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we consider the off-lattice two-dimensional q-state active clock model (acm) as a natural discretization of the vicsek model (vm) describing flocking. the acm consists of particles able to move in the plane in a discrete set of q equidistant angular directions, as in the active potts model (apm), with an alignment interaction inspired by the ferromagnetic equilibrium clock model. we find that for a small number of directions, the flocking transition of the acm has the same phenomenology as the apm, including macrophase separation and reorientation transition. for a larger number of directions, the flocking transition in the acm becomes equivalent to the one of the vm and displays microphase separation and only transverse bands, i.e., no re-orientation transition. concomitantly also the transition of the q → ∞ limit of the acm, the active xy model (axym), is in the same universality class as the vm. we also construct a coarse-grained hydrodynamic description for the acm and axym akin to the vm. | polar flocks with discretized directions: the active clock model approaching the vicsek model |
asteroid (101955) bennu, a near-earth object with a primitive carbonaceous chondrite-like composition, was observed by the origins, spectral interpretation, resource identification, and security-regolith explorer (osiris-rex) spacecraft to undergo multiple particle ejection events near perihelion between december 2018 and february 2019. the three largest events observed during this period, which all occurred 3.5 to 6 hr after local noon, placed numerous particles <10 cm on temporary orbits around bennu. here we examine whether these events could have been produced by sporadic meteoroid impacts using the national aeronautics and space administration's (nasa) meteoroid engineering model 3.0. most projectiles that impact bennu come from nearly isotropic or jupiter-family comets and have evolved toward the sun by poynting-robertson drag. we find that 7,000-j impacts on bennu occur with a biweekly cadence near perihelion, with a preference to strike in the late afternoon (~6 pm local time). this timing matches observations. crater scaling laws also indicate that these impact energies can reproduce the sizes and masses of the largest observed particles, provided the surface has the cohesive properties of weak, porous materials. bennu's ejection events could be caused by the same kinds of meteoroid impacts that created the moon's asymmetric debris cloud observed by the lunar atmosphere and dust environment explorer (ladee). our findings also suggest that fewer ejection events should take place as bennu moves further away from the sun, a result that can be tested with future observations. | meteoroid impacts as a source of bennu's particle ejection events |
the solar system currently possesses two remnant debris disks leftover from the planetary formation era in the form of the asteroid belt and the edgeworth-kuiper belt (ekb). similar to other stellar systems, these debris disks continually generate submillimeter-sized dust grains through processes such as mutual collisions, interstellar dust grain bombardment, and sublimation/sputtering of larger grains. here, we use recent in situ measurements by the new horizons student dust counter and an interplanetary dust dynamics model to constrain the overall structure and magnitude of the solar system’s debris disk, including the disk mass, optical depth, and surface brightness in both scattered light and thermal emission. we find that ∼99% of the solar system’s dust disk mass (grains with diameter <1 mm) is contained within ekb and oort cloud cometary grains outside of 30 au, with the remaining ∼1% mass in the form of jupiter-family cometary dust within 5 au. the total disk mass is estimated to be ∼8 × 10-7 m ⊕ with a total fractional luminosity of ∼5 × 10-7, confirming our solar system as a relatively dust-poor system compared to debris disks around similar-aged fgk stars. finally, we estimate that kuiper belt object collisional events such as that which created the haumea family could transiently increase the current surface brightness of our debris disk by a factor of only ∼6, far less than median brightnesses seen in other nearby disks. this further supports the idea that the ekb has been largely depleted of its primordial mass relative to other stellar systems by instabilities triggered by planetary migration. | constraining the solar system's debris disk with in situ new horizons measurements from the edgeworth-kuiper belt |
we have analyzed the orbits and ablation characteristics in the atmosphere of 59 earth-impacting fireballs, produced by meteoroids 1 m in diameter or larger, described here as meter-scale. using heights at peak luminosity as a proxy for strength, we determine that there is roughly an order of magnitude spread in strengths of the population of meter-scale impactors at the earth. we use fireballs producing recovered meteorites and well documented fireballs from ground-based camera networks to calibrate our ablation model interpretation of the observed peak height of luminosity as a function of speed. the orbits and physical strength of these objects are consistent with the majority being asteroidal bodies originating from the inner main asteroid belt. this is in contrast to earlier suggestions by ceplecha (ceplecha, z. [1994]. astron. astrophys. 286, 967-970) that the majority of meter-tens of meter sized meteoroids are ;… cometary bodies of the weakest known structure;. we find a lower limit of ∼10-15% of our objects have a possible cometary (jupiter-family comet and/or halley-type comet) origin based on orbital characteristics alone. only half this number, however, also show evidence for weaker than average structure. two events, sumava and usg 20131121, have exceptionally high (relative to the remainder of the population) heights of peak brightness. these are physically most consistent with high microporosity objects, though both were on asteroidal-type orbits. we also find three events, including the oct 8, 2009 airburst near sulawesi, indonesia, which display comparatively low heights of peak brightness, consistent with strong monolithic stones or iron meteoroids. based on orbital similarity, we find a probable connection among several events in our population with the taurid meteoroid complex; no other major meteoroid streams show probable linkages to the orbits of our meter-scale population. our impactors cover almost four orders of magnitude in mass, but no trend in height of peak brightness as a function of mass is evident, suggesting no strong trend in strength with size for meter-scale impactors consistent with the results of popova et al. (popova, o.p. et al. [2011]. meteorit. planet. sci. 46, 1525-1550). | orbital and physical characteristics of meter-scale impactors from airburst observations |
the discovery of over 50 planets around evolved stars and more than 35 debris discs orbiting white dwarfs highlight the increasing need to understand small body evolution around both early and asymptotic giant branch (gb) stars. pebbles and asteroids are susceptible to strong accelerations from the intense luminosity and winds of gb stars. here, we establish equations that can model time-varying gb stellar radiation, wind drag and mass-loss. we derive the complete three-dimensional equations of motion in orbital elements due to (1) the epstein and stokes regimes of stellar wind drag, (2) poynting-robertson drag, and (3) the yarkovsky drift with seasonal and diurnal components. we prove through averaging that the potential secular eccentricity and inclination excitation due to yarkovsky drift can exceed that from poynting-robertson drag and radiation pressure by at least three orders of magnitude, possibly flinging asteroids which survive yorp spin-up into a widely dispersed cloud around the resulting white dwarf. the gb yarkovsky effect alone may change an asteroid's orbital eccentricity by 10 per cent in just 1 myr. damping perturbations from stellar wind drag can be just as extreme, but are strongly dependent on the highly uncertain local gas density and mean free path length. we conclude that gb radiative and wind effects must be considered when modelling the post-main-sequence evolution of bodies smaller than about 1000 km. | the orbital evolution of asteroids, pebbles and planets from giant branch stellar radiation and winds |
from near-earth asteroids to superluminous supernovae and gravitational wave counterparts, the zwicky transient facility will soon scan for transient phenomena, explain eric bellm and shrinivas kulkarni. | the unblinking eye on the sky |
