia, and the outer edge is in 14:13 resonance with Ophelia. The masses of the moons need to be at least three times the mass of the ring to confine it effectively. The mass of the ε ring is estimated to be about 1016 kg.
δ (delta) ring | Rings of Uranus | Wikipedia | 369 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
The δ ring is circular and slightly inclined. It shows significant unexplained azimuthal variations in normal optical depth and width. One possible explanation is that the ring has an azimuthal wave-like structure, excited by a small moonlet just inside it. The sharp outer edge of the δ ring is in 23:22 resonance with Cordelia. The δ ring consists of two components: a narrow optically dense component and a broad inward shoulder with low optical depth. The width of the narrow component is 4.1–6.1 km and the equivalent depth is about 2.2 km, which corresponds to a normal optical depth of about 0.3–0.6. The ring's broad component is about 10–12 km wide and its equivalent depth is close to 0.3 km, indicating a low normal optical depth of 3 × 10−2. This is known only from occultation data because Voyager 2's imaging experiment failed to resolve the δ ring. When observed in forward-scattering geometry by Voyager 2, the δ ring appeared relatively bright, which is compatible with the presence of dust in its broad component. The broad component is geometrically thicker than the narrow component. This is supported by the observations of a ring plane-crossing event in 2007, when the δ ring remained visible, which is consistent with the behavior of a simultaneously geometrically thick and optically thin ring.
γ (gamma) ring
The γ ring is narrow, optically dense and slightly eccentric. Its orbital inclination is almost zero. The width of the ring varies in the range 3.6–4.7 km, although equivalent optical depth is constant at 3.3 km. The normal optical depth of the γ ring is 0.7–0.9. During a ring plane-crossing event in 2007 the γ ring disappeared, which means it is geometrically thin like the ε ring and devoid of dust. The width and normal optical depth of the γ ring show significant azimuthal variations. The mechanism of confinement of such a narrow ring is not known, but it has been noticed that the sharp inner edge of the γ ring is in a 6:5 resonance with Ophelia. | Rings of Uranus | Wikipedia | 445 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
η (eta) ring
The η ring has zero orbital eccentricity and inclination. Like the δ ring, it consists of two components: a narrow optically dense component and a broad outward shoulder with low optical depth. The width of the narrow component is 1.9–2.7 km and the equivalent depth is about 0.42 km, which corresponds to the normal optical depth of about 0.16–0.25. The broad component is about 40 km wide and its equivalent depth is close to 0.85 km, indicating a low normal optical depth of 2 × 10−2. It was resolved in Voyager 2 images. In forward-scattered light, the η ring looked bright, which indicated the presence of a considerable amount of dust in this ring, probably in the broad component. The broad component is much thicker (geometrically) than the narrow one. This conclusion is supported by the observations of a ring plane-crossing event in 2007, when the η ring demonstrated increased brightness, becoming the second brightest feature in the ring system. This is consistent with the behavior of a geometrically thick but simultaneously optically thin ring. Like the majority of other rings, the η ring shows significant azimuthal variations in the normal optical depth and width. The narrow component even vanishes in some places.
The η ring is located close to a 3:2 Lindblad resonance with Uranian moon Cressida, which makes the ring to take the shape with three maxima and three minima in the radius, rotating with a pattering speed equal to the Cressida's orbital motion. | Rings of Uranus | Wikipedia | 322 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
α (alpha) and β (beta) rings
After the ε ring, the α and β rings are the brightest of Uranus' rings. Like the ε ring, they exhibit regular variations in brightness and width. They are brightest and widest 30° from the apoapsis and dimmest and narrowest 30° from the periapsis. The α and β rings have sizable orbital eccentricity and non-negligible inclination. The widths of these rings are 4.8–10 km and 6.1–11.4 km, respectively. The equivalent optical depths are 3.29 km and 2.14 km, resulting in normal optical depths of 0.3–0.7 and 0.2–0.35, respectively. During a ring plane-crossing event in 2007 the rings disappeared, which means they are geometrically thin like the ε ring and devoid of dust. The same event revealed a thick and optically thin dust band just outside the β ring, which was also observed earlier by Voyager 2. The masses of the α and β rings are estimated to be about 5 kg (each)—half the mass of the ε ring.
Rings 6, 5 and 4
Rings 6, 5 and 4 are the innermost and dimmest of Uranus' narrow rings. They are the most inclined rings, and their orbital eccentricities are the largest excluding the ε ring. In fact, their inclinations (0.06°, 0.05° and 0.03°) were large enough for Voyager 2 to observe their elevations above the Uranian equatorial plane, which were 24–46 km. Rings 6, 5 and 4 are also the narrowest rings of Uranus, measuring 1.6–2.2 km, 1.9–4.9 km and 2.4–4.4 km wide, respectively. Their equivalent depths are 0.41 km, 0.91 and 0.71 km resulting in normal optical depth 0.18–0.25, 0.18–0.48 and 0.16–0.3. They were not visible during a ring plane-crossing event in 2007 due to their narrowness and lack of dust.
Dusty rings
λ (lambda) ring | Rings of Uranus | Wikipedia | 452 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
The λ ring was one of two rings discovered by Voyager 2 in 1986. It is a narrow, faint ring located just inside the ε ring, between it and the shepherd moon Cordelia. This moon clears a dark lane just inside the λ ring. When viewed in back-scattered light, the λ ring is extremely narrow—about 1–2 km—and has the equivalent optical depth 0.1–0.2 km at the wavelength 2.2 μm. The normal optical depth is 0.1–0.2. The optical depth of the λ ring shows strong wavelength dependence, which is atypical for the Uranian ring system. The equivalent depth is as high as 0.36 km in the ultraviolet part of the spectrum, which explains why λ ring was initially detected only in UV stellar occultations by Voyager 2. The detection during a stellar occultation at the wavelength 2.2 μm was only announced in 1996.
The appearance of the λ ring changed dramatically when it was observed in forward-scattered light in 1986. In this geometry the ring became the brightest feature of the Uranian ring system, outshining the ε ring. This observation, together with the wavelength dependence of the optical depth, indicates that the λ ring contains significant amount of micrometre-sized dust. The normal optical depth of this dust is 10−4–10−3. Observations in 2007 by the Keck telescope during the ring plane-crossing event confirmed this conclusion, because the λ ring became one of the brightest features in the Uranian ring system.
Detailed analysis of the Voyager 2 images revealed azimuthal variations in the brightness of the λ ring. The variations appear to be periodic, resembling a standing wave. The origin of this fine structure in the λ ring remains a mystery.
1986U2R/ζ (zeta) ring | Rings of Uranus | Wikipedia | 370 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
In 1986 Voyager 2 detected a broad and faint sheet of material inward of ring 6. This ring was given the temporary designation 1986U2R. It had a normal optical depth of 10−3 or less and was extremely faint. It was thought to be visible only in a single Voyager 2 image, until reanalysis of Voyager data in 2022 revealed the ring in post-encounter images. The ring was located between 37,000 and 39,500 km from the centre of Uranus, or only about 12,000 km above the clouds. It was not observed again until 2003–2004, when the Keck telescope found a broad and faint sheet of material just inside ring 6. This ring was dubbed the ζ ring. The position of the recovered ζ ring differs significantly from that observed in 1986. Now it is situated between 37,850 and 41,350 km from the centre of the planet. There is an inward gradually fading extension reaching to at least 32,600 km, or possibly even to 27,000 km—to the atmosphere of Uranus. These extensions are labelled as the ζc and ζcc rings respectively.
The ζ ring was observed again during the ring plane-crossing event in 2007 when it became the brightest feature of the ring system, outshining all other rings combined. The equivalent optical depth of this ring is near 1 km (0.6 km for the inward extension), while the normal optical depth is again less than 10−3. Rather different appearances of the 1986U2R and ζ rings may be caused by different viewing geometries: back-scattering geometry in 2003–2007 and side-scattering geometry in 1986. Changes during the past 20 years in the distribution of dust, which is thought to predominate in the ring, cannot be ruled out. | Rings of Uranus | Wikipedia | 361 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
Other dust bands
In addition to the 1986U2R/ζ and λ rings, there are other extremely faint dust bands in the Uranian ring system. They are invisible during occultations because they have negligible optical depth, though they are bright in forward-scattered light. Voyager 2'''s images of forward-scattered light revealed the existence of bright dust bands between the λ and δ rings, between the η and β rings, and between the α ring and ring 4. Many of these bands were detected again in 2003–2004 by the Keck Telescope and during the 2007 ring-plane crossing event in backscattered light, but their precise locations and relative brightnesses were different from during the Voyager observations. The normal optical depth of the dust bands is about 10−5 or less. The dust particle size distribution is thought to obey a power law with the index p = 2.5 ± 0.5.
In addition to separate dust bands the system of Uranian rings appears to be immersed into wide and faint sheet of dust with the normal optical depth not exceeding 10−3.
μ (mu) and ν (nu) rings
In 2003–2005, the Hubble Space Telescope detected a pair of previously unknown rings, now called the outer ring system, which brought the number of known Uranian rings to 13. These rings were subsequently named the μ (mu) and ν (nu) rings. The μ ring is the outermost of the pair, and is twice the distance from the planet as the bright η ring. The outer rings differ from the inner narrow rings in a number of respects. They are broad, 17,000 and 3,800 km wide, respectively, and very faint. Their peak normal optical depths are 8.5 × 10−6 and 5.4 × 10−6, respectively. The resulting equivalent optical depths are 0.14 km and 0.012 km. The rings have triangular radial brightness profiles. | Rings of Uranus | Wikipedia | 393 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
The peak brightness of the μ (mu) ring lies almost exactly on the orbit of the small Uranian moon Mab, which is probably the source of the ring’s particles. The ν (nu) ring is positioned between Portia and Rosalind and does not contain any moons inside it. A reanalysis of the Voyager 2 images of forward-scattered light clearly reveals the μ and ν rings. In this geometry the rings are much brighter, which indicates that they contain much micrometer-sized dust. The outer rings of Uranus may be similar to the G and E rings of Saturn as E ring is extremely broad and receives dust from Enceladus.
The μ ring may consist entirely of dust, without any large particles at all. This hypothesis is supported by observations performed by the Keck telescope, which failed to detect the μ ring in the near infrared at 2.2 μm, but detected the ν ring. This failure means that the μ ring is blue in color, which in turn indicates that very small (submicrometer) dust predominates within it. The dust may be made of water ice. In contrast, the ν ring is slightly red in color.
Dynamics and origin
An outstanding problem concerning the physics governing the narrow Uranian rings is their confinement. Without some mechanism to hold their particles together, the rings would quickly spread out radially. The lifetime of the Uranian rings without such a mechanism cannot be more than 1 million years. The most widely cited model for such confinement, proposed initially by Goldreich and Tremaine, is that a pair of nearby moons, outer and inner shepherds, interact gravitationally with a ring and act like sinks and donors, respectively, for excessive and insufficient angular momentum (or equivalently, energy). The shepherds thus keep ring particles in place, but gradually move away from the ring themselves. To be effective, the masses of the shepherds should exceed the mass of the ring by at least a factor of two to three. This mechanism is known to be at work in the case of the ε ring, where Cordelia and Ophelia serve as shepherds. Cordelia is also the outer shepherd of the δ ring, and Ophelia is the outer shepherd of the γ ring. No moon larger than 10 km is known in the vicinity of other rings. The current distance of Cordelia and Ophelia from the ε ring can be used to estimate the ring’s age. The calculations show that the ε ring cannot be older than 600 million years. | Rings of Uranus | Wikipedia | 512 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
Since the rings of Uranus appear to be young, they must be continuously renewed by the collisional fragmentation of larger bodies. The estimates show that the lifetime against collisional disruption of a moon with the size like that of Puck is a few billion years. The lifetime of a smaller satellite is much shorter. Therefore, all current inner moons and rings can be products of disruption of several Puck-sized satellites during the last four and half billion years. Every such disruption would have started a collisional cascade that quickly ground almost all large bodies into much smaller particles, including dust. Eventually the majority of mass was lost, and particles survived only in positions that were stabilized by mutual resonances and shepherding. The end product of such a disruptive evolution would be a system of narrow rings. A few moonlets must still be embedded within the rings at present. The maximum size of such moonlets is probably around 10 km.
The origin of the dust bands is less problematic. The dust has a very short lifetime, 100–1000 years, and should be continuously replenished by collisions between larger ring particles, moonlets and meteoroids from outside the Uranian system. The belts of the parent moonlets and particles are themselves invisible due to their low optical depth, while the dust reveals itself in forward-scattered light. The narrow main rings and the moonlet belts that create dust bands are expected to differ in particle size distribution. The main rings have more centimeter to meter-sized bodies. Such a distribution increases the surface area of the material in the rings, leading to high optical density in back-scattered light. In contrast, the dust bands have relatively few large particles, which results in low optical depth.
Exploration
The rings were thoroughly investigated by the Voyager 2 spacecraft in January 1986. Two new faint rings—λ and 1986U2R—were discovered bringing the total number then known to eleven. Rings were studied by analyzing results of radio, ultraviolet and optical occultations. Voyager 2'' observed the rings in different geometries relative to the Sun, producing images with back-scattered, forward-scattered and side-scattered light. Analysis of these images allowed derivation of the complete phase function, geometrical and Bond albedo of ring particles. Two rings—ε and η—were resolved in the images revealing a complicated fine structure. Analysis of Voyager's images also led to discovery of eleven inner moons of Uranus, including the two shepherd moons of the ε ring—Cordelia and Ophelia. | Rings of Uranus | Wikipedia | 502 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
List of properties
This table summarizes the properties of the planetary ring system of Uranus. | Rings of Uranus | Wikipedia | 19 | 979232 | https://en.wikipedia.org/wiki/Rings%20of%20Uranus | Physical sciences | Solar System | Astronomy |
The rings of Neptune consist primarily of five principal rings. They were first discovered (as "arcs") by simultaneous observations of a stellar occultation on 22 July 1984 by André Brahic's and William B. Hubbard's teams at La Silla Observatory (ESO) and at Cerro Tololo Interamerican Observatory in Chile. They were eventually imaged in 1989 by the Voyager 2 spacecraft. At their densest, they are comparable to the less dense portions of Saturn's main rings such as the C ring and the Cassini Division, but much of Neptune's ring system is quite faint and dusty, in some aspects more closely resembling the rings of Jupiter. Neptune's rings are named after astronomers who contributed important work on the planet: Galle, Le Verrier, Lassell, Arago, and Adams. Neptune also has a faint unnamed ring coincident with the orbit of the moon Galatea. Three other moons orbit between the rings: Naiad, Thalassa and Despina.