the earth is impacted by 35-40 metre-scale objects every year. these meteoroids are the low-mass end of impactors that can do damage on the ground. despite this they are very poorly surveyed and characterized, too infrequent for ground-based fireball observation efforts, and too small to be efficiently detected by neo telescopic surveys whilst still in interplanetary space. we want to evaluate the suitability of different instruments for characterizing metre-scale impactors and where they come from. we use data collected over the first 3 yr of operation of the continent-scale desert fireball network, and compare results with other published results as well as orbital sensors. we find that although the orbital sensors have the advantage of using the entire planet as collecting area, there are several serious problems with the accuracy of the data, notably the reported velocity vector, which is key to getting an accurate pre-impact orbit and calculating meteorite fall positions. we also outline dynamic range issues that fireball networks face when observing large meteoroid entries. | observation of metre-scale impactors by the desert fireball network |
we present the results of melting experiments on a suite of carbonaceous and ordinary chondritic compositions (cv, cm, ci, h and ll) performed at low pressure (0.1-13.1 mpa) and over a range of oxygen fugacity (log fo2 - (log fo2)iw = -2.5 to -1 and +0.8, iw being the iron-wustite buffer). these experiments constrain the composition of partial melts (f = 5-25 wt.%) of chondritic planetesimals. most experiments (iw -2.5 to -1) were conducted in molybdenum-hafnium carbide pressure vessels, which prevented the loss of alkali elements from the melt. the results show that all planetesimals not significantly depleted in moderately volatile elements relative to the sun's photosphere (e.g. ci, h and ll compositions) produced low-degree melts (<15 wt.%) rich in sio2, al2o3 and alkali elements, regardless of the fo2. despite their high apparent viscosities (104-5 pa.s), such low-density partial melts (2400-2500 kg/m3) were mobilized and, upon crystallizing, formed rocks containing up to 80 vol.% of plagioclase an10-30 (i.e. oligoclase) such as the trachyandesite achondrites graves nunataks 06128/9, northwest africa 6698 and 11575, the almahata sitta clast alm-a, as well as smaller "albitic clasts" in polymict ureilites and "alkali-silica-rich" inclusions in non-magmatic iron meteorites. we suggest that silica- and alkali- rich melts were widespread in small bodies of the early solar system but that much evidence was erased by subsequent stages of melting and planetary accretion and differentiation. | widespread production of silica- and alkali-rich melts at the onset of planetesimal melting |
pulsar-timing analyses are sensitive to errors in the solar-system ephemerides (sses) that timing models utilize to estimate the location of the solar-system barycentre, the quasi-inertial reference frame to which all recorded pulse times-of-arrival are referred. any error in the sse will affect all pulsars, therefore pulsar timing arrays (ptas) are a suitable tool to search for such errors and impose independent constraints on relevant physical parameters. we employ the first data release of the international pulsar timing array to constrain the masses of the planet-moons systems and to search for possible unmodelled objects (umos) in the solar system. we employ 10 sses from two independent research groups, derive and compare mass constraints of planetary systems, and derive the first pta mass constraints on asteroid-belt objects. constraints on planetary-system masses have been improved by factors of up to 20 from the previous relevant study using the same assumptions, with the mass of the jovian system measured at 9.5479189(3) × 10-4 m⊙. the mass of the dwarf planet ceres is measured at 4.7(4) × 10-10 m⊙. we also present the first sensitivity curves using real data that place generic limits on the masses of umos, which can also be used as upper limits on the mass of putative exotic objects. for example, upper limits on dark-matter clumps are comparable to published limits using independent methods. while the constraints on planetary masses derived with all employed sses are consistent, we note and discuss differences in the associated timing residuals and umo sensitivity curves. | studying the solar system with the international pulsar timing array |
we report 491 new near-infrared spectroscopic measurements of 420 near-earth objects (neos) collected on the nasa infrared telescope facility as part of the mit-hawaii neo spectroscopic survey. these measurements were combined with previously published data from binzel et al. and bias-corrected to derive the intrinsic compositional distribution of the overall neo population, as well as of subpopulations coming from various escape routes (ers) in the asteroid belt and beyond. the resulting distributions reflect well the overall compositional gradient of the asteroid belt, with decreasing fractions of silicate-rich (s- and q-type) bodies and increasing fractions of carbonaceous (b-, c-, d- and p-type) bodies as a function of increasing er distance from the sun. the close compositional match between neos and their predicted source populations validates dynamical models used to identify ers and argues against any strong composition change with size in the asteroid belt between ~5 km and ~100 m. a notable exception comes from the overabundance of d-type neos from the 5:2j and, to a lesser extend, the 3:1j and ν 6 ers, hinting at the presence of a large population of small d-type asteroids in the main belt. alternatively, this excess may indicate preferential spectral evolution from d-type surfaces to c and p types as a consequence of space weathering, or point to the fact that d-type objects fragment more often than other spectral types in the neo space. no further evidence for the existence of collisional families in the main belt, below the detection limit of current main-belt surveys, was found in this work. | the debiased compositional distribution of mithneos: global match between the near-earth and main-belt asteroid populations, and excess of d-type near-earth objects |
in this work we identify spectral similarities between asteroids and meteorites. using spectral features such as absorption bands and spectral curvature, we identify spectral matches between 500 asteroid spectra and over 1,000 samples of relab meteorite spectra over visible plus near-infrared wavelengths (0.45- 2 . 5 μm). we reproduce and confirm many major and previously known meteorite-asteroid connections and find possible new, more rare or less-established connections. well-established connections include: ordinary chondrites with s-complex asteroids; pristine cm carbonaceous chondrites with ch-type asteroids and heated cms with c-type asteroids ; hed meteorites with v-types; enstatite chondrites with xc-type asteroids; cv meteorites with k-type asteroids; brachinites, pallasites and r chondrites with olivine-dominated a-type asteroids. in addition to the link between ordinary chondrite meteorites with s-complex asteroids, we find a trend from q, sq, s, sr to sv correlates with ll to h, with q-types matching predominately to l and ll ordinary chondrites, and sr and sv matching predominantly with l and h ordinary chondrites. we find ordinary chondrite samples that match to the x-complex. these are measurements of slabs and many are labeled as dark or black (shocked) ordinary chondrites. we find carbonaceous chondrite samples having spectral slopes large enough to match d-type asteroid spectra. we find in many cases the asteroid type to meteorite type links are not unique, for classes with and without distinct spectral features. while there are examples of dominant matches between an asteroid class and meteorite class that are well established, there are less common but still spectrally compatible matches between many asteroid types and meteorite types. this result emphasizes the diversity of asteroid and meteorite compositions and highlights the degeneracy of classification by spectral features alone requiring additional measurements to firmly establish asteroid-meteorite links. recent and upcoming spacecraft missions will shed light on the compositions of many of the asteroid classes, particularly those without diagnostic features, (c-, b-, x-, and d-types), with measurements of c-type ceres, c-type ryugu, b-type bennu, m-type psyche, and c-, p-, and d-types as part of the lucy mission. | connecting asteroids and meteorites with visible and near-infrared spectroscopy |