The rings of Neptune are made of extremely dark material, likely organic compounds processed by radiation, similar to those found in the rings of Uranus. The proportion of dust in the rings (between 20% and 70%) is high, while their optical depth is low to moderate, at less than 0.1. Uniquely, the Adams ring includes five distinct arcs, named Fraternité, Égalité 1 and 2, Liberté, and Courage. The arcs occupy a narrow range of orbital longitudes and are remarkably stable, having changed only slightly since their initial detection in 1980. How the arcs are stabilized is still under debate. However, their stability is probably related to the resonant interaction between the Adams ring and its inner shepherd moon, Galatea.
Discovery and observations | Rings of Neptune | Wikipedia | 363 | 979237 | https://en.wikipedia.org/wiki/Rings%20of%20Neptune | Physical sciences | Solar System | Astronomy |
The first mention of rings around Neptune dates back to 1846 when William Lassell, the discoverer of Neptune's largest moon, Triton, thought he had seen a ring around the planet. However, his claim was never confirmed and it is likely that it was an observational artifact. The first reliable detection of a ring was made in 1968 by stellar occultation, although that result would go unnoticed until 1977 when the rings of Uranus were discovered. Soon after the Uranus discovery, a team from Villanova University led by Harold J. Reitsema began searching for rings around Neptune. On 24 May 1981, they detected a dip in a star's brightness during one occultation; however, the manner in which the star dimmed did not suggest a ring. Later, after the Voyager fly-by, it was found that the occultation was due to the small Neptunian moon Larissa, a highly unusual event.
In the 1980s, significant occultations were much rarer for Neptune than for Uranus, which lay near the Milky Way at the time and was thus moving against a denser field of stars. Neptune's next occultation, on 12 September 1983, resulted in a possible detection of a ring. However, ground-based results were inconclusive. Over the next six years, approximately 50 other occultations were observed with only about one-third of them yielding positive results. Something (probably incomplete arcs) definitely existed around Neptune, but the features of the ring system remained a mystery. The Voyager 2 spacecraft made the definitive discovery of the Neptunian rings during its fly-by of Neptune in 1989, passing by as close as above the planet's atmosphere on 25 August. It confirmed that occasional occultation events observed before were indeed caused by the arcs within the Adams ring (see below). After the Voyager fly-by the previous terrestrial occultation observations were reanalyzed yielding features of the ring's arcs as they were in 1980s, which matched those found by Voyager 2 almost perfectly. | Rings of Neptune | Wikipedia | 412 | 979237 | https://en.wikipedia.org/wiki/Rings%20of%20Neptune | Physical sciences | Solar System | Astronomy |
Since Voyager 2s fly-by, the brightest rings (Adams and Le Verrier) have been imaged with the Hubble Space Telescope and Earth-based telescopes, owing to advances in resolution and light-gathering power. They are visible, slightly above background noise levels, at methane-absorbed wavelengths in which the glare from Neptune is significantly reduced. The fainter rings are still far below the visibility threshold for these instruments. In 2022 the rings were imaged by the James Webb Space Telescope, which made the first observation of the fainter rings since the Voyager 2s fly-by.
General properties
Neptune possesses five distinct rings named, in order of increasing distance from the planet, Galle, Le Verrier, Lassell, Arago and Adams. In addition to these well-defined rings, Neptune may also possess an extremely faint sheet of material stretching inward from the Le Verrier to the Galle ring, and possibly farther in toward the planet. Three of the Neptunian rings are narrow, with widths of about 100 km or less; in contrast, the Galle and Lassell rings are broad—their widths are between 2,000 and 5,000 km. The Adams ring consists of five bright arcs embedded in a fainter continuous ring. Proceeding counterclockwise, the arcs are: Fraternité, Égalité 1 and 2, Liberté, and Courage. The first four names come from "liberty, equality, fraternity", the motto of the French Revolution and Republic. The terminology was suggested by their original discoverers, who had found them during stellar occultations in 1984 and 1985. Four small Neptunian moons have orbits inside the ring system: Naiad and Thalassa orbit in the gap between the Galle and Le Verrier rings; Despina is just inward of the Le Verrier ring; and Galatea lies slightly inward of the Adams ring, embedded in an unnamed faint, narrow ringlet. | Rings of Neptune | Wikipedia | 397 | 979237 | https://en.wikipedia.org/wiki/Rings%20of%20Neptune | Physical sciences | Solar System | Astronomy |
The Neptunian rings contain a large quantity of micrometer-sized dust: the dust fraction by cross-section area is between 20% and 70%. In this respect they are similar to the rings of Jupiter, in which the dust fraction is 50%–100%, and are very different from the rings of Saturn and Uranus, which contain little dust (less than 0.1%). The particles in Neptune's rings are made from a dark material; probably a mixture of ice with radiation-processed organics. The rings are reddish in color, and their geometrical (0.05) and Bond (0.01–0.02) albedos are similar to those of the Uranian rings' particles and the inner Neptunian moons. The rings are generally optically thin (transparent); their normal optical depths do not exceed 0.1. As a whole, the Neptunian rings resemble those of Jupiter; both systems consist of faint, narrow, dusty ringlets and even fainter broad dusty rings.
The rings of Neptune, like those of Uranus, are thought to be relatively young; their age is probably significantly less than that of the Solar System. Also, like those of Uranus, Neptune's rings probably resulted from the collisional fragmentation of onetime inner moons. Such events create moonlet belts, which act as the sources of dust for the rings. In this respect the rings of Neptune are similar to faint dusty bands observed by Voyager 2 between the main rings of Uranus.
Inner rings
Galle ring
The innermost ring of Neptune is called the Galle ring after Johann Gottfried Galle, the first person to see Neptune through a telescope (1846). It is about 2,000 km wide and orbits 41,000–43,000 km from the planet. It is a faint ring with an average normal optical depth of around 10−4, and with an equivalent depth of 0.15 km. The fraction of dust in this ring is estimated from 40% to 70%. | Rings of Neptune | Wikipedia | 416 | 979237 | https://en.wikipedia.org/wiki/Rings%20of%20Neptune | Physical sciences | Solar System | Astronomy |
Le Verrier ring
The next ring is named the Le Verrier ring after Urbain Le Verrier, who predicted Neptune's position in 1846. With an orbital radius of about 53,200 km, it is narrow, with a width of about 113 km. Its normal optical depth is 0.0062 ± 0.0015, which corresponds to an equivalent depth of 0.7 ± 0.2 km. The dust fraction in the Le Verrier ring ranges from 40% to 70%. The small moon Despina, which orbits just inside of it at 52,526 km, may play a role in the ring's confinement by acting as a shepherd.
Lassell ring
The Lassell ring, also known as the plateau, is the broadest ring in the Neptunian system. Its namesake is William Lassell, the English astronomer who discovered Neptune's largest moon, Triton. This ring is a faint sheet of material occupying the space between the Le Verrier ring at about 53,200 km and the Arago ring at 57,200 km. Its average normal optical depth is around 10−4, which corresponds to an equivalent depth of 0.4 km. The ring's dust fraction is in the range from 20% to 40%.
Potential ring
There is a small peak of brightness near the outer edge of the Lassell ring, located at 57,200 km from Neptune and less than 100 km wide, which some planetary scientists call the Arago ring after François Arago, a French mathematician, physicist, astronomer and politician. However, many publications do not mention the Arago ring at all.
Adams ring | Rings of Neptune | Wikipedia | 333 | 979237 | https://en.wikipedia.org/wiki/Rings%20of%20Neptune | Physical sciences | Solar System | Astronomy |
The outer Adams ring, with an orbital radius of about 63,930 km, is the best studied of Neptune's rings. It is named after John Couch Adams, who predicted the position of Neptune independently of Le Verrier. This ring is narrow, slightly eccentric and inclined, with total width of about 35 km (15–50 km), and its normal optical depth is around 0.011 ± 0.003 outside the arcs, which corresponds to the equivalent depth of about 0.4 km. The fraction of dust in this ring is from 20% to 40%—lower than in other narrow rings. Neptune's small moon Galatea, which orbits just inside of the Adams ring at 61,953 km, acts like a shepherd, keeping ring particles inside a narrow range of orbital radii through a 42:43 outer Lindblad resonance. Galatea's gravitational influence creates 42 radial wiggles in the Adams ring with an amplitude of about 30 km, which have been used to infer Galatea's mass.
Arcs
The brightest parts of the Adams ring, the ring arcs, were the first elements of Neptune's ring system to be discovered. The arcs are discrete regions within the ring in which the particles that it comprises are mysteriously clustered together. The Adams ring is known to comprise five short arcs, which occupy a relatively narrow range of longitudes from 247° to 294°. In 1986 they were located between longitudes of:
247–257° (Fraternité),
261–264° (Égalité 1),
265–266° (Égalité 2),
276–280° (Liberté),
284.5–285.5° (Courage).
The brightest and longest arc was Fraternité; the faintest was Courage. The normal optical depths of the arcs are estimated to lie in the range 0.03–0.09 (0.034 ± 0.005 for the leading edge of Liberté arc as measured by stellar occultation); the radial widths are approximately the same as those of the continuous ring—about 30 km. The equivalent depths of arcs vary in the range 1.25–2.15 km (0.77 ± 0.13 km for the leading edge of Liberté arc). The fraction of dust in the arcs is from 40% to 70%. The arcs in the Adams ring are somewhat similar to the arc in Saturn's G ring. | Rings of Neptune | Wikipedia | 504 | 979237 | https://en.wikipedia.org/wiki/Rings%20of%20Neptune | Physical sciences | Solar System | Astronomy |
The highest resolution Voyager 2 images revealed a pronounced clumpiness in the arcs, with a typical separation between visible clumps of 0.1° to 0.2°, which corresponds to 100–200 km along the ring. Because the clumps were not resolved, they may or may not include larger bodies, but are certainly associated with concentrations of microscopic dust as evidenced by their enhanced brightness when backlit by the Sun.
The arcs are quite stable structures. They were detected by ground-based stellar occultations in the 1980s, by Voyager 2 in 1989 and by Hubble Space Telescope and ground-based telescopes in 1997–2005 and remained at approximately the same orbital longitudes. However some changes have been noticed. The overall brightness of arcs decreased since 1986. The Courage arc jumped forward by 8° to 294° (it probably jumped over to the next stable co-rotation resonance position) while the Liberté arc had almost disappeared by 2003. The Fraternité and Égalité (1 and 2) arcs have demonstrated irregular variations in their relative brightness. Their observed dynamics is probably related to the exchange of dust between them. Courage, a very faint arc found during the Voyager flyby, was seen to flare in brightness in 1998; it was back to its usual dimness by June 2005. Visible light observations show that the total amount of material in the arcs has remained approximately constant, but they are dimmer in the infrared light wavelengths where previous observations were taken.
Confinement
The arcs in the Adams ring remain unexplained. Their existence is a puzzle because basic orbital dynamics imply that they should spread out into a uniform ring over a matter of years. Several hypotheses about the arcs' confinement have been suggested, the most widely publicized of which holds that Galatea confines the arcs via its 42:43 co-rotational inclination resonance (CIR). The resonance creates 84 stable sites along the ring's orbit, each 4° long, with arcs residing in the adjacent sites. However measurements of the rings' mean motion with Hubble and Keck telescopes in 1998 led to the conclusion that the rings are not in CIR with Galatea. | Rings of Neptune | Wikipedia | 437 | 979237 | https://en.wikipedia.org/wiki/Rings%20of%20Neptune | Physical sciences | Solar System | Astronomy |
A later model suggested that confinement resulted from a co-rotational eccentricity resonance (CER). The model takes into account the finite mass of the Adams ring, which is necessary to move the resonance closer to the ring. A byproduct of this hypothesis is a mass estimate for the Adams ring—about 0.002 of the mass of Galatea. A third hypothesis proposed in 1986 requires an additional moon orbiting inside the ring; the arcs in this case are trapped in its stable Lagrangian points. However Voyager 2'''s observations placed strict constraints on the size and mass of any undiscovered moons, making such a hypothesis unlikely. Some other more complicated hypotheses hold that a number of moonlets are trapped in co-rotational resonances with Galatea, providing confinement of the arcs and simultaneously serving as sources of the dust.
Exploration
The rings were investigated in detail during the Voyager 2 spacecraft's flyby of Neptune in August 1989. They were studied with optical imaging, and through observations of occultations in ultraviolet and visible light. The spaceprobe observed the rings in different geometries relative to the Sun, producing images of back-scattered, forward-scattered and side-scattered light. Analysis of these images allowed derivation of the phase function (dependence of the ring's reflectivity on the angle between the observer and Sun), and geometrical and Bond albedo of ring particles. Analysis of Voyager's images also led to discovery of six inner moons of Neptune, including the Adams ring shepherd Galatea.
Properties *A question mark means that the parameter is not known.'' | Rings of Neptune | Wikipedia | 330 | 979237 | https://en.wikipedia.org/wiki/Rings%20of%20Neptune | Physical sciences | Solar System | Astronomy |
In astrodynamics, orbital station-keeping is keeping a spacecraft at a fixed distance from another spacecraft or celestial body. It requires a series of orbital maneuvers made with thruster burns to keep the active craft in the same orbit as its target. For many low Earth orbit satellites, the effects of non-Keplerian forces, i.e. the deviations of the gravitational force of the Earth from that of a homogeneous sphere, gravitational forces from Sun/Moon, solar radiation pressure and air drag, must be counteracted.
For spacecraft in a halo orbit around a Lagrange point, station-keeping is even more fundamental, as such an orbit is unstable; without an active control with thruster burns, the smallest deviation in position or velocity would result in the spacecraft leaving orbit completely.
Perturbations
The deviation of Earth's gravity field from that of a homogeneous sphere and gravitational forces from the Sun and Moon will in general perturb the orbital plane. For a Sun-synchronous orbit, the precession of the orbital plane caused by the oblateness of the Earth is a desirable feature that is part of mission design but the inclination change caused by the gravitational forces of the Sun and Moon is undesirable. For geostationary spacecraft, the inclination change caused by the gravitational forces of the Sun and Moon must be counteracted by a rather large expense of fuel, as the inclination should be kept sufficiently small for the spacecraft to be tracked by non-steerable antennae.