we have presented in this communication a new solving procedure for the dynamics of non-rigid asteroid rotation, considering the final spin state of rotation for a small celestial body (asteroid). the last condition means the ultimate absence of the applied external torques (including short-term effect from torques during collisions, long-term yorp effect, etc.). fundamental law of angular momentum conservation has been used for the aforementioned solving procedure. the system of euler equations for dynamics of non-rigid asteroid rotation has been explored with regard to the existence of an analytic way of presentation of the approximated solution. despite of various perturbations (such as collisions, yorp effect) which destabilize the rotation of asteroid via deviating from the current spin state, the inelastic (mainly, tidal) dissipation reduces kinetic energy of asteroid. so, evolution of the spinning asteroid should be resulting by the rotation about maximal-inertia axis with the proper spin state corresponding to minimal energy with a fixed angular momentum. basing on the aforesaid assumption (component k1 is supposed to be fluctuating near the given appropriate constant of the fixed angular momentum), we have obtained that 2-nd component k2 is the solution of appropriate riccati ordinary differential equation of 1-st order, whereas component k3 should be determined via expression for k2. | on the dynamics of non-rigid asteroid rotation |
in growing active matter systems, a large collection of engineered or living autonomous units metabolize free energy and create order at different length scales as they proliferate and migrate collectively. one such example is bacterial biofilms, surface-attached aggregates of bacterial cells embedded in an extracellular matrix that can exhibit community-scale orientational order. however, how bacterial growth coordinates with cell-surface interactions to create distinctive, long-range order during biofilm development remains elusive. here we report a collective cell reorientation cascade in growing vibrio cholerae biofilms that leads to a differentially ordered, spatiotemporally coupled core-rim structure reminiscent of a blooming aster. cell verticalization in the core leads to a pattern of differential growth that drives radial alignment of the cells in the rim, while the growing rim generates compressive stresses that expand the verticalized core. such self-patterning disappears in nonadherent mutants but can be restored through opto-manipulation of growth. agent-based simulations and two-phase active nematic modeling jointly reveal the strong interdependence of the driving forces underlying the differential ordering. our findings offer insight into the developmental processes that shape bacterial communities and provide ways to engineer phenotypes and functions in living active matter. | mechanical forces drive a reorientation cascade leading to biofilm self-patterning |
the solar power sail is an original japanese concept in which electric power is generated by thin-film solar cells attached on the solar sail membrane. japan aerospace exploration agency (jaxa) successfully demonstrated the world's first solar power sail technology through ikaros (interplanetary kite-craft accelerated by radiation of the sun) mission in 2010. ikaros demonstrated photon propulsion and power generation using thin-film solar cells during its interplanetary cruise. scaled up, solar power sails can generate enough power to drive high specific impulse ion thrusters in the outer planetary region. with this concept, we propose a landing or sample return mission to directly explore a jupiter trojan asteroid using solar power sail-craft okeanos (oversize kite-craft for exploration and astronautics in the outer solar system). after rendezvousing with a trojan asteroid, a lander separates from okeanos to collect samples, and perform in-situ analyses in three proposed mission sequences, including sending samples back to earth. this paper proposes a system design for okeanos and includes analyses of the latest mission. | solar power sail mission of okeanos |
osiris-rex is the first nasa mission to return a sample of an asteroid to earth. navigation and flight dynamics for the mission to acquire and return a sample of asteroid 101955 bennu establish many firsts for space exploration. these include relatively small orbital maneuvers that are precise to ∼1 mm/s, close-up operations in a captured orbit about an asteroid that is small in size and mass, and planning and orbit phasing to revisit the same spot on bennu in similar lighting conditions. after preliminary surveys and close approach flyovers of bennu, the sample site will be scientifically characterized and selected. a robotic shock-absorbing arm with an attached sample collection head mounted on the main spacecraft bus acquires the sample, requiring navigation to bennu's surface. a touch-and-go sample acquisition maneuver will result in the retrieval of at least 60 grams of regolith, and up to several kilograms. the flight activity concludes with a return cruise to earth and delivery of the sample return capsule (src) for landing and sample recovery at the utah test and training range (uttr). | osiris-rex flight dynamics and navigation design |
we present archival observations of main-belt asteroid (248370) 2005 qn173 (also designated 433p) that demonstrate this recently discovered active asteroid (a body with a dynamically asteroidal orbit displaying a tail or coma) has had at least one additional apparition of activity near perihelion during a prior orbit. we discovered evidence of this second activity epoch in an image captured 2016 july 22 with the decam on the 4 m blanco telescope at the cerro tololo inter-american observatory in chile. as of this writing, (248370) 2005 qn173 is just the eighth active asteroid demonstrated to undergo recurrent activity near perihelion. our analyses demonstrate (248370) 2005 qn173 is likely a member of the active asteroid subset known as main-belt comets, a group of objects that orbit in the main asteroid belt that exhibit activity that is specifically driven by sublimation. we implement an activity detection technique, wedge photometry, that has the potential to detect tails in images of solar system objects and quantify their agreement with computed antisolar and antimotion vectors normally associated with observed tail directions. we present a catalog and an image gallery of archival observations. the object will soon become unobservable as it passes behind the sun as seen from earth, and when it again becomes visible (late 2022) it will be farther than 3 au from the sun. our findings suggest (248370) 2005 qn173 is most active interior to 2.7 au (0.3 au from perihelion), so we encourage the community to observe and study this special object before 2021 december. | recurrent activity from active asteroid (248370) 2005 qn173: a main-belt comet |
in marine and freshwater oxygen-deficient zones, the remineralization of sinking organic matter from the photic zone is central to driving nitrogen loss. deep blooms of photosynthetic bacteria, which form the suboxic/anoxic chlorophyll maximum (acm), widespread in aquatic ecosystems, may also contribute to the local input of organic matter. yet, the influence of the acm on nitrogen and carbon cycling remains poorly understood. using a suite of stable isotope tracer experiments, we examined the transformation of nitrogen and carbon under an acm (comprising of chlorobiaceae and synechococcales) and a non-acm scenario in the anoxic zone of lake tanganyika. we find that the acm hosts a tight coupling of photo/litho-autotrophic and heterotrophic processes. in particular, the acm was a hotspot of organic matter remineralization that controlled an important supply of ammonium driving a nitrification-anammox coupling, and thereby played a key role in regulating nitrogen loss in the oxygen-deficient zone. | anoxic chlorophyll maximum enhances local organic matter remineralization and nitrogen loss in lake tanganyika |
potentially hazardous asteroids and comets have hit earth throughout its history, with catastrophic consequences in the case of the chicxulub impact. here we reexamine one of the mechanisms that allow an impact to have a global effect—the release of climate-active gases from sedimentary rocks. we use the sova hydrocode and model ejected materials for a sufficient time after impact to quantify the volume of gases that reach high enough altitudes (> 25 km) to have global consequences. we vary impact angle, sediment thickness and porosity, water depth, and shock pressure for devolatilization and present the results in a dimensionless form so that the released gases can be estimated for any impact into a sedimentary target. using new constraints on the chicxulub impact angle and target composition, we estimate that 325 ± 130 gt of sulfur and 425 ± 160 gt co2 were ejected and produced severe changes to the global climate. | quantifying the release of climate-active gases by large meteorite impacts with a case study of chicxulub |