For spacecraft in a low orbit, the effects of atmospheric drag must often be compensated for, often to avoid re-entry; for missions requiring the orbit to be accurately synchronized with the Earth’s rotation, this is necessary to prevent a shortening of the orbital period.
Solar radiation pressure will in general perturb the eccentricity (i.e. the eccentricity vector); see Orbital perturbation analysis (spacecraft). For some missions, this must be actively counter-acted with maneuvers. For geostationary spacecraft, the eccentricity must be kept sufficiently small for a spacecraft to be tracked with a non-steerable antenna. Also for Earth observation spacecraft for which a very repetitive orbit with a fixed ground track is desirable, the eccentricity vector should be kept as fixed as possible. A large part of this compensation can be done by using a frozen orbit design, but often thrusters are needed for fine control maneuvers.
Low Earth orbit | Orbital station-keeping | Wikipedia | 496 | 979306 | https://en.wikipedia.org/wiki/Orbital%20station-keeping | Physical sciences | Orbital mechanics | Astronomy |
For spacecraft in a very low orbit, the atmospheric drag is sufficiently strong to cause a re-entry before the intended end of mission if orbit raising maneuvers are not executed from time to time.
An example of this is the International Space Station (ISS), which has an operational altitude above Earth's surface of between 400 and 430 km (250-270 mi). Due to atmospheric drag the space station is constantly losing orbital energy. In order to compensate for this loss, which would eventually lead to a re-entry of the station, it has to be reboosted to a higher orbit from time to time. The chosen orbital altitude is a trade-off between the average thrust needed to counter-act the air drag and the impulse needed to send payloads and people to the station.
GOCE which orbited at 255 km (later reduced to 235 km) used ion thrusters to provide up to 20 mN of thrust to compensate for the drag on its frontal area of about 1 m2.
Earth observation spacecraft
For Earth observation spacecraft typically operated in an altitude above the Earth surface of about 700 – 800 km the air-drag is very faint and a re-entry due to air-drag is not a concern. But if the orbital period should be synchronous with the Earth's rotation to maintain a fixed ground track, the faint air-drag at this high altitude must also be counter-acted by orbit raising maneuvers in the form of thruster burns tangential to the orbit. These maneuvers will be very small, typically in the order of a few mm/s of delta-v. If a frozen orbit design is used these very small orbit raising maneuvers are sufficient to also control the eccentricity vector.
To maintain a fixed ground track it is also necessary to make out-of-plane maneuvers to compensate for the inclination change caused by Sun/Moon gravitation. These are executed as thruster burns orthogonal to the orbital plane. For Sun-synchronous spacecraft having a constant geometry relative to the Sun, the inclination change due to the solar gravitation is particularly large; a delta-v in the order of 1–2 m/s per year can be needed to keep the inclination constant.
Geostationary orbit | Orbital station-keeping | Wikipedia | 452 | 979306 | https://en.wikipedia.org/wiki/Orbital%20station-keeping | Physical sciences | Orbital mechanics | Astronomy |
For geostationary spacecraft, thruster burns orthogonal to the orbital plane must be executed to compensate for the effect of the lunar/solar gravitation that perturbs the orbit pole with typically 0.85 degrees per year. The delta-v needed to compensate for this perturbation keeping the inclination to the equatorial plane amounts to in the order 45 m/s per year. This part of the GEO station-keeping is called North-South control.
The East-West control is the control of the orbital period and the eccentricity vector performed by making thruster burns tangential to the orbit. These burns are then designed to keep the orbital period perfectly synchronous with the Earth rotation and to keep the eccentricity sufficiently small. Perturbation of the orbital period results from the imperfect rotational symmetry of the Earth relative the North/South axis, sometimes called the ellipticity of the Earth equator. The eccentricity (i.e. the eccentricity vector) is perturbed by the solar radiation pressure. The fuel needed for this East-West control is much less than what is needed for the North-South control.
To extend the life-time of geostationary spacecraft with little fuel left one sometimes discontinues the North-South control only continuing with the East-West control. As seen from an observer on the rotating Earth the spacecraft will then move North-South with a period of 24 hours. When this North-South movement gets too large a steerable antenna is needed to track the spacecraft. An example of this is Artemis.
To save weight, it is crucial for GEO satellites to have the most fuel-efficient propulsion system. Almost all modern satellites are therefore employing a high specific impulse system like plasma or ion thrusters.
Lagrange points
Orbits of spacecraft are also possible around Lagrange points—also referred to as libration points—five equilibrium points that exist in relation to two larger solar system bodies. For example, there are five of these points in the Sun-Earth system, five in the Earth-Moon system, and so on. Spacecraft may orbit around these points with a minimum of propellant required for station-keeping purposes. Two orbits that have been used for such purposes include halo and Lissajous orbits. | Orbital station-keeping | Wikipedia | 456 | 979306 | https://en.wikipedia.org/wiki/Orbital%20station-keeping | Physical sciences | Orbital mechanics | Astronomy |
One important Lagrange point is Earth-Sun , and three heliophysics missions have been orbiting L1 since approximately 2000. Station-keeping propellant use can be quite low, facilitating missions that can potentially last decades should other spacecraft systems remain operational. The three spacecraft—Advanced Composition Explorer (ACE), Solar Heliospheric Observatory (SOHO), and the Global Geoscience WIND satellite—each have annual station-keeping propellant requirements of approximately 1 m/s or less.
Earth-Sun —approximately 1.5 million kilometers from Earth in the anti-sun direction—is another important Lagrange point, and the ESA Herschel space observatory operated there in a Lissajous orbit during 2009–2013, at which time it ran out of coolant for the space telescope. Small station-keeping orbital maneuvers were executed approximately monthly to maintain the spacecraft in the station-keeping orbit.
The James Webb Space Telescope will use propellant to maintain its halo orbit around the Earth-Sun L2, which provides an upper limit to its designed lifetime: it is being designed to carry enough for ten years. However, the precision of trajectory following launch by an Ariane 5 is credited with potentially doubling the lifetime of the telescope by leaving more hydrazine propellant on-board than expected.
The CAPSTONE orbiter and the planned Lunar Gateway is stationed along a 9:2 synodically resonant Near Rectilinear Halo Orbit (NRHO) around the Earth-Moon L2 Lagrange point. | Orbital station-keeping | Wikipedia | 306 | 979306 | https://en.wikipedia.org/wiki/Orbital%20station-keeping | Physical sciences | Orbital mechanics | Astronomy |
The Sculptor Galaxy (also known as the Silver Coin Galaxy, Silver Dollar Galaxy, NGC 253, or Caldwell 65) is an intermediate spiral galaxy in the constellation Sculptor. The Sculptor Galaxy is a starburst galaxy, which means that it is currently undergoing a period of intense star formation.
Observation
Observational history
The galaxy was discovered by Caroline Herschel in 1783 during one of her systematic comet searches. About half a century later, John Herschel observed it using his 18-inch metallic mirror reflector at the Cape of Good Hope. He wrote: "very bright and large (24′ in length); a superb object.... Its light is somewhat streaky, but I see no stars in it except 4 large and one very small one, and these seem not to belong to it, there being many near..."
In 1961, Allan Sandage wrote in the Hubble Atlas of Galaxies that the Sculptor Galaxy is "the prototype example of a special subgroup of Sc systems....photographic images of galaxies of the group are dominated by the dust pattern. Dust lanes and patches of great complexity are scattered throughout the surface. Spiral arms are often difficult to trace.... The arms are defined as much by the dust as by the spiral pattern." Bernard Y. Mills, working out of Sydney, discovered that the Sculptor Galaxy is also a fairly strong radio source.
In 1998, the Hubble Space Telescope took a detailed image of NGC 253.
Amateur
As one of the brightest galaxies in the sky, the Sculptor Galaxy can be seen through binoculars and is near the star Beta Ceti. It is considered one of the most easily viewed galaxies in the sky after the Andromeda Galaxy.
The Sculptor Galaxy is a good target for observation with a telescope with a 300 mm diameter or larger. In such telescopes, it appears as a galaxy with a long, oval bulge and a mottled galactic disc. Although the bulge appears only slightly brighter than the rest of the galaxy, it is fairly extended compared to the disk. In 400 mm scopes and larger, a dark dust lane northwest of the nucleus is visible, and over a dozen faint stars can be seen superimposed on the bulge. Some people claim to have observed the galaxy with the unaided eye under exceptional viewing conditions.
Features | Sculptor Galaxy | Wikipedia | 463 | 979452 | https://en.wikipedia.org/wiki/Sculptor%20Galaxy | Physical sciences | Notable galaxies | Astronomy |
The Sculptor Galaxy is located at the center of the Sculptor Group, one of the nearest groups of galaxies to the Milky Way. The Sculptor Galaxy (the brightest galaxy in the group and one of the intrinsically brightest galaxies in the vicinity of ours, only surpassed by the Andromeda Galaxy and the Sombrero Galaxy) and the companion galaxies NGC 247, PGC 2881, PGC 2933, Sculptor-dE1, and UGCA 15 form a gravitationally-bound core near the center of the group. Most other galaxies associated with the Sculptor Group are only weakly gravitationally bound to this core.
Starburst
NGC 253's starburst has created several super star clusters on NGC 253's center (discovered with the aid of the Hubble Space Telescope): one with a mass of solar masses, and absolute magnitude of at least −15, and two others with solar masses and absolute magnitudes around −11; later studies have discovered an even more massive cluster heavily obscured by NGC 253's interstellar dust with a mass of solar masses, an age of around years, and rich in Wolf-Rayet stars. The super star clusters are arranged in an ellipse around the center of NGC 253, which from the Earth's perspective appears as a flat line.
Star formation is also high in the northeast of NGC 253's disk, where a number of red supergiant stars can be found, and in its halo there are young stars as well as some amounts of neutral hydrogen. This, along with other peculiarities found in NGC 253, suggest that a gas-rich dwarf galaxy collided with it 200 million years ago, disturbing its disk and starting the present starburst.
As happens in other galaxies suffering strong star formation such as Messier 82, NGC 4631, or NGC 4666, the stellar winds of the massive stars produced in the starburst as well as their deaths as supernovae have blown out material to NGC 253's halo in the form of a superwind that seems to be inhibiting star formation in the galaxy.
Novae and Supernovae
Although supernovae are generally associated with starburst galaxies, only one has been detected within the Sculptor Galaxy. SN 1940E (type unknown, mag. 14) was discovered by Fritz Zwicky on 22 November 1940, located approximately 54″ southwest of the galaxy's nucleus. | Sculptor Galaxy | Wikipedia | 485 | 979452 | https://en.wikipedia.org/wiki/Sculptor%20Galaxy | Physical sciences | Notable galaxies | Astronomy |
NGC 253 is close enough that classical novae can also be detected. The first confirmed nova in this galaxy was discovered by BlackGEM at magnitude 19.6 on 12 July 2024, and designated AT 2024pid.
Central black hole
Research suggests the presence of a supermassive black hole in the center of this galaxy with a mass estimated to be 5 million times that of the Sun, which is slightly heavier than Sagittarius A*.
Distance estimates
At least two techniques have been used to measure distances to Sculptor in the past ten years.
Using the planetary nebula luminosity function method, an estimate of 10.89 million light years (or Mly; 3.34 Megaparsecs, or Mpc) was achieved in 2005.
The Sculptor Galaxy is close enough that the tip of the red-giant branch (TRGB) method may also be used to estimate its distance. The estimated distance to Sculptor using this technique in 2004 yielded ().
A weighted average of the most reliable distance estimates gives a distance of ().
Satellite
An international team of researchers has used the Subaru Telescope to identify a faint dwarf galaxy disrupted by NGC 253. The satellite galaxy is called NGC 253-dw2 and may not survive its next passage by its much larger host. The host galaxy may suffer some damage too if the dwarf is massive enough. The interplay between the two galaxies is responsible for the disturbance in NGC 253's structure. | Sculptor Galaxy | Wikipedia | 292 | 979452 | https://en.wikipedia.org/wiki/Sculptor%20Galaxy | Physical sciences | Notable galaxies | Astronomy |
The Bosphorus Bridge (), known officially as the 15 July Martyrs Bridge () and colloquially as the First Bridge (), is the oldest and southernmost of the three suspension bridges spanning the Bosphorus strait (Turkish: Boğaziçi) in Istanbul, Turkey, thus connecting Europe and Asia (alongside the Fatih Sultan Mehmet Bridge and Yavuz Sultan Selim Bridge). The bridge extends between Ortaköy (in Europe) and Beylerbeyi (in Asia).
It is a gravity-anchored suspension bridge with steel towers and inclined hangers. The aerodynamic deck hangs on steel cables. It is long with a deck width of . The distance between the towers (main span) is and the total height of the towers is . The clearance of the bridge from sea level is .
Upon its completion in 1973, the Bosphorus Bridge had the fourth-longest suspension bridge span in the world, and the longest outside the United States (only the Verrazano-Narrows Bridge, Golden Gate Bridge and Mackinac Bridge had a longer span in 1973). The Bosphorus Bridge remained the longest suspension bridge in Europe until the completion of the Humber Bridge in 1981, and the longest suspension bridge in Asia until the completion of the Fatih Sultan Mehmet Bridge (Second Bosphorus Bridge) in 1988 (which was surpassed by the Minami Bisan-Seto Bridge in 1989). Currently, the Bosphorus Bridge has the 40th-longest suspension bridge span in the world.
After a group of soldiers took control and partially closed off the bridge during the military coup d'état attempt on 15 July 2016, Prime Minister Binali Yıldırım proclaimed on 25 July 2016 the decision of the Cabinet of Turkey that the bridge will be formally renamed as the 15 Temmuz Şehitler Köprüsü (July 15th Martyrs Bridge) in memory of those killed while resisting the attempted coup.