magnetite (fe3+(fe2+fe3+)2o4) is ubiquitous in earth and planetary materials, forming in igneous, metamorphic, and sedimentary settings, sometimes influenced by microbiology. magnetite can be used to study many and varied planetary processes, such as the oxidation state of magmas, paleomagnetism, water-rock interactions such as serpentinization, alteration and metamorphism occurring on meteorite parent bodies, and for astrobiology. the spectral reflectance signature of magnetite in the ultraviolet, visible, and near-infrared is somewhat unusual compared to common planetary materials, suggesting that remote detection and characterization of magnetite should be possible. here we present a systematic investigation of the reflectance spectral properties of magnetite using natural and synthetic samples. we investigate the effects of chemical substitutions, grain size variations, and mixtures with other phases in order to better constrain remote spectral searches for, and interpretation of, magnetite-bearing lithologies. magnetite is characterized by high extinction over the entire wavelength range considered here, and therefore surface scattering dominates over volume scattering. magnetite reflectance spectra are strongly influenced by the presence of delocalized electrons above the verwey transition temperature (∼120 k), leading to metal-like scattering behavior, that is, high extinction, surface scattering dominant, and a general increase in reflectance with increasing wavelength, i.e. "red-sloped and featureless". superimposed upon the metal-like reflectance are local reflectance maxima which we ascribe to fresnel reflectance peaks corresponding to fe-o oxygen-metal charge transfer processes (∼0.27 and ∼0.39 μm) and fe-related field-d orbital transitions (∼0.65 μm). we also find a systematic shift in the wavelength position of the 0.65 μm fresnel peak with increasing chemical impurity in magnetite. magnetite reflectance spectra are most similar to those of titanomagnetite and wüstite, and unlike those of other fe-(ti) oxides, such as ilmenite, hæmatite, ulvospinel, maghemite, pseudobrookite, and armalcolite. | spectral reflectance properties of magnetites: implications for remote sensing |
the source of water (h2o) and hydroxyl radicals (oh), identified on the lunar surface, represents a fundamental, unsolved puzzle. the interaction of solar-wind protons with silicates and oxides has been proposed as a key mechanism, but laboratory experiments yield conflicting results that suggest that proton implantation alone is insufficient to generate and liberate water. here, we demonstrate in laboratory simulation experiments combined with imaging studies that water can be efficiently generated and released through rapid energetic heating like micrometeorite impacts into anhydrous silicates implanted with solar-wind protons. these synergistic effects of solar-wind protons and micrometeorites liberate water at mineral temperatures from 10 to 300 k via vesicles, thus providing evidence of a key mechanism to synthesize water in silicates and advancing our understanding on the origin of water as detected on the moon and other airless bodies in our solar system such as mercury and asteroids. | untangling the formation and liberation of water in the lunar regolith |
the delivery of water to the inner solar system, including earth, is still a debated topic. a preferential role for hydrated asteroids in this process is supported by isotopic measurements. carbonaceous chondrite (cc) meteorites represent our main source of information about these volatile-rich asteroids. however, the destruction of weaker materials during atmospheric entry creates a bias in our cc data. the return of surface materials from the c-type asteroid 162173 ryugu by the hayabusa2 spacecraft provides a unique opportunity to study high-porosity, low-density, primitive materials, unrepresented in the meteorite record. we measured the bulk oxygen isotope composition from four ryugu particles and show that they most closely resemble the rare ci (cc ivuna-type) chondrites, but with some differences that we attribute to the terrestrial contamination of the ci meteorites. we suggest that ci-related material is widespread among carbonaceous asteroids and a more important source of earth's water and other volatiles than its limited presence in our meteoritic collection indicates. | oxygen isotope evidence from ryugu samples for early water delivery to earth by ci chondrites |
this work explores the target selection and trajectory design of the mission candidate for isas/jaxa's small science satellite series, destiny plus or destiny+. this mission combines unique aspects of the latest satellite technology and exploration of transition bodies to fill a technical and scientific gap in the japanese space science program. the spacecraft is targeted to study the comet-asteroid transition body (3200) phaethon through a combination of low-thrust propulsion and earth gravity assist. the trajectory design concept is presented in details together with the launch window and flyby date analysis. alternative targets for a possible mission extension scenario are also explored. | destiny+ trajectory design to (3200) phaethon |
asteroid (162173) ryugu is the target object of hayabusa2, an asteroid exploration and sample return mission led by japan aerospace exploration agency (jaxa). ground-based observations indicate that ryugu is a c-type near-earth asteroid with a diameter of less than 1 km, but the knowledge of its detailed properties is very limited prior to hayabusa2 observation. this paper summarizes our best understanding of the physical and dynamical properties of ryugu based on ground-based remote sensing and theoretical modeling before the hayabusa2's arrival at the asteroid. this information is used to construct a design reference model of the asteroid that is used for the formulation of mission operation plans in advance of asteroid arrival. particular attention is given to the surface properties of ryugu that are relevant to sample acquisition. this reference model helps readers to appropriately interpret the data that will be directly obtained by hayabusa2 and promotes scientific studies not only for ryugu itself and other small bodies but also for the solar system evolution that small bodies shed light on. [figure not available: see fulltext.] | asteroid ryugu before the hayabusa2 encounter |
in the last decades, the necessity to process massive amounts of textual data fueled the development of compressed text indexes: data structures efficiently answering queries on a given text while occupying space proportional to the compressed representation of the text. a widespread phenomenon in compressed indexing is that more powerful queries require larger indexes. for example, random access, the most basic query, can be supported in $o(\delta\log\frac{n\log\sigma}{\delta\log n})$ space (where $n$ is the text length, $\sigma$ is the alphabet size, and $\delta$ is text's substring complexity), which is the asymptotically smallest space to represent a string, for all $n$, $\sigma$, and $\delta$ (kociumaka, navarro, prezza; ieee trans. inf. theory 2023). the other end of the hierarchy is occupied by indexes supporting the powerful suffix array (sa) queries. the currently smallest one takes $o(r\log\frac{n}{r})$ space, where $r\geq\delta$ is the number of runs in the bwt of the text (gagie, navarro, prezza; j. acm 2020). we present a new compressed index that needs only $o(\delta\log\frac{n\log\sigma}{\delta\log n})$ space to support sa functionality in $o(\log^{4+\epsilon} n)$ time. this collapses the hierarchy of compressed data structures into a single point: the space required to represent the text is simultaneously sufficient for efficient sa queries. our result immediately improves the space complexity of dozens of algorithms, which can now be executed in optimal compressed space. in addition, we show how to construct our index in $o(\delta\text{ polylog } n)$ time from the lz77 parsing of the text. for highly repetitive texts, this is up to exponentially faster than the previously best algorithm. to obtain our results, we develop numerous techniques of independent interest, including the first $o(\delta\log\frac{n\log\sigma}{\delta\log n})$-size index for lce queries. | collapsing the hierarchy of compressed data structures: suffix arrays in optimal compressed space |