The Bosphorus Bridge is famous for its important transport routes, connecting parts of Europe to Turkey. | Bosphorus Bridge | Wikipedia | 421 | 4096049 | https://en.wikipedia.org/wiki/Bosphorus%20Bridge | Technology | Bridges | null |
Precedents and proposals
The idea of a bridge crossing the Bosphorus dates back to antiquity. The Greek writer Herodotus says in his Histories that, on the orders of Emperor Darius the Great of the Achaemenid Empire (522 BC–485 BC), Mandrocles of Samos once engineered a pontoon bridge across the Bosphorus, linking Asia to Europe; this bridge enabled Darius to pursue the fleeing Scythians as well as position his army in the Balkans to overwhelm Macedon. The first modern project for a permanent bridge across the Bosphorus was proposed to Sultan Abdul Hamid II of the Ottoman Empire by the Bosphorus Railroad Company in 1900, which included a rail link between the continents.
Construction
The decision to build a bridge across the Bosphorus was taken in 1957 by Prime Minister Adnan Menderes. For the structural engineering work, a contract was signed with the British firm Freeman Fox & Partners in 1968. The bridge was designed by the British civil engineers Gilbert Roberts, William Brown and Michael Parsons, who also designed the Humber Bridge, Severn Bridge, and Forth Road Bridge. David B Steinman, an American engineer who had recently designed the Mackinac Bridge was also contracted, but died early on in the design process in 1960. Construction started in February 1970 and ceremonies were attended by President Cevdet Sunay and Prime Minister Süleyman Demirel. The bridge was built by the Turkish firm Enka Construction & Industry Co. along with the co-contractors Cleveland Bridge & Engineering Company (England) and Hochtief AG (Germany).
The bridge was completed on 30 October 1973, one day after the 50th anniversary of the founding of the Republic of Turkey, and opened by President Fahri Korutürk and Prime Minister Naim Talu. The cost of the bridge was US$200 million ($ in dollars).
Upon the bridge's opening, it was often defined by the media as the first bridge between Asia and Europe since the pontoon bridge of Xerxes in 480 BC. That bridge, however, spanned the Hellespont (Dardanelles) strait to the southwest of the Bosphorus, across the Sea of Marmara, and was in fact the second pontoon bridge between Asia and Europe after an earlier one built by Darius the Great across the Bosphorus strait in 513 BC.
Operation and tolls | Bosphorus Bridge | Wikipedia | 496 | 4096049 | https://en.wikipedia.org/wiki/Bosphorus%20Bridge | Technology | Bridges | null |
The bridge highway is eight lanes wide. Three standard lanes, one emergency lane and one pedestrian lane serve each direction. On weekday mornings, most commuter traffic flows westbound to Europe, so four of the six lanes run westbound and only two eastbound. Conversely, on weekday evenings, four lanes are dedicated to eastbound traffic and two lanes, to westbound traffic.
For the first three years, pedestrians could walk over the bridge, reaching it with elevators inside the towers on both sides. No pedestrians or commercial vehicles, such as trucks, are allowed to use the bridge today.
Today, around 180,000 vehicles pass daily in both directions, with almost 85% being cars. On 29 December 1997, the one-billionth vehicle passed the bridge. Fully loaded, the bridge sags about in the middle of the span.
It is a toll bridge. A toll is charged for passing from Europe to Asia, but not for passing in the reverse direction.
Between 1999 and 2006, some of the toll booths (#9 - #13), which were located to the far left as motorists approached them, were unmanned and equipped only with a remote payment system (Turkish: OGS). In addition to the OGS system, another toll pay system with special contactless smart cards (Turkish: KGS) was installed at specific toll booths in 2005. Toll payments in cash were stopped on 3 April 2006.
Between 2006 and 2012, toll booths accepted only OGS or KGS. An OGS device or KGS card could be obtained at various stations before reaching the toll plazas of highways and bridges. In 2006, the toll was 3.00 TL or about $2.00.
Since April 2007, a computerised LED lighting system of changing colours and patterns, developed by Philips, illuminates the bridge at night.
On 17 September 2012, the KGS system on the Bosphorus Bridge was replaced by the new HGS system (Turkish: Hızlı Geçiş Sistemi), which also replaced the OGS system a decade later, on 31 March 2022. The HGS system requires a batteryless front window sticker with a passive radio-frequency identification (RFID) chip, whereas the older OGS system required a small RFID device with a battery that was sticked to the front window. | Bosphorus Bridge | Wikipedia | 471 | 4096049 | https://en.wikipedia.org/wiki/Bosphorus%20Bridge | Technology | Bridges | null |
In 2017, the toll increased by nearly 50% from 4.75 to 7 TRY. After 21 months, in late 2019, the toll went up another 20% to 10.50 TRY. Tolls need to be increased almost every year to keep up with high producers' price inflation.
Notable events
The bridge was depicted on the reverse of the Turkish 1000 lira banknotes of 1978–1986.
Since 1979, every October, the annual Intercontinental Istanbul Eurasia Marathon crosses the bridge on its way from Asia to Europe. During the marathon, the bridge is closed to vehicular traffic.
On 15 May 2005 at 07:00 local time, U.S. tennis star Venus Williams played a show game with Turkish player İpek Şenoğlu on the bridge, the first tennis match played on two continents. The event promoted the upcoming 2005 WTA İstanbul Cup and lasted five minutes. After the exhibition, they both threw a tennis ball into the Bosphorus.
On 17 July 2005 at 10:30 local time, British Formula One driver David Coulthard drove his Red Bull racing car across the bridge from the European side to the Asian side, then, after turning with a powerslide at the toll plaza, back to the European side for show. He parked his car in the garden of Dolmabahçe Palace where his ride had started. While crossing the bridge with his Formula 1 car, Coulthard was picked up by the automatic surveillance system and charged with a fine of 20 Euros because he passed through the toll booths without payment. His team agreed to pay for him.
On 5 November 2013, World No. 1 golfer Tiger Woods, visiting for the 2013 Turkish Airlines Open golf tournament held between 7 and 10 November, was brought to the bridge by helicopter and made a couple of show shots on the bridge, hitting balls from the Asian side to the European side on one side of the bridge, which was closed to traffic for about one hour. | Bosphorus Bridge | Wikipedia | 391 | 4096049 | https://en.wikipedia.org/wiki/Bosphorus%20Bridge | Technology | Bridges | null |
On 15 July 2016, the bridge was blocked by a rogue faction of the Turkish Armed Forces during a coup attempt. They arrested civilians and police officers. The soldiers involved surrendered to police and to civilians the next day. On 25 July 2016, Binali Yıldırım, Turkey's last prime minister before a presidential system was adopted with a referendum in 2017, announced that the bridge would be renamed as the 15 Temmuz Şehitler Köprüsü (15 July Martyrs Bridge). In honor of the victims who were martyred while resisting the coup attempt, a monument, museum and mosque were built on a roadside hill near the Asian (Anatolian) end of the bridge. | Bosphorus Bridge | Wikipedia | 142 | 4096049 | https://en.wikipedia.org/wiki/Bosphorus%20Bridge | Technology | Bridges | null |
A rocker box (also known as a cradle or a big box) is a gold mining implement for separating alluvial placer gold from sand and gravel which was used in placer mining in the 19th century. It consists of a high-sided box, which is open on one end and on top, and was placed on rockers.
The inside bottom of the box is lined with riffles and usually a carpet (called Miner's Moss) similar to a sluice box. On top of the box is a classifier sieve (usually with half-inch or quarter-inch openings) which screens-out larger pieces of rock and other material, allowing only finer sand and gravel through. Between the sieve and the lower sluice section is a baffle, which acts as another trap for fine gold and also ensures that the aggregate material being processed is evenly distributed before it enters the sluice section. It sits at an angle and points towards the closed back of the box. Traditionally, the baffle consisted of a flexible apron made of canvas or a similar material, which had a sag of about an inch and a half in the center, to act as a collection pocket for fine gold. Later rockers (including most modern ones) dispensed with the flexible apron and used a pair of solid wood or metal baffle boards. These are sometimes covered with carpet to trap fine gold. The entire device sits on rockers at a slight gradient, which allows it to be rocked side to side.
Today, the rocker box is not used as extensively as the sluice, but still is an effective method of recovering gold in areas where there is not enough available water to operate a sluice effectively. Like a sluice box, the rocker box has riffles and a carpet in it to trap gold. It was designed to be used in areas with less water than a sluice box. The mineral processing involves pouring water out of a small cup and then rocking the small sluice box like a cradle, thus the name rocker box or cradle. | Rocker box | Wikipedia | 423 | 4101529 | https://en.wikipedia.org/wiki/Rocker%20box | Technology | Metallurgy | null |
Rocker boxes must be manipulated carefully, to prevent losing the gold. Although big, and difficult to move, the rocker can pick up twice the amount of the gravel, and therefore more gold in one day than an ordinary gold mining pan. The rocker, like the pan, is used extensively in small-scale placer work, in sampling, and for washing sluice concentrates and material cleaned by hand from bedrock in other placer operations. One to three cubic yards, bank measure, can be dug and washed in a rocker per man-shift, depending upon the distance the gravel or water has to be carried, the character of the gravel, and the size of the rocker.
Rockers are usually homemade and display a variety of designs. A favorite design consists essentially of a combination washing box and screen, a canvas or carpet apron under the screen, a short sluice with two or more riffles, and rockers under the sluice. The bottom of the washing box consists of sheet metal with holes about a half an inch in diameter punched in it, or a half-inch mesh screen can be used. Dimensions shown are satisfactory, but variations are possible. The bottom of the rocker should be made of a single wide, smooth board, which will greatly facilitate cleanups. The materials for building a rocker cost only a few dollars, depending mainly on the source of lumber. | Rocker box | Wikipedia | 279 | 4101529 | https://en.wikipedia.org/wiki/Rocker%20box | Technology | Metallurgy | null |
Seine fishing (or seine-haul fishing; ) is a method of fishing that employs a surrounding net, called a seine, that hangs vertically in the water with its bottom edge held down by weights and its top edge buoyed by floats. Seine nets can be deployed from the shore as a beach seine, or from a boat.
Boats deploying seine nets are known as seiners. Two main types of seine net are deployed from seiners: purse seines and Danish seines. A seine differs from a gillnet, in that a seine encloses fish, where a gillnet directly snares fish.
Etymology
The word seine has its origins in the Old English segne, which entered the language via Latin sagena, from the original Greek σαγήνη sagēnē (a drag-net).
History
Seines have been used widely in the past, including by Stone Age societies. For example, the Māori used large canoes to deploy seine nets which could be over a kilometer long. The nets were woven from green flax, with stone weights and light wood or gourd floats, and could require hundreds of men to haul.
Native Americans on the Columbia River wove seine nets from spruce root fibers or wild grass, again using stones as weights. For floats they used sticks made of cedar which moved in a way which frightened the fish and helped keep them together.
Arrian's description of Alexander the Great's expedition on the Makran coast in 325 B.C. includes a detailed description of seine fishing by a tribe known as the Ichthyophagi (Fish-eaters).
Seine nets are also well documented in ancient cultures in the Mediterranean region. They appear in Egyptian tomb paintings from 3000 BCE. In ancient Roman literature, the poet Ovid makes many references to seine nets, including the use of cork floats and lead weights.
Beach seine
The beach seine is employed by anchoring a section of netting on the shoreline, then dragging the net into the water and surrounding the fish, before pulling it ashore. Several countries have prohibited the use of the seines. Kenya outlawed the use of beach seines in 2001.
Purse seine | Seine fishing | Wikipedia | 440 | 1513400 | https://en.wikipedia.org/wiki/Seine%20fishing | Technology | Hunting and fishing | null |
A common type of seine is a purse seine, named such because along the bottom are a number of rings. A line (referred to as a purse-line) passes through all the rings, and when pulled, draws the rings close to one another, preventing the fish from "sounding", or swimming down to escape the net. This operation is similar to a traditional style purse, which has a drawstring.
The purse seine is a preferred technique for capturing fish species which school, or aggregate, close to the surface: sardines, mackerel, anchovies, herring, and certain species of tuna (schooling); and salmon soon before they swim up rivers and streams to spawn (aggregation). Boats equipped with purse seines are called purse seiners.
Purse seines are ranked by experts as one of the most sustainable commercial fishing methods when compared with other options. Purse seine fishing can result in smaller amounts of by-catch (unintentionally caught fish), especially when used to catch large species of fish (like herring or mackerel) that shoal tightly together. When used to catch fish that shoal together with other species, or when used in parallel with fish aggregating devices, the percentage of by-catch greatly increases.
Use of purse seines is regulated by many countries; in Sri Lanka, for example, using this type of net within of the shore is illegal. However, they can be used in the deep sea, after obtaining permission from authorities. Purse seine fishing can have negative impacts on fish stocks because it can involve the bycatch of non-target species and it can put too much pressure on fish stocks.
Power block
The power block is a mechanized pulley used on some seiners to haul in the nets. According to the UN Food and Agriculture Organization, no single invention has contributed more to the effectiveness of purse seine net hauling than the power block.
The Puretic power block line was introduced in the 1950s and was the key factor in the mechanization of purse seining. The combination of these blocks with advances in fluid hydraulics and the new large synthetic nets changed the character of purse seine fishing. The original Puretic power block was driven by an endless rope from the warping head of a winch. Nowadays, power blocks are usually driven by hydraulic pumps powered by the main or auxiliary engine. Their rpm, pull and direction can be controlled remotely. | Seine fishing | Wikipedia | 490 | 1513400 | https://en.wikipedia.org/wiki/Seine%20fishing | Technology | Hunting and fishing | null |
A minimum of three people are required for power block seining; the skipper, skiff operator, and corkline stacker. In many operations a fourth person stacks the leadline, and often a fifth person stacks the web.
Drum
In certain parts of the western United States as well as Canada, specifically on the coast of British Columbia, drum seining is a method of seine fishing which was adopted in the late 1950s and is used exclusively in that region.
The drum seine uses a horizontally mounted drum to haul and store the net instead of a power block. The net is pulled in over a roller, which spans the stern, and then passes through a spooling gear with upright rollers. The spooling gear is moved from side to side across the stern which allows the net to be guided and wound tightly on the drum.
There are several advantages to the drum seine over the power block. The net can be hauled very quickly - at more than twice the speed of using a power block, the net does not require overhead handling, and the process is therefore safer. The most important advantage is that the drum system can be operated with fewer deckhands. However, it is illegal to use a seine drum in the state of Alaska.
Danish seine
A Danish seine, also occasionally called an anchor seine, consists of a conical net with two long wings with a bag where the fish collect. Drag lines extend from the wings, and are long so they can surround an area.