a class of graphs $\mathscr{c}$ is monadically stable if for any unary expansion $\widehat{\mathscr{c}}$ of $\mathscr{c}$, one cannot interpret, in first-order logic, arbitrarily long linear orders in graphs from $\widehat{\mathscr{c}}$. it is known that nowhere dense graph classes are monadically stable; these encompass most of the studied concepts of sparsity in graphs, including graph classes that exclude a fixed topological minor. on the other hand, monadic stability is a property expressed in purely model-theoretic terms and hence it is also suited for capturing structure in dense graphs. for several years, it has been suspected that one can create a structure theory for monadically stable graph classes that mirrors the theory of nowhere dense graph classes in the dense setting. in this work we provide a step in this direction by giving a characterization of monadic stability through the flipper game: a game on a graph played by flipper, who in each round can complement the edge relation between any pair of vertex subsets, and connector, who in each round localizes the game to a ball of bounded radius. this is an analog of the splitter game, which characterizes nowhere dense classes of graphs (grohe, kreutzer, and siebertz, j.acm'17). we give two different proofs of our main result. the first proof uses tools from model theory, and it exposes an additional property of monadically stable graph classes that is close in spirit to definability of types. also, as a byproduct, we give an alternative proof of the recent result of braunfeld and laskowski (arxiv 2209.05120) that monadic stability for graph classes coincides with existential monadic stability. the second proof relies on the recently introduced notion of flip-wideness (dreier, mählmann, siebertz, and toruńczyk, arxiv 2206.13765) and provides an efficient algorithm to compute flipper's moves in a winning strategy. | flipper games for monadically stable graph classes |
(10) hygiea is the fourth largest main belt asteroid and the only known asteroid whose surface composition appears similar to that of the dwarf planet (1) ceres1,2, suggesting a similar origin for these two objects. hygiea suffered a giant impact more than 2 gyr ago3 that is at the origin of one of the largest asteroid families. however, hygeia has never been observed with sufficiently high resolution to resolve the details of its surface or to constrain its size and shape. here, we report high-angular-resolution imaging observations of hygiea with the vlt/sphere instrument (~20 mas at 600 nm) that reveal a basin-free nearly spherical shape with a volume-equivalent radius of 217 ± 7 km, implying a density of 1,944 ± 250 kg m-3 to 1σ. in addition, we have determined a new rotation period for hygiea of ~13.8 h, which is half the currently accepted value. numerical simulations of the family-forming event show that hygiea's spherical shape and family can be explained by a collision with a large projectile (diameter ~75-150 km). by comparing hygiea's sphericity with that of other solar system objects, it appears that hygiea is nearly as spherical as ceres, opening up the possibility for this object to be reclassified as a dwarf planet. | a basin-free spherical shape as an outcome of a giant impact on asteroid hygiea |
many isolated white dwarfs (wds) show spectral evidence of atmospheric metal pollution. since heavy-element sedimentation timescales are short, this most likely indicates ongoing accretion. accreted metals encounter a variety of mixing processes at the wd surface: convection, gravitational sedimentation, overshoot, and thermohaline instability. we present mesa wd models that explore each of these processes and their implications for inferred accretion rates. we provide diffusion timescales for many individual metals, and we quantify the regimes in which thermohaline mixing dominates over gravitational sedimentation in setting the effective settling rate of the heavy elements. we build upon and confirm earlier work finding that accretion rates as high as {10}13 {{g}} {{{s}}}-1 are needed to explain the observed pollution in da wds for t eff > 15,000 k, and we provide tabulated results from our models that enable accretion rate inferences from observations of polluted da wds. if these rates are representative of young wds, we estimate that the total mass of planetesimal material accreted over a wd lifetime may be as high as {10}28 {{g}}, though this estimate is susceptible to potential selection biases and uncertainties about the nature of disk processes that supply accretion to the wd surface. we also find that polluted db wds experience much less thermohaline mixing than da wds, and we do not expect thermohaline instability to be active for polluted db wds with {t}eff}< {{18,000}} {{k}}. | polluted white dwarfs: mixing regions and diffusion timescales |
we address the expressions for the rates of the keplerian orbital elements within a two-body problem perturbed by the tides in both partners. formulae for these rates appeared in the literature in various forms, at times with errors. we reconsider, from scratch, the derivation of these rates and arrive at the lagrange-type equations which, in some details, differ from the corresponding equations obtained previously by kaula (rev geophys 2:661-684, 1964). we also write down detailed expressions for {d}a{/}{d}t, {d}e{/}{d}t and {d}i{/}{d}t, to order e^4. they differ from kaula's expressions which contain a redundant factor of m{/}(m+m^{ ' }), with m and m^{ ' } being the masses of the primary and the secondary. as kaula was interested in the earth-moon system, this redundant factor was close to unity and was unimportant in his developments. this factor, however, must be removed when kaula's theory is applied to a binary composed of partners of comparable masses. we have found that while it is legitimate to simply sum the primary's and secondary's inputs in {d}a{/}{d}t or {d}e{/}{d}t, this is not the case for {d}i{/}{d}t. so our expression for {d}i{/}{d}t differs from that of kaula in two regards. first, the contribution due to the dissipation in the secondary averages out when the apsidal precession is uniform. second, we have obtained an additional term which emerges owing to the conservation of the angular momentum: a change in the inclination of the orbit causes a change of the primary's plane of equator. | tidal evolution of the keplerian elements |
the near-earth objects (neos) (3200) phaethon and (155140) 2005 ud are thought to share a common origin, with the former exhibiting dust activity at perihelion that is thought to directly supply the geminid meteor stream. both of these objects currently have very small perihelion distances (0 . 140 au and 0 . 163 au for phaethon and 2005 ud, respectively), which results in them having perihelion temperatures around 1000 k . a comparison between neo population models to discovery statistics suggests that low-perihelion objects are destroyed over time by a, possibly temperature-dependent, mechanism that is efficient at heliocentric distances less than 0 . 3 au . by implication, the current activity from phaethon is linked to the destruction mechanism of neos close to the sun. we model the past thermal characteristics of phaethon and 2005 ud using a combination of a thermophysical model (tpm) and orbital integrations of each object. temperature characteristics such as maximum daily temperature, maximum thermal gradient, and temperature at different depths are extracted from the model, which is run for a predefined set of semi-major axis and eccentricity values. next, dynamical integrations of orbital clones of phaethon and 2005 ud are used to estimate the past orbital elements of each object. these dynamical results are then combined with the temperature characteristics to model the past evolution of thermal characteristics such as maximum (and minimum) surface temperature and thermal gradient. the orbital histories of phaethon and 2005 ud are characterized by cyclic changes in e, resulting in perihelia values periodically shifting between present-day values and 0 . 3 au . currently, phaethon is experiencing relatively large degrees of heating when compared to the recent 20 , 000 yr . we find that the subsurface temperatures are too large over this timescale for water ice to be stable, unless actively supplied somehow. the near-surface thermal gradients strongly suggest that thermal fracturing may be very effective at breaking down and ejecting dust particles. observations by the destiny+ flyby mission will provide important constraints on the mechanics of dust-loss from phaethon and, potentially, reveal signs of activity from 2005 ud. in addition to simulating the recent dynamical evolution of these objects, we use orbital integrations that start from the main belt to assess their early dynamical evolution (origin and delivery mechanism). we find that dwarf planet (2) pallas is unlikely to be the parent body for phaethon and 2005 ud, and it is more likely that the source is in the inner part of the asteroid belt in the families of, e.g., (329) svea or (142) polana. | dynamical evolution and thermal history of asteroids (3200) phaethon and (155140) 2005 ud |
this paper shows that $k_t$-minor-free (and $k_{s, t}$-minor-free) graphs $g$ are subgraphs of products of a tree-like graph $h$ (of bounded treewidth) and a complete graph $k_m$. our results include optimal bounds on the treewidth of $h$ and optimal bounds (to within a constant factor) on $m$ in terms of the number of vertices of $g$ and the treewidth of $g$. these results follow from a more general theorem whose corollaries include a strengthening of the celebrated separator theorem of alon, seymour, and thomas [j. amer. math. soc. 1990] and the planar graph product structure theorem of dujmović et al. [j. acm 2020]. | product structure of graphs with an excluded minor |
wd 1145+017 was observed from 2015 november to 2016 july, for the purpose of characterizing transit behaviour of the white dwarf by dust clouds thought to be produced by fragments of an asteroid in close orbit with the star. fortuitously, most of these observations were carried out during a time when the overall 'dip' activity was dramatically enhanced over that during its discovery with kepler k2. by the end of our reported observations, the dip activity had declined to a level close to its k2 discovery state. three notable events were observed. in 2016 january, a large number of dust clouds appeared that had an orbital period of 4.4912 h, and this event also marked the end of a three-month interval dominated by the k2 'a' period. the second event was a 2016 april 21 appearance of four dip features with drift lines in a waterfall (date versus phase) diagram that diverged from their origin date, at a location away from the 'a' asteroid, and which lasted for two weeks. the third event was the sudden appearance of a dip feature with a period of 4.6064 h, which is essentially the same as the k2 'b' period. the evolution of dip shape, depth, and total fade amount provide constraints on dust production and loss mechanisms. collisions can account for the sudden appearance of dust clouds, and the sudden increase in dust amount, but another mechanism for continual dust production is also required. | wd 1145+017 photometric observations during eight months of high activity |
given the current transformative potential of research that sits at the intersection of deep learning (dl) and software engineering (se), an nsf-sponsored community workshop was conducted in co-location with the 34th ieee/acm international conference on automated software engineering (ase'19) in san diego, california. the goal of this workshop was to outline high priority areas for cross-cutting research. while a multitude of exciting directions for future work were identified, this report provides a general summary of the research areas representing the areas of highest priority which were discussed at the workshop. the intent of this report is to serve as a potential roadmap to guide future work that sits at the intersection of se & dl. | deep learning & software engineering: state of research and future directions |
sitian is an ambitious ground-based all-sky optical monitoring project, developed by the chinese academy of sciences. the concept is an integrated network of dozens of 1-m-class telescopes deployed partly in china and partly at various other sites around the world. the main science goals are the detection, identification and monitoring of optical transients (such as gravitational wave events, fast radio bursts, supernovae) on the largely unknown timescales of less than 1 day; sitian will also provide a treasure trove of data for studies of agn, quasars, variable stars, planets, asteroids, and microlensing events. to achieve those goals, sitian will scan at least 10,000 square deg of sky every 30 min, down to a detection limit of $v \approx 21$ mag. the scans will produce simultaneous light-curves in 3 optical bands. in addition, sitian will include at least three 4-m telescopes specifically allocated for follow-up spectroscopy of the most interesting targets. we plan to complete the installation of 72 telescopes by 2030 and start full scientific operations in 2032. | the sitian project |
a significant fraction of white dwarfs show metal lines indicative of pollution with planetary material but the accretion process remains poorly understood. the main aim of this paper is to produce a road-map illustrating several potential routes for white dwarf pollution and to link these paths to observational outcomes. our proposed main road begins with the tidal disruption of a scattered asteroid and the formation of a highly eccentric tidal disc with a wide range of fragment sizes. accretion of these fragments by poynting-robertson (pr) drag alone is too slow to explain the observed rates. instead, in the second stage, several processes including differential apsidal precession cause high-velocity collisions between the eccentric fragments. large asteroids produce more fragments when they disrupt, causing rapid grind-down and generating short and intense bursts of dust production, whereas smaller asteroids grind down over longer periods of time. in the final stage, the collisionally produced dust circularizes and accretes on to the white dwarf via drag forces. we show that optically thin dust accretion by pr drag produces large infrared (ir) excesses when the accretion rate exceeds 107 g s-1. we hypothesize that around white dwarfs accreting at a high rate, but with no detected ir excess, dust circularization requires enhanced drag - for instance due to the presence of gas near the disc's pericentre. | a road-map to white dwarf pollution: tidal disruption, eccentric grind-down, and dust accretion |
nasa's double asteroid redirection test (dart) mission will impact its target asteroid, dimorphos, at an oblique angle that will not be known prior to the impact. we computed isale-3d simulations of dart-like impacts on asteroid surfaces at different impact angles and found that the vertical momentum transfer efficiency, β, is similar for different impact angles, however, the imparted momentum is reduced as the impact angle decreases. it is expected that the momentum imparted from a 45∘ impact is reduced by up to 50% compared to a vertical impact. the direction of the ejected momentum is not normal to the surface, however it is observed to 'straighten up' with crater growth. isale-2d simulations of vertical impacts provide context for the isale-3d simulation results and show that the ejection angle varies with both target properties and with crater growth. while the ejection angle is relatively insensitive to the target porosity, it varies by up to 30∘ with target coefficient of internal friction. the simulation results presented in this paper can help constrain target properties from the dart crater ejecta cone, which will be imaged by the liciacube. the results presented here represent the basis for an empirical scaling relationship for oblique impacts and can be used as a framework to determine an analytical approximation of the vertical component of the ejecta momentum, β - 1 , given known target properties. | ejecta distribution and momentum transfer from oblique impacts on asteroid surfaces |
asteroid interiors play a key role in our understanding of asteroid formation and evolution. as no direct interior probing has been done yet, characterisation of asteroids' interiors relies on interpretations of external properties. here we show, by numerical simulations, that the top-shaped rubble-pile asteroid (101955) bennu's geophysical response to spinup is highly sensitive to its material strength. this allows us to infer bennu's interior properties and provide general implications for top-shaped rubble piles' structural evolution. we find that low-cohesion (≲0.78 pa at surface and ≲1.3 pa inside) and low-friction (friction angle ≲ 35∘) structures with several high-cohesion internal zones can consistently account for all the known geophysical characteristics of bennu and explain the absence of moons. furthermore, we reveal the underlying mechanisms that lead to different failure behaviours and identify the reconfiguration pathways of top-shaped asteroids as functions of their structural properties that either facilitate or prevent the formation of moons. | inferring interiors and structural history of top-shaped asteroids from external properties of asteroid (101955) bennu |