A Danish seine is similar to a small trawl net, but the wire warps are much longer and there are no otter boards. The seine boat drags the warps and the net in a circle around the fish. The motion of the warps herds the fish into the central net.
Danish seiner vessels are usually larger than purse seiners, though they are often accompanied by a smaller vessel. The drag lines are often stored on drums or coiled onto the deck by a coiling machine. A brightly coloured buoy, anchored as a "marker", serves as a fixed point when hauling the seine. A power block, usually mounted on a boom or a slewing deck crane, hauls the seine net. | Seine fishing | Wikipedia | 444 | 1513400 | https://en.wikipedia.org/wiki/Seine%20fishing | Technology | Hunting and fishing | null |
Danish seining works best on demersal fish which are either scattered on or close to the bottom of the sea, or are aggregated (schooling). They are used when there are flat but rough seabeds which are not trawlable. It is especially useful in northern regions, but not much in tropical to sub-tropical areas.
The net is deployed, with one end attached to an anchored dan (marker) buoy, by the main vessel, the seiner, or by a smaller auxiliary boat. A drag line is paid out, followed by a net wing. As the seiner sweeps in a big circle returning to the buoy, the deployment continues with the seine bag and the remaining wing, finishing with the remaining drag line. In this way a large area can be surrounded. Next the drag lines are hauled in using rope-coiling machines until the catch bag can be secured.
The seine netting method developed in Denmark. Scottish seining ("fly dragging") was a later modification. The original procedure is much the same as fly dragging except for the use of an anchored marker buoy when hauling, and closing the net and warps and net by winch.
Other images | Seine fishing | Wikipedia | 244 | 1513400 | https://en.wikipedia.org/wiki/Seine%20fishing | Technology | Hunting and fishing | null |
Column chromatography in chemistry is a chromatography method used to isolate a single chemical compound from a mixture. Chromatography is able to separate substances based on differential absorption of compounds to the adsorbent; compounds move through the column at different rates, allowing them to be separated into fractions. The technique is widely applicable, as many different adsorbents (normal phase, reversed phase, or otherwise) can be used with a wide range of solvents. The technique can be used on scales from micrograms up to kilograms. The main advantage of column chromatography is the relatively low cost and disposability of the stationary phase used in the process. The latter prevents cross-contamination and stationary phase degradation due to recycling. Column chromatography can be done using gravity to move the solvent, or using compressed gas to push the solvent through the column.
A thin-layer chromatograph can show how a mixture of compounds will behave when purified by column chromatography. The separation is first optimised using thin-layer chromatography before performing column chromatography.
Column preparation
A column is prepared by packing a solid adsorbent into a cylindrical glass or plastic tube. The size will depend on the amount of compound being isolated. The base of the tube contains a filter, either a cotton or glass wool plug, or glass frit to hold the solid phase in place. A solvent reservoir may be attached at the top of the column.
Two methods are generally used to prepare a column: the dry method and the wet method. For the dry method, the column is first filled with dry stationary phase powder, followed by the addition of mobile phase, which is flushed through the column until it is completely wet, and from this point is never allowed to run dry. For the wet method, a slurry is prepared of the eluent with the stationary phase powder and then carefully poured into the column. The top of the silica should be flat, and the top of the silica can be protected by a layer of sand. Eluent is slowly passed through the column to advance the organic material. | Column chromatography | Wikipedia | 444 | 1514469 | https://en.wikipedia.org/wiki/Column%20chromatography | Physical sciences | Chromatography | Chemistry |
The individual components are retained by the stationary phase differently and separate from each other while they are running at different speeds through the column with the eluent. At the end of the column they elute one at a time. During the entire chromatography process the eluent is collected in a series of fractions. Fractions can be collected automatically by means of fraction collectors. The productivity of chromatography can be increased by running several columns at a time. In this case multi stream collectors are used. The composition of the eluent flow can be monitored and each fraction is analyzed for dissolved compounds, e.g. by analytical chromatography, UV absorption spectra, or fluorescence. Colored compounds (or fluorescent compounds with the aid of a UV lamp) can be seen through the glass wall as moving bands.
Stationary phase
The stationary phase or adsorbent in column chromatography is a solid. The most common stationary phase for column chromatography is silica gel, the next most common being alumina. Cellulose powder has often been used in the past. A wide range of stationary phases are available in order to perform ion exchange chromatography, reversed-phase chromatography (RP), affinity chromatography or expanded bed adsorption (EBA). The stationary phases are usually finely ground powders or gels and/or are microporous for an increased surface, though in EBA a fluidized bed is used. There is an important ratio between the stationary phase weight and the dry weight of the analyte mixture that can be applied onto the column. For silica column chromatography, this ratio lies within 20:1 to 100:1, depending on how close to each other the analyte components are being eluted. | Column chromatography | Wikipedia | 372 | 1514469 | https://en.wikipedia.org/wiki/Column%20chromatography | Physical sciences | Chromatography | Chemistry |
Mobile phase (eluent)
The mobile phase or eluent is a solvent or a mixture of solvents used to move the compounds through the column. It is chosen so that the retention factor value of the compound of interest is roughly around 0.2 - 0.3 in order to minimize the time and the amount of eluent to run the chromatography. The eluent has also been chosen so that the different compounds can be separated effectively. The eluent is optimized in small scale pretests, often using thin layer chromatography (TLC) with the same stationary phase, using solvents of different polarity until a suitable solvent system is found. Common mobile phase solvents, in order of increasing polarity, include hexane, dichloromethane, ethyl acetate, acetone, and methanol. A common solvent system is a mixture of hexane and ethyl acetate, with proportions adjusted until the target compound has a retention factor of 0.2 - 0.3. Contrary to common misconception, methanol alone can be used as an eluent for highly polar compounds, and does not dissolve silica gel.
There is an optimum flow rate for each particular separation. A faster flow rate of the eluent minimizes the time required to run a column and thereby minimizes diffusion, resulting in a better separation. However, the maximum flow rate is limited because a finite time is required for the analyte to equilibrate between the stationary phase and mobile phase, see Van Deemter's equation. A simple laboratory column runs by gravity flow. The flow rate of such a column can be increased by extending the fresh eluent filled column above the top of the stationary phase or decreased by the tap controls. Faster flow rates can be achieved by using a pump or by using compressed gas (e.g. air, nitrogen, or argon) to push the solvent through the column (flash column chromatography).
The particle size of the stationary phase is generally finer in flash column chromatography than in gravity column chromatography. For example, one of the most widely used silica gel grades in the former technique is mesh 230 – 400 (40 – 63 μm), while the latter technique typically requires mesh 70 – 230 (63 – 200 μm) silica gel. | Column chromatography | Wikipedia | 495 | 1514469 | https://en.wikipedia.org/wiki/Column%20chromatography | Physical sciences | Chromatography | Chemistry |
A spreadsheet that assists in the successful development of flash columns has been developed. The spreadsheet estimates the retention volume and band volume of analytes, the fraction numbers expected to contain each analyte, and the resolution between adjacent peaks. This information allows users to select optimal parameters for preparative-scale separations before the flash column itself is attempted.
Automated systems
Column chromatography is an extremely time-consuming stage in any lab and can quickly become the bottleneck for any process lab. Many manufacturers like Biotage, Buchi, Interchim and Teledyne Isco have developed automated flash chromatography systems (typically referred to as LPLC, low pressure liquid chromatography, around ) that minimize human involvement in the purification process. Automated systems will include components normally found on more expensive high performance liquid chromatography (HPLC) systems such as a gradient pump, sample injection ports, a UV detector and a fraction collector to collect the eluent. Typically these automated systems can separate samples from a few milligrams up to an industrial many kilogram scale and offer a much cheaper and quicker solution to doing multiple injections on prep-HPLC systems.
The resolution (or the ability to separate a mixture) on an LPLC system will always be lower compared to HPLC, as the packing material in an HPLC column can be much smaller, typically only 5 micrometre thus increasing stationary phase surface area, increasing surface interactions and giving better separation. However, the use of this small packing media causes the high back pressure and is why it is termed high pressure liquid chromatography. The LPLC columns are typically packed with silica of around 50 micrometres, thus reducing back pressure and resolution, but it also removes the need for expensive high pressure pumps. Manufacturers are now starting to move into higher pressure flash chromatography systems and have termed these as medium pressure liquid chromatography (MPLC) systems which operate above .
Column chromatogram resolution calculation | Column chromatography | Wikipedia | 412 | 1514469 | https://en.wikipedia.org/wiki/Column%20chromatography | Physical sciences | Chromatography | Chemistry |
Typically, column chromatography is set up with peristaltic pumps, flowing buffers and the solution sample through the top of the column. The solutions and buffers pass through the column where a fraction collector at the end of the column setup collects the eluted samples. Prior to the fraction collection, the samples that are eluted from the column pass through a detector such as a spectrophotometer or mass spectrometer so that the concentration of the separated samples in the sample solution mixture can be determined.
For example, if you were to separate two different proteins with different binding capacities to the column from a solution sample, a good type of detector would be a spectrophotometer using a wavelength of 280 nm. The higher the concentration of protein that passes through the eluted solution through the column, the higher the absorbance of that wavelength.
Because the column chromatography has a constant flow of eluted solution passing through the detector at varying concentrations, the detector must plot the concentration of the eluted sample over a course of time. This plot of sample concentration versus time is called a chromatogram.
The ultimate goal of chromatography is to separate different components from a solution mixture. The resolution expresses the extent of separation between the components from the mixture. The higher the resolution of the chromatogram, the better the extent of separation of the samples the column gives. This data is a good way of determining the column's separation properties of that particular sample. The resolution can be calculated from the chromatogram.
The separate curves in the diagram represent different sample elution concentration profiles over time based on their affinity to the column resin. To calculate resolution, the retention time and curve width are required.
Retention time is the time from the start of signal detection by the detector to the peak height of the elution concentration profile of each different sample.
Curve width is the width of the concentration profile curve of the different samples in the chromatogram in units of time. | Column chromatography | Wikipedia | 408 | 1514469 | https://en.wikipedia.org/wiki/Column%20chromatography | Physical sciences | Chromatography | Chemistry |
A simplified method of calculating chromatogram resolution is to use the plate model. The plate model assumes that the column can be divided into a certain number of sections, or plates and the mass balance can be calculated for each individual plate. This approach approximates a typical chromatogram curve as a Gaussian distribution curve. By doing this, the curve width is estimated as 4 times the standard deviation of the curve, 4σ. The retention time is the time from the start of signal detection to the time of the peak height of the Gaussian curve.
From the variables in the figure above, the resolution, plate number, and plate height of the column plate model can be calculated using the equations:
Resolution (Rs):
Rs = 2(tRB – tRA)/(wB + wA),
where:
tRB = retention time of solute B
tRA = retention time of solute A
wB = Gaussian curve width of solute B
wA = Gaussian curve width of solute A
Plate Number (N):
N = (tR)2/(w/4)2
Plate Height (H):
H = L/N
where L is the length of the column.
Column adsorption equilibrium
For an adsorption column, the column resin (the stationary phase) is composed of microbeads. Even smaller particles such as proteins, carbohydrates, metal ions, or other chemical compounds are conjugated onto the microbeads. Each binding particle that is attached to the microbead can be assumed to bind in a 1:1 ratio with the solute sample sent through the column that needs to be purified or separated.
Binding between the target molecule to be separated and the binding molecule on the column beads can be modeled using a simple equilibrium reaction Keq = [CS]/([C][S]) where Keq is the equilibrium constant, [C] and [S] are the concentrations of the target molecule and the binding molecule on the column resin, respectively. [CS] is the concentration of the complex of the target molecule bound to the column resin.
Using this as a basis, three different isotherms can be used to describe the binding dynamics of a column chromatography: linear, Langmuir, and Freundlich.
The linear isotherm occurs when the solute concentration needed to be purified is very small relative to the binding molecule. Thus, the equilibrium can be defined as: | Column chromatography | Wikipedia | 508 | 1514469 | https://en.wikipedia.org/wiki/Column%20chromatography | Physical sciences | Chromatography | Chemistry |
[CS] = Keq[C].
For industrial scale uses, the total binding molecules on the column resin beads must be factored in because unoccupied sites must be taken into account. The Langmuir isotherm and Freundlich isotherm are useful in describing this equilibrium. The Langmuir isotherm is given by:
[CS] = (KeqStot[C])/(1 + Keq[C]), where Stot is the total binding molecules on the beads.
The Freundlich isotherm is given by:
[CS] = Keq[C]1/n
The Freundlich isotherm is used when the column can bind to many different samples in the solution that needs to be purified. Because the many different samples have different binding constants to the beads, there are many different Keqs. Therefore, the Langmuir isotherm is not a good model for binding in this case. | Column chromatography | Wikipedia | 205 | 1514469 | https://en.wikipedia.org/wiki/Column%20chromatography | Physical sciences | Chromatography | Chemistry |
A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of three types – a ridge (R), trench (T) or transform fault (F) – and triple junctions can be described according to the types of plate margin that meet at them (e.g. fault–fault–trench, ridge–ridge–ridge, or abbreviated F-F-T, R-R-R). Of the ten possible types of triple junctions only a few are stable through time (stable in this context means that the geometrical configuration of the triple junction will not change through geologic time). The meeting of four or more plates is also theoretically possible, but junctions will only exist instantaneously.
History
The first scientific paper detailing the triple-junction concept was published in 1969 by Dan McKenzie and W. Jason Morgan. The term had traditionally been used for the intersection of three divergent boundaries or spreading ridges. These three divergent boundaries ideally meet at near 120° angles.
In plate tectonics theory during the breakup of a continent, three divergent boundaries form, radiating out from a central point (the triple junction). One of these divergent plate boundaries fails (see aulacogen) and the other two continue spreading to form an ocean. The opening of the south Atlantic Ocean started at the south of the South American and African continents, reaching a triple junction in the present Gulf of Guinea, from where it continued to the west. The NE-trending Benue Trough is the failed arm of this junction.