thermophysical models allow for improved constraints on the physical and thermal surface properties of asteroids beyond what can be inferred from more simple thermal modeling, provided that a sufficient number of observations is available. we present thermophysical modeling results of observations from the near-earth object wise (neowise) mission for two near-earth asteroids which are the targets of the destiny+ flyby mission: (3200) phaethon and (155140) 2005 ud. our model assumes a rotating, cratered, spherical surface, and employs a monte carlo markov chain to explore the multidimensional parameter space of the fit. we find an effective spherical diameter for phaethon of {4.6}-0.3+0.2 km, a geometric albedo of pv= 0.16 ± 0.02, and a thermal inertia γ = 880 {}-330+580, using five epochs of neowise observations. the best model fit for (155140) 2005 ud was less well constrained due to only having two neowise observation epochs, giving a diameter of 1.2 ± 0.4 km and a geometric albedo of pv= 0.14 ± 0.09. | thermophysical modeling of neowise observations of destiny+ targets phaethon and 2005 ud |
in this paper we update, extend, and improve upon the recent paper on near-earth asteroid (nea) population by harris and d'abramo (2015). we update the population estimate taking into account discoveries to august 3, 2020. shortly after the previous paper was published, we identified a problem in our previous studies due to rounding off of absolute magnitude h by the minor planet center to 0.1 magnitude that implicitly shifted our bin boundaries by 0.05 magnitude. here we correct the problem by choosing h bin boundaries at 0.25-0.75 magnitude, rather than 0.00-0.50 magnitude thereby eliminating the round-off shift. we also introduce an updated model distribution of nea orbits (granvik et al. 2018) in our survey simulations. this new population model includes orbital distributions as a function of size, allowing us to test our presumption that distributions are homologous with respect to size. the change in distribution of orbits between that used by harris and d'abramo (2015) and the newer granvik et al. (2018) distributions was substantially greater than the range over size given in the latter, but made almost no difference in population estimates, suggesting that the re-detection algorithm is robust and does not depend strongly on the detailed orbit distribution, and also validates our presumption of homologous distribution over size. for the current analysis, we have added atlas and zwicky transient facility (ztf) to our analysis. we have separately analyzed the re-detection ratio versus size (h magnitude) for each of four surveys (catalina + mt. lemmon, pan-starrs, atlas and ztf) to investigate any differences between the various systems and cadences, and found none of significance, thus again, the re-detection algorithm appears very robust and mostly independent of survey parameters. with these changes and more recent discovery statistics to 2020, we estimate the number of neas of absolute magnitude h < 17.75 (nominally d > 1 km) to be 940 ± 10. following the updated population estimate, we examine more closely the rates of discovery of the largest neas (absolute magnitude h < 17.75, nominally diameter d > 1 km) to make a more detailed estimate of the numbers of these large objects that remain undiscovered. lastly we use the distribution of albedos of neas from neowise observations (mainzer et al. 2011) to transform our population estimate from number versus h, n(<h), to number versus d, n(>d). unfortunately this transformation depends substantially on only minor uncertainties in the albedo distribution, so n(>d) remains much less certain than n(<h). | the population of near-earth asteroids revisited and updated |
ancient layers of impact spherules provide a record of earth's early bombardment history. here, we compare different bombardment histories to the spherule layer record and show that 3.2-3.5 ga the flux of large impactors (10-100 km in diameter) was likely 20-40 times higher than today. the e-belt model of early solar system dynamics suggests that an increased impactor flux during the archean is the result of the destabilization of an inward extension of the main asteroid belt (bottke et al., 2012). here, we find that the nominal flux predicted by the e-belt model is 7-19 times too low to explain the spherule layer record. moreover, rather than making most lunar basins younger than 4.1 gyr old, the nominal e-belt model, coupled with a corrected crater diameter scaling law, only produces two lunar basins larger than 300 km in diameter. we also show that the spherule layer record when coupled with the lunar cratering record and careful consideration of crater scaling laws can constrain the size distribution of ancient terrestrial impactors. the preferred population is main-belt-like up to ∼50 km in diameter transitioning to a steep distribution going to larger sizes. | spherule layers, crater scaling laws, and the population of ancient terrestrial impactors |
we present imaging and spectroscopic observations of 6478 gault, a ∼6 km diameter inner main-belt asteroid currently exhibiting strong, comet-like characteristics. three distinct tails indicate that ultra-slow dust (ejection speed 0.15 ± 0.05 m s-1) was emitted from gault in separate episodes beginning ut 2018 october 28 ± 5 (tail a), ut 2018 december 31 ± 5 (tail b), and ut 2019 february 10 ± 7 (tail c), with durations of δt ∼ 10-20 days. with a mean particle radius \overline{a} ∼ 200 μm, the estimated masses of the tails are ma∼ 4 × 107 kg, mb∼ 6 × 106 kg, and mc∼ 6 × 105 kg, respectively, and the mass-loss rates from the nucleus are 20-40 kg s-1 for tail a, 4-6 kg s-1 for tail b, and ∼0.4 kg s-1 for tail c. in its optical colors gault is more similar to c-type asteroids than to s-types, even though the latter are numerically dominant in the inner asteroid belt. a spectroscopic upper limit to the production of gas is set at 1 kg s-1. discrete emission in three protracted episodes effectively rules out an impact origin for the observed activity. sublimation driven activity is unlikely given the inner-belt orbit and the absence of detectable gas. in any case, sublimation would not easily account for the observed multiple ejections. the closest similarity is between gault and active asteroid 311p/(2013 p5), an object showing repeated but aperiodic ejections of dust over a 9 month period. while gault is 10 times larger than 311p/(2013 p5), and the relevant timescale for spin-up by radiation torques is ∼100 times longer, its properties are likewise most consistent with episodic emission from a body rotating near breakup. | episodically active asteroid 6478 gault |
the origin and evolution of solar system bodies, including water on the earth, have been discussed based on the assumption that the relevant ingredients were simply silicates and ices. however, large amounts of organic matter have been found in cometary and interplanetary dust, which are recognized as remnants of interstellar/precometary grains. precometary organic matter may therefore be a potential source of water; however, to date, there have been no experimental investigations into this possibility. here, we experimentally demonstrate that abundant water and oil are formed via the heating of a precometary-organic-matter analog under conditions appropriate for the parent bodies of meteorites inside the snow line. this implies that h2o ice is not required as the sole source of water on planetary bodies inside the snow line. further, we can explain the change in the oxidation state of the earth from an initially reduced state to a final oxidized state. our study also suggests that petroleum was present in the asteroids and is present in icy satellites and dwarf planets. | precometary organic matter: a hidden reservoir of water inside the snow line |