In the years since, the term triple-junction has come to refer to any point where three tectonic plates meet. | Triple junction | Wikipedia | 349 | 1514751 | https://en.wikipedia.org/wiki/Triple%20junction | Physical sciences | Tectonics | Earth science |
Interpretation
The properties of triple junctions are most easily understood from the purely kinematic point of view where the plates are rigid and moving over the surface of the Earth. No knowledge of the Earth's interior or the geological details of the crust are then needed. Another useful simplification is that the kinematics of triple junctions on a flat Earth are essentially the same as those on the surface of a sphere. On a sphere, plate motions are described as relative rotations about Euler poles (see Plate reconstruction), and the relative motion at every point along a plate boundary can be calculated from this rotation. But the area around a triple junction is small enough (relative to the size of the sphere) and (usually) far enough from the pole of rotation, that the relative motion across a boundary can be assumed to be constant along that boundary. Thus, analysis of triple junctions can usually be done on a flat surface with motions defined by vectors.
Stability
Triple junctions may be described and their stability assessed without use of the geological details but simply by defining the properties of the ridges, trenches and transform faults involved, making some simplifying assumptions and applying simple velocity calculations. This assessment can generalise to most actual triple junction settings provided the assumptions and definitions broadly apply to the real Earth.
A stable junction is one at which the geometry of the junction is retained with time as the plates involved move. This places restrictions on relative velocities and plate boundary orientation. An unstable triple junction will change with time, either to become another form of triple junction (RRF junctions easily evolve to FFR junctions), will change geometry or are simply not feasible (as in the case of FFF junctions). The inherent instability of an FFF junction is believed to have caused the formation of the Pacific plate about 190 million years ago.
By assuming that plates are rigid and that the Earth is spherical, Leonhard Euler's theorem of motion on a sphere can be used to reduce the stability assessment to determining boundaries and relative motions of the interacting plates. The rigid assumption holds very well in the case of oceanic crust, and the radius of the Earth at the equator and poles only varies by a factor of roughly one part in 300 so the Earth approximates very well to a sphere. | Triple junction | Wikipedia | 457 | 1514751 | https://en.wikipedia.org/wiki/Triple%20junction | Physical sciences | Tectonics | Earth science |
McKenzie and Morgan first analysed the stability of triple junctions using these assumptions with the additional assumption that the Euler poles describing the motions of the plates were such that they approximated to straight line motion on a flat surface. This simplification applies when the Euler poles are distant from the triple junction concerned. The definitions they used for R, T and F are as follows:
R – structures that produce lithosphere symmetrically and perpendicular to the relative velocity of the plates on either side (this does not always apply, for example in the Gulf of Aden).
T – structures that consume lithosphere from one side only. The relative velocity vector can be oblique to the plate boundary.
F – active faults parallel to the slip vector.
Stability criteria
For a triple junction between the plates A, B and C to exist, the following condition must be satisfied:
AvB + BvC + CvA = 0
where AvB is the relative motion of B with respect to A.
This condition can be represented in velocity space by constructing a velocity triangle ABC where the lengths AB, BC and CA are proportional to the velocities AvB, BvC and CvA respectively.
Further conditions must also be met for the triple junction to exist stably – the plates must move in a way that leaves their individual geometries unchanged. Alternatively the triple junction must move in such a way that it remains on all three of the plate boundaries involved.
McKenzie and Morgan demonstrated that these criteria can be represented on the same velocity space diagrams in the following way. The lines ab, bc and ca join points in velocity space which will leave the geometry of AB, BC and CA unchanged. These lines are the same as those that join points in velocity space at which an observer could move at the given velocity and still remain on the plate boundary. When these are drawn onto the diagram containing the velocity triangle these lines must be able to meet at a single point, for the triple junction to exist stably.
These lines necessarily are parallel to the plate boundaries as to remain on the plate boundaries the observer must either move along the plate boundary or remain stationary on it. | Triple junction | Wikipedia | 433 | 1514751 | https://en.wikipedia.org/wiki/Triple%20junction | Physical sciences | Tectonics | Earth science |
For a ridge the line constructed must be the perpendicular bisector of the relative motion vector as to remain in the middle of the ridge an observer would have to move at half the relative speeds of the plates either side but could also move in a perpendicular direction along the plate boundary.
For a transform fault the line must be parallel to the relative motion vector as all of the motion is parallel to the boundary direction and so the line ab must lie along AB for a transform fault separating the plates A and B.
For an observer to remain on a trench boundary they must walk along the strike of the trench but remaining on the overriding plate. Therefore, the line constructed will lie parallel to the plate boundary but passing through the point in velocity space occupied by the overriding plate.
The point at which these lines meet, J, gives the overall motion of the triple junction with respect to the Earth.
Using these criteria it can easily be shown why the FFF triple junction is not stable: the only case in which three lines lying along the sides of a triangle can meet at a point is the trivial case in which the triangle has sides lengths zero, corresponding to zero relative motion between the plates. As faults are required to be active for the purpose of this assessment, an FFF junction can never be stable.
Types
McKenzie and Morgan determined that there were 16 types of triple junction theoretically possible, though several of these are speculative and have not necessarily been seen on Earth. These junctions were classified firstly by the types of plate boundaries meeting – for example RRR, TTR, RRT, FFT etc. – and secondly by the relative motion directions of the plates involved. Some configurations such as RRR can only have one set of relative motions whereas TTT junctions may be classified into TTT(a) and TTT(b). These differences in motion direction affect the stability criteria.
McKenzie and Morgan claimed that of these 16 types, 14 were stable with FFF and RRF configurations unstable, however, York later showed that the RRF configuration could be stable under certain conditions.
Ridge–ridge–ridge junctions | Triple junction | Wikipedia | 421 | 1514751 | https://en.wikipedia.org/wiki/Triple%20junction | Physical sciences | Tectonics | Earth science |
An RRR junction is always stable using these definitions and therefore very common on Earth, though in a geological sense ridge spreading is usually discontinued in one direction leaving a failed rift zone. There are many examples of these present both now and in the geological past such as the South Atlantic opening with ridges spreading North and South to form the Mid-Atlantic Ridge, and an associated aulacogen, the Benue Trough, in the Niger Delta region of Africa. RRR junctions are also common as rifting along three fractures at 120° is the best way to relieve stresses from uplift at the surface of a sphere; on Earth, stresses similar to these are believed to be caused by the mantle hotspots thought to initiate rifting in continents.
The stability of RRR junctions is demonstrated below – as the perpendicular bisectors of the sides of a triangle always meet at a single point, the lines ab, bc and ca can always be made to meet regardless of relative velocities.
Ridge–trench–fault junctions
RTF junctions are less common, an unstable junction of this type (an RTF(a)) is thought to have existed at roughly 12Ma at the mouth of the Gulf of California where the East Pacific Rise currently meets the San Andreas Fault zone. The Guadeloupe and Farallon microplates were previously being subducted under the North American plate and the northern end of this boundary met the San Andreas Fault. Material for this subduction was provided by a ridge equivalent to the modern East Pacific Rise slightly displaced to the west of the trench. As the ridge itself was subducted an RTF triple junction momentarily existed but subduction of the ridge caused the subducted lithosphere to weaken and 'tear' from the point of the triple junction. The loss of slab pull caused by the detachment of this lithosphere ended the RTF junction giving the present day ridge – fault system. An RTF(a) is stable if ab goes through the point in velocity space C, or if ac and bc are colinear.
Trench–trench–trench junctions
A TTT(a) junction can be found in central Japan where the Eurasian plate overrides the Philippine and Pacific plates, with the Philippine plate also overriding the Pacific. Here the Japan Trench effectively branches to form the Ryukyu and Bonin arcs. The stability criteria for this type of junction are either ab and ac form a straight line or that the line bc is parallel to CA.
Examples | Triple junction | Wikipedia | 502 | 1514751 | https://en.wikipedia.org/wiki/Triple%20junction | Physical sciences | Tectonics | Earth science |
The junction of the Red Sea, the Gulf of Aden and the East African Rift centered in the Afar Triangle (the Afar triple junction) is the only R-R-R triple junction above sea level.
The Rodrigues triple junction is a R-R-R triple junction in the southern Indian Ocean, where the African, the Indo-Australian and the Antarctic Plates meet.
The Galapagos triple junction is an R-R-R triple junction where the Nazca, the Cocos, and the Pacific plates meet. The East Pacific Rise extends north and south from this junction and the Cocos–Nazca spreading centre goes to the east. This example is made more complex by the Galapagos Microplate which is a small separate plate on the rise just to the southeast of the triple junction.
Chiapas coast off Tapachula where Guatemala, North America and Pacific join and small earthquakes occur weekly. This is pushed eastward by the Cocos plate.
On the west coast of North America is another unstable triple junction offshore of Cape Mendocino. To the south, the San Andreas Fault, a strike-slip fault and transform plate boundary, separates the Pacific plate and the North American plate. To the north lies the Cascadia subduction zone, where a section of the Juan de Fuca plate called the Gorda plate is being subducted under the North American plate, forming a trench (T). Another transform fault, the Mendocino Fault (F), runs along the boundary between the Pacific plate and the Gorda plate. Where the three intersect is the seismically active, F-F-T Mendocino triple junction.
The Amurian plate, the Okhotsk microplate, and the Philippine Sea plate meet in Japan near Mount Fuji. (see Mount Fuji's Geology)
The Azores triple junction is a geologic triple junction where the boundaries of three tectonic plates intersect: the North American plate, the Eurasian plate and the African plate, R-R-R.
The Boso triple junction offshore of Japan is a T-T-T triple junction between the Okhotsk microplate, Pacific plate and Philippine Sea plate.
The North Sea is located at the extinct triple junction of three former continental plates of the Palaeozoic era: Avalonia, Laurentia and Baltica.
The South Greenland triple junction was an R-R-R triple junction where the Eurasian, Greenland and North American plates diverged during the Paleogene. | Triple junction | Wikipedia | 501 | 1514751 | https://en.wikipedia.org/wiki/Triple%20junction | Physical sciences | Tectonics | Earth science |
The Chile triple junction is where the South American plate, the Nazca plate, and the Antarctic plate meet. | Triple junction | Wikipedia | 23 | 1514751 | https://en.wikipedia.org/wiki/Triple%20junction | Physical sciences | Tectonics | Earth science |
The Stokes parameters are a set of values that describe the polarization state of electromagnetic radiation. They were defined by George Gabriel Stokes in 1851, as a mathematically convenient alternative to the more common description of incoherent or partially polarized radiation in terms of its total intensity (I), (fractional) degree of polarization (p), and the shape parameters of the polarization ellipse. The effect of an optical system on the polarization of light can be determined by constructing the Stokes vector for the input light and applying Mueller calculus, to obtain the Stokes vector of the light leaving the system. They can be determined from directly observable phenomena. The original Stokes paper was discovered independently by Francis Perrin in 1942 and by Subrahamanyan Chandrasekhar in 1947, who named it as the Stokes parameters.
Definitions
The relationship of the Stokes parameters S0, S1, S2, S3 to intensity and polarization ellipse parameters is shown in the equations below and the figure on the right.
Here , and are the spherical coordinates of the three-dimensional vector of cartesian coordinates . is the total intensity of the beam, and is the degree of polarization, constrained by . The factor of two before represents the fact that any polarization ellipse is indistinguishable from one rotated by 180°, while the factor of two before indicates that an ellipse is indistinguishable from one with the semi-axis lengths swapped accompanied by a 90° rotation. The phase information of the polarized light is not recorded in the Stokes parameters. The four Stokes parameters are sometimes denoted I, Q, U and V, respectively.
Given the Stokes parameters, one can solve for the spherical coordinates with the following equations:
Stokes vectors
The Stokes parameters are often combined into a vector, known as the Stokes vector:
The Stokes vector spans the space of unpolarized, partially polarized, and fully polarized light. For comparison, the Jones vector only spans the space of fully polarized light, but is more useful for problems involving coherent light. The four Stokes parameters are not a preferred coordinate system of the space, but rather were chosen because they can be easily measured or calculated. | Stokes parameters | Wikipedia | 451 | 1515472 | https://en.wikipedia.org/wiki/Stokes%20parameters | Physical sciences | Optics | Physics |
Note that there is an ambiguous sign for the component depending on the physical convention used. In practice, there are two separate conventions used, either defining the Stokes parameters when looking down the beam towards the source (opposite the direction of light propagation) or looking down the beam away from the source (coincident with the direction of light propagation). These two conventions result in different signs for , and a convention must be chosen and adhered to.
Examples
Below are shown some Stokes vectors for common states of polarization of light.
{|
|-
| || Linearly polarized (horizontal)
|-
| || Linearly polarized (vertical)
|-
| || Linearly polarized (+45°)
|-
| || Linearly polarized (−45°)
|-
| || Right-hand circularly polarized
|-
| || Left-hand circularly polarized
|-
| || Unpolarized
|}
Alternative explanation
A monochromatic plane wave is specified by its propagation vector, , and the complex amplitudes of the electric field, and , in a basis . The pair is called a Jones vector. Alternatively, one may specify the propagation vector, the phase, , and the polarization state, , where is the curve traced out by the electric field as a function of time in a fixed plane. The most familiar polarization states are linear and circular, which are degenerate cases of the most general state, an ellipse.
One way to describe polarization is by giving the semi-major and semi-minor axes of the polarization ellipse, its orientation, and the direction of rotation (See the above figure). The Stokes parameters , , , and , provide an alternative description of the polarization state which is experimentally convenient because each parameter corresponds to a sum or difference of measurable intensities. The next figure shows examples of the Stokes parameters in degenerate states.
Definitions
The Stokes parameters are defined by
where the subscripts refer to three different bases of the space of Jones vectors: the standard Cartesian basis (), a Cartesian basis rotated by 45° (), and a circular basis (). The circular basis is defined so that , . | Stokes parameters | Wikipedia | 454 | 1515472 | https://en.wikipedia.org/wiki/Stokes%20parameters | Physical sciences | Optics | Physics |
The symbols ⟨⋅⟩ represent expectation values. The light can be viewed as a random variable taking values in the space C2 of Jones vectors . Any given measurement yields a specific wave (with a specific phase, polarization ellipse, and magnitude), but it keeps flickering and wobbling between different outcomes. The expectation values are various averages of these outcomes. Intense, but unpolarized light will have I > 0 but
Q = U = V = 0, reflecting that no polarization type predominates. A convincing waveform is depicted at the article on coherence.
The opposite would be perfectly polarized light which, in addition, has a fixed, nonvarying amplitude—a pure sine curve. This is represented by a random variable with only a single possible value, say . In this case one may replace the brackets by absolute value bars, obtaining a well-defined quadratic map
from the Jones vectors to the corresponding Stokes vectors; more convenient forms are given below. The map takes its image in the cone defined by |I |2 = |Q |2 + |U |2 + |V |2, where the purity of the state satisfies p = 1 (see below).
The next figure shows how the signs of the Stokes parameters are determined by the helicity and the orientation of the semi-major axis of the polarization ellipse.
Representations in fixed bases
In a fixed () basis, the Stokes parameters when using an increasing phase convention are
while for , they are
and for , they are
Properties
For purely monochromatic coherent radiation, it follows from the above equations that
whereas for the whole (non-coherent) beam radiation, the Stokes parameters are defined as averaged quantities, and the previous equation becomes an inequality:
However, we can define a total polarization intensity , so that
where is the total polarization fraction.
Let us define the complex intensity of linear polarization to be
Under a rotation of the polarization ellipse, it can be shown that and are invariant, but
With these properties, the Stokes parameters may be thought of as constituting three generalized intensities:
where is the total intensity, is the intensity of circular polarization, and is the intensity of linear polarization. The total intensity of polarization is , and the orientation and sense of rotation are given by
Since and , we have
Relation to the polarization ellipse
In terms of the parameters of the polarization ellipse, the Stokes parameters are
Inverting the previous equation gives
Measurement | Stokes parameters | Wikipedia | 511 | 1515472 | https://en.wikipedia.org/wiki/Stokes%20parameters | Physical sciences | Optics | Physics |
The Stokes parameters (and thus the polarization of some electromagnetic radiation) can be directly determined from observation. Using a linear polarizer and a quarter-wave plate, the following system of equations relating the Stokes parameters to measured intensity can be obtained:
where is the irradiance of the radiation at a point when the linear polarizer is rotated at an angle of , and similarly is the irradiance at a point when the quarter-wave plate is rotated at an angle of . A system can be implemented using both plates at once at different angles to measure the parameters. This can give a more accurate measure of the relative magnitudes of the parameters (which is often the main result desired) due to all parameters being affected by the same losses.
Relationship to Hermitian operators and quantum mixed states
From a geometric and algebraic point of view, the Stokes parameters stand in one-to-one correspondence with the closed, convex, 4-real-dimensional cone of nonnegative Hermitian operators on the Hilbert space C2. The parameter I serves as the trace of the operator, whereas the entries of the matrix of the operator are simple linear functions of the four parameters I, Q, U, V, serving as coefficients in a linear combination of the Stokes operators. The eigenvalues and eigenvectors of the operator can be calculated from the polarization ellipse parameters I, p, ψ, χ.
The Stokes parameters with I set equal to 1 (i.e. the trace 1 operators) are in one-to-one correspondence with the closed unit 3-dimensional ball of mixed states (or density operators) of the quantum space C2, whose boundary is the Bloch sphere. The Jones vectors correspond to the underlying space C2, that is, the (unnormalized) pure states of the same system. Note that the overall phase (i.e. the common phase factor between the two component waves on the two perpendicular polarization axes) is lost when passing from a pure state |φ⟩ to the corresponding mixed state |φ⟩⟨φ|, just as it is lost when passing from a Jones vector to the corresponding Stokes vector. | Stokes parameters | Wikipedia | 434 | 1515472 | https://en.wikipedia.org/wiki/Stokes%20parameters | Physical sciences | Optics | Physics |
In the basis of horizontal polarization state and vertical polarization state , the +45° linear polarization state is , the -45° linear polarization state is , the left hand circular polarization state is , and the right hand circular polarization state is . It's easy to see that these states are the eigenvectors of Pauli matrices, and that the normalized Stokes parameters (U/I, V/I, Q/I) correspond to the coordinates of the Bloch vector (, , ). Equivalently, we have , , , where is the density matrix of the mixed state.
Generally, a linear polarization at angle θ has a pure quantum state ; therefore, the transmittance of a linear polarizer/analyzer at angle θ for a mixed state light source with density matrix is , with a maximum transmittance of at if , or at if ; the minimum transmittance of is reached at the perpendicular to the maximum transmittance direction. Here, the ratio of maximum transmittance to minimum transmittance is defined as the extinction ratio , where the degree of linear polarization is . Equivalently, the formula for the transmittance can be rewritten as , which is an extended form of Malus's law; here, are both non-negative, and is related to the extinction ratio by . Two of the normalized Stokes parameters can also be calculated by .
It's also worth noting that a rotation of polarization axis by angle θ corresponds to the Bloch sphere rotation operator . For example, the horizontal polarization state would rotate to . The effect of a quarter-wave plate aligned to the horizontal axis is described by , or equivalently the Phase gate S, and the resulting Bloch vector becomes . With this configuration, if we perform the rotating analyzer method to measure the extinction ratio, we will be able to calculate and also verify . For this method to work, the fast axis and the slow axis of the waveplate must be aligned with the reference directions for the basis states.
The effect of a quarter-wave plate rotated by angle θ can be determined by Rodrigues' rotation formula as , with . The transmittance of the resulting light through a linear polarizer (analyzer plate) along the horizontal axis can be calculated using the same Rodrigues' rotation formula and focusing on its components on and : | Stokes parameters | Wikipedia | 474 | 1515472 | https://en.wikipedia.org/wiki/Stokes%20parameters | Physical sciences | Optics | Physics |
The above expression is the theory basis of many polarimeters. For unpolarized light, T=1/2 is a constant. For purely circularly polarized light, T has a sinusoidal dependence on angle θ with a period of 180 degrees, and can reach absolute extinction where T=0. For purely linearly polarized light, T has a sinusoidal dependence on angle θ with a period of 90 degrees, and absolute extinction is only reachable when the original light's polarization is at 90 degrees from the polarizer (i.e. ). In this configuration, and , with a maximum of 1/2 at θ=45°, and an extinction point at θ=0°. This result can be used to precisely determine the fast or slow axis of a quarter-wave plate, for example, by using a polarizing beam splitter to obtain a linearly polarized light aligned to the analyzer plate and rotating the quarter-wave plate in between.
Similarly, the effect of a half-wave plate rotated by angle θ is described by , which transforms the density matrix to:
The above expression demonstrates that if the original light is of pure linear polarization (i.e. ), the resulting light after the half-wave plate is still of pure linear polariztion (i.e. without component) with a rotated major axis. Such rotation of the linear polarization has a sinusoidal dependence on angle θ with a period of 90 degrees. | Stokes parameters | Wikipedia | 303 | 1515472 | https://en.wikipedia.org/wiki/Stokes%20parameters | Physical sciences | Optics | Physics |
A satellite navigation or satnav system is a system that uses satellites to provide autonomous geopositioning. A satellite navigation system with global coverage is termed global navigation satellite system (GNSS). , four global systems are operational: the United States's Global Positioning System (GPS), Russia's Global Navigation Satellite System (GLONASS), China's BeiDou Navigation Satellite System (BDS), and the European Union's Galileo.
Satellite-based augmentation systems (SBAS), designed to enhance the accuracy of GNSS, include Japan's Quasi-Zenith Satellite System (QZSS), India's GAGAN and the European EGNOS, all of them based on GPS.
Previous iterations of the BeiDou navigation system and the present Indian Regional Navigation Satellite System (IRNSS), operationally known as NavIC, are examples of stand-alone operating regional navigation satellite systems (RNSS).
Satellite navigation devices determine their location (longitude, latitude, and altitude/elevation) to high precision (within a few centimeters to meters) using time signals transmitted along a line of sight by radio from satellites. The system can be used for providing position, navigation or for tracking the position of something fitted with a receiver (satellite tracking). The signals also allow the electronic receiver to calculate the current local time to a high precision, which allows time synchronisation. These uses are collectively known as Positioning, Navigation and Timing (PNT). Satnav systems operate independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the positioning information generated.
Global coverage for each system is generally achieved by a satellite constellation of 18–30 medium Earth orbit (MEO) satellites spread between several orbital planes. The actual systems vary, but all use orbital inclinations of >50° and orbital periods of roughly twelve hours (at an altitude of about ).
Classification
GNSS systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows: | Satellite navigation | Wikipedia | 415 | 1515653 | https://en.wikipedia.org/wiki/Satellite%20navigation | Technology | Navigation | null |
is the first generation system and is the combination of existing satellite navigation systems (GPS and GLONASS), with Satellite Based Augmentation Systems (SBAS) or Ground Based Augmentation Systems (GBAS). In the United States, the satellite-based component is the Wide Area Augmentation System (WAAS); in Europe, it is the European Geostationary Navigation Overlay Service (EGNOS); in Japan, it is the Multi-Functional Satellite Augmentation System (MSAS); and in India, it is the GPS-aided GEO augmented navigation (GAGAN). Ground-based augmentation is provided by systems like the Local Area Augmentation System (LAAS).
is the second generation of systems that independently provide a full civilian satellite navigation system, exemplified by the European Galileo positioning system. These systems will provide the accuracy and integrity monitoring necessary for civil navigation; including aircraft. Initially, this system consisted of only Upper L Band frequency sets (L1 for GPS, E1 for Galileo, and G1 for GLONASS). In recent years, GNSS systems have begun activating Lower L Band frequency sets (L2 and L5 for GPS, E5a and E5b for Galileo, and G3 for GLONASS) for civilian use; they feature higher aggregate accuracy and fewer problems with signal reflection. As of late 2018, a few consumer-grade GNSS devices are being sold that leverage both. They are typically called "Dual band GNSS" or "Dual band GPS" devices. | Satellite navigation | Wikipedia | 321 | 1515653 | https://en.wikipedia.org/wiki/Satellite%20navigation | Technology | Navigation | null |
By their roles in the navigation system, systems can be classified as:
There are four global satellite navigation systems, currently GPS (United States), GLONASS (Russian Federation), Beidou (China) and Galileo (European Union).
Global Satellite-Based Augmentation Systems (SBAS) such as OmniSTAR and StarFire.
Regional SBAS including WAAS (US), EGNOS (EU), MSAS (Japan), GAGAN (India) and SDCM (Russia).
Regional Satellite Navigation Systems such as India's NAVIC, and Japan's QZSS.
Continental scale Ground Based Augmentation Systems (GBAS) for example the Australian GRAS and the joint US Coast Guard, Canadian Coast Guard, US Army Corps of Engineers and US Department of Transportation National Differential GPS (DGPS) service.
Regional scale GBAS such as CORS networks.
Local GBAS typified by a single GPS reference station operating Real Time Kinematic (RTK) corrections.
As many of the global GNSS systems (and augmentation systems) use similar frequencies and signals around L1, many "Multi-GNSS" receivers capable of using multiple systems have been produced. While some systems strive to interoperate with GPS as well as possible by providing the same clock, others do not.
History
Ground-based radio navigation is decades old. The DECCA, LORAN, GEE and Omega systems used terrestrial longwave radio transmitters which broadcast a radio pulse from a known "master" location, followed by a pulse repeated from a number of "slave" stations. The delay between the reception of the master signal and the slave signals allowed the receiver to deduce the distance to each of the slaves, providing a fix. | Satellite navigation | Wikipedia | 360 | 1515653 | https://en.wikipedia.org/wiki/Satellite%20navigation | Technology | Navigation | null |
The first satellite navigation system was Transit, a system deployed by the US military in the 1960s. Transit's operation was based on the Doppler effect: the satellites travelled on well-known paths and broadcast their signals on a well-known radio frequency. The received frequency will differ slightly from the broadcast frequency because of the movement of the satellite with respect to the receiver. By monitoring this frequency shift over a short time interval, the receiver can determine its location to one side or the other of the satellite, and several such measurements combined with a precise knowledge of the satellite's orbit can fix a particular position. Satellite orbital position errors are caused by radio-wave refraction, gravity field changes (as the Earth's gravitational field is not uniform), and other phenomena. A team, led by Harold L Jury of Pan Am Aerospace Division in Florida from 1970 to 1973, found solutions and/or corrections for many error sources. Using real-time data and recursive estimation, the systematic and residual errors were narrowed down to accuracy sufficient for navigation.
Principles
Part of an orbiting satellite's broadcast includes its precise orbital data. Originally, the US Naval Observatory (USNO) continuously observed the precise orbits of these satellites. As a satellite's orbit deviated, the USNO sent the updated information to the satellite. Subsequent broadcasts from an updated satellite would contain its most recent ephemeris.
Modern systems are more direct. The satellite broadcasts a signal that contains orbital data (from which the position of the satellite can be calculated) and the precise time the signal was transmitted. Orbital data include a rough almanac for all satellites to aid in finding them, and a precise ephemeris for this satellite. The orbital ephemeris is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission of three (at sea level) or four (which allows an altitude calculation also) different satellites, measuring the time-of-flight to each satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time using an adapted version of trilateration: see GNSS positioning calculation for details. | Satellite navigation | Wikipedia | 476 | 1515653 | https://en.wikipedia.org/wiki/Satellite%20navigation | Technology | Navigation | null |
Each distance measurement, regardless of the system being used, places the receiver on a spherical shell centred on the broadcaster, at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where the shells meet, a fix is generated. However, in the case of fast-moving receivers, the position of the receiver moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver's angle to the satellite, because that angle corresponds to the distance which the signal travels through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centred on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators, and then using techniques such as Kalman filtering to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.
Einstein's theory of general relativity is applied to GPS time correction, the net result is that time on a GPS satellite clock advances faster than a clock on the ground by about 38 microseconds per day.
Applications
The original motivation for satellite navigation was for military applications. Satellite navigation allows precision in the delivery of weapons to targets, greatly increasing their lethality whilst reducing inadvertent casualties from mis-directed weapons. (See Guided bomb). Satellite navigation also allows forces to be directed and to locate themselves more easily, reducing the fog of war.
Now a global navigation satellite system, such as Galileo, is used to determine users location and the location of other people or objects at any given moment. The range of application of satellite navigation in the future is enormous, including both the public and private sectors across numerous market segments such as science, transport, agriculture, insurance, energy, etc.
The ability to supply satellite navigation signals is also the ability to deny their availability. The operator of a satellite navigation system potentially has the ability to degrade or eliminate satellite navigation services over any territory it desires.
Global navigation satellite systems
In order of first launch year:
GPS
First launch year: 1978
The United States' Global Positioning System (GPS) consists of up to 32 medium Earth orbit satellites in six different orbital planes. The exact number of satellites varies as older satellites are retired and replaced. Operational since 1978 and globally available since 1994, GPS is the world's most utilized satellite navigation system.
GLONASS
First launch year: 1982 | Satellite navigation | Wikipedia | 504 | 1515653 | https://en.wikipedia.org/wiki/Satellite%20navigation | Technology | Navigation | null |
The formerly Soviet, and now Russian, Global'naya Navigatsionnaya Sputnikovaya Sistema, (GLObal NAvigation Satellite System or GLONASS), is a space-based satellite navigation system that provides a civilian radionavigation-satellite service and is also used by the Russian Aerospace Defence Forces. GLONASS has full global coverage since 1995 and with 24 active satellites.
BeiDou
First launch year: 2000
BeiDou started as the now-decommissioned Beidou-1, an Asia-Pacific local network on the geostationary orbits. The second generation of the system BeiDou-2 became operational in China in December 2011. The BeiDou-3 system is proposed to consist of 30 MEO satellites and five geostationary satellites (IGSO). A 16-satellite regional version (covering Asia and Pacific area) was completed by December 2012. Global service was completed by December 2018. On 23 June 2020, the BDS-3 constellation deployment is fully completed after the last satellite was successfully launched at the Xichang Satellite Launch Center.
Galileo
First launch year: 2011
The European Union and European Space Agency agreed in March 2002 to introduce their own alternative to GPS, called the Galileo positioning system. Galileo became operational on 15 December 2016 (global Early Operational Capability, EOC). At an estimated cost of €10 billion, the system of 30 MEO satellites was originally scheduled to be operational in 2010. The original year to become operational was 2014. The first experimental satellite was launched on 28 December 2005. Galileo is expected to be compatible with the modernized GPS system. The receivers will be able to combine the signals from both Galileo and GPS satellites to greatly increase the accuracy. The full Galileo constellation consists of 24 active satellites, the last of which was launched in December 2021. The main modulation used in Galileo Open Service signal is the Composite Binary Offset Carrier (CBOC) modulation.
Regional navigation satellite systems
NavIC | Satellite navigation | Wikipedia | 390 | 1515653 | https://en.wikipedia.org/wiki/Satellite%20navigation | Technology | Navigation | null |
The NavIC (acronym for Navigation with Indian Constellation) is an autonomous regional satellite navigation system developed by the Indian Space Research Organisation (ISRO). The Indian government approved the project in May 2006. It consists of a constellation of 7 navigational satellites. Three of the satellites are placed in geostationary orbit (GEO) and the remaining 4 in geosynchronous orbit (GSO) to have a larger signal footprint and lower number of satellites to map the region. It is intended to provide an all-weather absolute position accuracy of better than throughout India and within a region extending approximately around it. An Extended Service Area lies between the primary service area and a rectangle area enclosed by the 30th parallel south to the 50th parallel north and the 30th meridian east to the 130th meridian east, 1,500–6,000 km beyond borders. A goal of complete Indian control has been stated, with the space segment, ground segment and user receivers all being built in India.
The constellation was in orbit as of 2018, and the system was available for public use in early 2018. NavIC provides two levels of service, the "standard positioning service", which will be open for civilian use, and a "restricted service" (an encrypted one) for authorized users (including military). There are plans to expand NavIC system by increasing constellation size from 7 to 11.
India plans to make the NavIC global by adding 24 more MEO satellites. The Global NavIC will be free to use for the global public.
Early BeiDou
The first two generations of China's BeiDou navigation system were designed to provide regional coverage.
Augmentation
GNSS augmentation is a method of improving a navigation system's attributes, such as accuracy, reliability, and availability, through the integration of external information into the calculation process, for example, the Wide Area Augmentation System, the European Geostationary Navigation Overlay Service, the Multi-functional Satellite Augmentation System, Differential GPS, GPS-aided GEO augmented navigation (GAGAN) and inertial navigation systems.
QZSS
The Quasi-Zenith Satellite System (QZSS) is a four-satellite regional time transfer system and enhancement for GPS covering Japan and the Asia-Oceania regions. QZSS services were available on a trial basis as of January 12, 2018, and were started in November 2018. The first satellite was launched in September 2010. An independent satellite navigation system (from GPS) with 7 satellites is planned for 2023. | Satellite navigation | Wikipedia | 511 | 1515653 | https://en.wikipedia.org/wiki/Satellite%20navigation | Technology | Navigation | null |
EGNOS
Comparison of systems
Using multiple GNSS systems for user positioning increases the number of visible satellites, improves precise point positioning (PPP) and shortens the average convergence time.
The signal-in-space ranging error (SISRE) in November 2019 were 1.6 cm for Galileo, 2.3 cm for GPS, 5.2 cm for GLONASS and 5.5 cm for BeiDou when using real-time corrections for satellite orbits and clocks. The average SISREs of the BDS-3 MEO, IGSO, and GEO satellites were 0.52 m, 0.90 m and 1.15 m, respectively. Compared to the four major global satellite navigation systems consisting of MEO satellites, the SISRE of the BDS-3 MEO satellites was slightly inferior to 0.4 m of Galileo, slightly superior to 0.59 m of GPS, and remarkably superior to 2.33 m of GLONASS. The SISRE of BDS-3 IGSO was 0.90 m, which was on par with the 0.92 m of QZSS IGSO. However, as the BDS-3 GEO satellites were newly launched and not completely functioning in orbit, their average SISRE was marginally worse than the 0.91 m of the QZSS GEO satellites.
Related techniques
DORIS
Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) is a French precision navigation system. Unlike other GNSS systems, it is based on static emitting stations around the world, the receivers being on satellites, in order to precisely determine their orbital position. The system may be used also for mobile receivers on land with more limited usage and coverage. Used with traditional GNSS systems, it pushes the accuracy of positions to centimetric precision (and to millimetric precision for altimetric application and also allows monitoring very tiny seasonal changes of Earth rotation and deformations), in order to build a much more precise geodesic reference system.
LEO satellites
The two current operational low Earth orbit (LEO) satellite phone networks are able to track transceiver units with accuracy of a few kilometres using doppler shift calculations from the satellite. The coordinates are sent back to the transceiver unit where they can be read using AT commands or a graphical user interface. This can also be used by the gateway to enforce restrictions on geographically bound calling plans. | Satellite navigation | Wikipedia | 491 | 1515653 | https://en.wikipedia.org/wiki/Satellite%20navigation | Technology | Navigation | null |
International regulation
The International Telecommunication Union (ITU) defines a radionavigation-satellite service (RNSS) as "a radiodetermination-satellite service used for the purpose of radionavigation. This service may also include feeder links necessary for its operation".
RNSS is regarded as a safety-of-life service and an essential part of navigation which must be protected from interferences.
Aeronautical radionavigation-satellite (ARNSS) is – according to Article 1.47 of the International Telecommunication Union's (ITU) Radio Regulations (RR) – defined as «A radionavigation service in which earth stations are located on board aircraft.»
Maritime radionavigation-satellite service (MRNSS) is – according to Article 1.45 of the International Telecommunication Union's (ITU) Radio Regulations (RR) – defined as «A radionavigation-satellite service in which earth stations are located on board ships.»
Classification
ITU Radio Regulations (article 1) classifies radiocommunication services as:
Radiodetermination service (article 1.40)
Radiodetermination-satellite service (article 1.41)
Radionavigation service (article 1.42)
Radionavigation-satellite service (article 1.43)
Maritime radionavigation service (article 1.44)
Maritime radionavigation-satellite service (article 1.45)
Aeronautical radionavigation service (article 1.46)
Aeronautical radionavigation-satellite service (article 1.47)
Examples of RNSS use
Augmentation system GNSS augmentation
Automatic Dependent Surveillance–Broadcast
BeiDou Navigation Satellite System (BDS)
GALILEO, European GNSS
Global Positioning System (GPS), with Differential GPS (DGPS)
GLONASS
NAVIC
Quasi-Zenith Satellite System (QZSS)
Frequency allocation
The allocation of radio frequencies is provided according to Article 5 of the ITU Radio Regulations (edition 2012).
To improve harmonisation in spectrum utilisation, most service allocations are incorporated in national Tables of Frequency Allocations and Utilisations within the responsibility of the appropriate national administration. Allocations are:
primary: indicated by writing in capital letters
secondary: indicated by small letters
exclusive or shared utilization: within the responsibility of administrations. | Satellite navigation | Wikipedia | 474 | 1515653 | https://en.wikipedia.org/wiki/Satellite%20navigation | Technology | Navigation | null |
Pye-dog, or sometimes pariah dog, is a term used to describe an ownerless, half-wild, free-ranging dog that lives in or close to human settlements throughout Asia. The term is derived from the Sanskrit para, which translates to "outsider".
The United Kennel Club uses the term pariah dog to classify various breeds in a sighthound and pariah group.
Gallery | Pye-dog | Wikipedia | 82 | 1515866 | https://en.wikipedia.org/wiki/Pye-dog | Biology and health sciences | Dogs | Animals |
Thermodynamics is expressed by a mathematical framework of thermodynamic equations which relate various thermodynamic quantities and physical properties measured in a laboratory or production process. Thermodynamics is based on a fundamental set of postulates, that became the laws of thermodynamics.
Introduction
One of the fundamental thermodynamic equations is the description of thermodynamic work in analogy to mechanical work, or weight lifted through an elevation against gravity, as defined in 1824 by French physicist Sadi Carnot. Carnot used the phrase motive power for work. In the footnotes to his famous On the Motive Power of Fire, he states: “We use here the expression motive power to express the useful effect that a motor is capable of producing. This effect can always be likened to the elevation of a weight to a certain height. It has, as we know, as a measure, the product of the weight multiplied by the height to which it is raised.” With the inclusion of a unit of time in Carnot's definition, one arrives at the modern definition for power:
During the latter half of the 19th century, physicists such as Rudolf Clausius, Peter Guthrie Tait, and Willard Gibbs worked to develop the concept of a thermodynamic system and the correlative energetic laws which govern its associated processes. The equilibrium state of a thermodynamic system is described by specifying its "state". The state of a thermodynamic system is specified by a number of extensive quantities, the most familiar of which are volume, internal energy, and the amount of each constituent particle (particle numbers). Extensive parameters are properties of the entire system, as contrasted with intensive parameters which can be defined at a single point, such as temperature and pressure. The extensive parameters (except entropy) are generally conserved in some way as long as the system is "insulated" to changes to that parameter from the outside. The truth of this statement for volume is trivial, for particles one might say that the total particle number of each atomic element is conserved. In the case of energy, the statement of the conservation of energy is known as the first law of thermodynamics. | Thermodynamic equations | Wikipedia | 456 | 1515898 | https://en.wikipedia.org/wiki/Thermodynamic%20equations | Physical sciences | Thermodynamics | Physics |
A thermodynamic system is in equilibrium when it is no longer changing in time. This may happen in a very short time, or it may happen with glacial slowness. A thermodynamic system may be composed of many subsystems which may or may not be "insulated" from each other with respect to the various extensive quantities. If we have a thermodynamic system in equilibrium in which we relax some of its constraints, it will move to a new equilibrium state. The thermodynamic parameters may now be thought of as variables and the state may be thought of as a particular point in a space of thermodynamic parameters. The change in the state of the system can be seen as a path in this state space. This change is called a thermodynamic process. Thermodynamic equations are now used to express the relationships between the state parameters at these different equilibrium state.
The concept which governs the path that a thermodynamic system traces in state space as it goes from one equilibrium state to another is that of entropy. The entropy is first viewed as an extensive function of all of the extensive thermodynamic parameters. If we have a thermodynamic system in equilibrium, and we release some of the extensive constraints on the system, there are many equilibrium states that it could move to consistent with the conservation of energy, volume, etc. The second law of thermodynamics specifies that the equilibrium state that it moves to is in fact the one with the greatest entropy. Once we know the entropy as a function of the extensive variables of the system, we will be able to predict the final equilibrium state.
Notation
Some of the most common thermodynamic quantities are:
The conjugate variable pairs are the fundamental state variables used to formulate the thermodynamic functions.
The most important thermodynamic potentials are the following functions:
Thermodynamic systems are typically affected by the following types of system interactions. The types under consideration are used to classify systems as open systems, closed systems, and isolated systems.
Common material properties determined from the thermodynamic functions are the following:
The following constants are constants that occur in many relationships due to the application of a standard system of units.
Laws of thermodynamics
The behavior of a thermodynamic system is summarized in the laws of Thermodynamics, which concisely are: | Thermodynamic equations | Wikipedia | 510 | 1515898 | https://en.wikipedia.org/wiki/Thermodynamic%20equations | Physical sciences | Thermodynamics | Physics |
Zeroth law of thermodynamics
If A, B, C are thermodynamic systems such that A is in thermal equilibrium with B and B is in thermal equilibrium with C, then A is in thermal equilibrium with C.
The zeroth law is of importance in thermometry, because it implies the existence of temperature scales. In practice, C is a thermometer, and the zeroth law says that systems that are in thermodynamic equilibrium with each other have the same temperature. The law was actually the last of the laws to be formulated.
First law of thermodynamics
where is the infinitesimal increase in internal energy of the system, is the infinitesimal heat flow into the system, and is the infinitesimal work done by the system.
The first law is the law of conservation of energy. The symbol instead of the plain d, originated in the work of German mathematician Carl Gottfried Neumann and is used to denote an inexact differential and to indicate that Q and W are path-dependent (i.e., they are not state functions). In some fields such as physical chemistry, positive work is conventionally considered work done on the system rather than by the system, and the law is expressed as .
Second law of thermodynamics
The entropy of an isolated system never decreases: for an isolated system.
A concept related to the second law which is important in thermodynamics is that of reversibility. A process within a given isolated system is said to be reversible if throughout the process the entropy never increases (i.e. the entropy remains unchanged).
Third law of thermodynamics
when
The third law of thermodynamics states that at the absolute zero of temperature, the entropy is zero for a perfect crystalline structure.
Onsager reciprocal relations – sometimes called the Fourth law of thermodynamics
The fourth law of thermodynamics is not yet an agreed upon law (many supposed variations exist); historically, however, the Onsager reciprocal relations have been frequently referred to as the fourth law.
The fundamental equation | Thermodynamic equations | Wikipedia | 437 | 1515898 | https://en.wikipedia.org/wiki/Thermodynamic%20equations | Physical sciences | Thermodynamics | Physics |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.