the analysis of the optimal control law that steers a solar sail-based spacecraft from a given initial condition toward a final target state is typically carried out using either indirect or direct approaches. both these methods are usually time-consuming and require good initial guesses of costates or state vector. this paper presents a procedure requiring minimum user-computer interaction to compute an approximate three-dimensional optimal trajectory using a shape-based approach. to that end, novel shaping functions are introduced to describe the time evolution of the spacecraft state vector. the optimization problem is solved using a genetic algorithm, in which a set of shape coefficients and the initial and final spacecraft position are computed while enforcing suitable constraints on the magnitude and direction of the propulsive acceleration vector. numerical simulations of transfers from earth to potentially hazardous asteroids show that this method provides good estimates of solar sail trajectories, which can be used as guesses for more refined direct optimization approaches. | shape-based approach for solar sail trajectory optimization |
anhydrous pyroxene-rich interplanetary dust particles (idps) have been proposed as surface analogs for about two-thirds of all c-complex asteroids. however, this suggestion appears to be inconsistent with the presence of hydrated silicates on the surfaces of some of these asteroids, including ceres. here, we report the presence of enstatite (pyroxene) on the surface of two c-type asteroids (ceres and eugenia) based on their spectral properties in the mid-infrared range. the presence of this component is particularly unexpected in the case of ceres, because most thermal evolution models predict a surface consisting of hydrated compounds only. the most plausible scenario is that ceres’ surface has been partially contaminated by exogenous enstatite-rich material, possibly coming from the beagle asteroid family. this scenario questions a similar origin for ceres and the remaining c-types, and it possibly supports recent results obtained by the dawn mission (nasa) that ceres may have formed in the very outer solar system. concerning the smaller d ∼ 200 km c-types such as eugenia, both their derived surface composition (enstatite and amorphous silicates) and low density (<1.5 g cm-3) suggest that these bodies accreted from the same building blocks, namely chondritic porous, pyroxene-rich idps and volatiles (mostly water ice), and that a significant volume fraction of these bodies has remained unaffected by hydrothermal activity likely implying a late accretion. in addition, their current heliocentric distance may best explain the presence or absence of water ice at their surfaces. finally, we raise the possibility that ci chondrites, tagish-lake-like material, or hydrated idps may be representative samples of the cores of these bodies. | different origins or different evolutions? decoding the spectral diversity among c-type asteroids |
’oumuamua was discovered passing through our solar system on a hyperbolic orbit. it presents an apparent contradiction, with colors similar to those of volatile-rich solar system bodies but with no visible outgassing or activity during its close approach to the sun. here, we show that this contradiction can be explained by the dynamics of planetesimal ejection by giant planets. we propose that ’oumuamua is an extinct fragment of a comet-like planetesimal born a planet-forming disk that also formed neptune- to jupiter-mass giant planets. on its pathway to ejection ’oumuamua’s parent body underwent a close encounter with a giant planet and was tidally disrupted into small pieces, similar to comet shoemaker-levy 9’s disruption after passing close to jupiter. we use dynamical simulations to show that 0.1%-1% of cometary planetesimals undergo disruptive encounters prior to ejection. rocky asteroidal planetesimals are unlikely to disrupt due to their higher densities. after disruption, the bulk of fragments undergo enough close passages to their host stars to lose their surface volatiles and become extinct. planetesimal fragments such as ’oumuamua contain little of the mass in the population of interstellar objects but dominate by number. our model makes predictions that will be tested in the coming decade by the large synoptic survey telescope. | interstellar object ’oumuamua as an extinct fragment of an ejected cometary planetesimal |
the most heavily polluted white dwarfs often show excess infrared radiation from circumstellar dust disks, which are modeled as a result of tidal disruption of extrasolar minor planets. interaction of dust, gas, and disintegrating objects can all contribute to the dynamical evolution of these dust disks. here, we report two infrared variable dusty white dwarfs, sdss j1228+1040 and g29-38. for sdss j1228+1040, compared to the first measurements in 2007, the irac [3.6] and [4.5] fluxes decreased by 20% before 2014 to a level also seen in the recent 2018 observations. for g29-38, the infrared flux of the 10 μm silicate emission feature became 10% stronger between 2004 and 2007, we explore several scenarios that could account for these changes, including tidal disruption events, perturbation from a companion, and runaway accretion. no satisfactory causes are found for the flux drop in sdss j1228+1040 due to the limited time coverage. continuous tidal disruption of small planetesimals could increase the mass of small grains and concurrently change the strength of the 10 μm feature of g29-38. dust disks around white dwarfs are actively evolving and we speculate that there could be different mechanisms responsible for the temporal changes of these disks. | infrared variability of two dusty white dwarfs |
to investigate the origin of fine-grained rims around chondrules (fgrs), we compared presolar grain abundances, elemental compositions and mineralogies in fine-grained interstitial matrix material and individual fgrs in the primitive co3.0 chondrites allan hills a77307, lapaz icefield 031117 and dominion range 08006. the observation of similar overall o-anomalous (∼155 ppm) and c-anomalous grain abundances (∼40 ppm) in all three co3.0 chondrites suggests that they all accreted from a nebular reservoir with similar presolar grain abundances. the presence of presolar silicate grains in fgrs combined with the observation of similar estimated porosity between interstitial matrix regions and fgrs in lap 031117 and alha77307, as well as the identification of a composite fgr (a small rimmed chondrule within a larger chondrule rim) in alha77307, all provide evidence for a formation of fgrs by accretion of dust grains onto freely-floating chondrules in the solar nebula before their aggregation into their parent body asteroids. our study also shows systematically lower abundances of presolar silicate grains in the fgrs than in the matrix regions of co3 chondrites, while the abundances of sic grains are the same in all areas, within errors. this trend differs from cr2 chondrites in which the presolar silicate abundances are higher in the fgrs than in the matrix, but similar to each other within 2σ errors. this observation combined with the identification of localized (micrometer-scaled) aqueous alteration in a fgr of lap 031117 suggests that the lower abundance of presolar silicates in fgrs reflects pre-accretionary aqueous alteration of the fine-grained material in the fgrs. this pre-accretionary alteration could be due to either hydration and heating of freely floating rimmed chondrules in icy regions of the solar nebula or melted water ice associated with 26al-related heating inside precursor planetesimals, followed by aggregation of fgrs into the co chondrite parent-body. | presolar silicates in the matrix and fine-grained rims around chondrules in primitive co3.0 chondrites: evidence for pre-accretionary aqueous alteration of the rims in the solar nebula |
over the past 3 decades, there has been an exponential increase in work done in the newly emerging field of matter at extreme states of deformation and compression. this accelerating progress is due to the confluence of new experimental facilities, experimental techniques, theory, and simulations. regimes of science hitherto thought out of reach in terrestrial settings are now being accessed routinely. high-pressure macroscopic states of matter are being experimentally studied on high-power lasers and pulsed power facilities, and next-generation light sources are probing the quantum response of matter at the atomic level. combined, this gives experimental access to the properties and dynamics of matter from femtoseconds to microseconds in time scale and from kilobars to gigabars in pressure. there are a multitude of new regimes of science that are now accessible in laboratory settings. examples include planetary formation dynamics, asteroid and meteor impact dynamics, space hardware response to hypervelocity dust and debris impacts, nuclear reactor component response to prolonged exposure to radiation damage, advanced research into light weight armor, capsule dynamics in inertial confinement fusion research, and the basic high energy density properties of matter. we review highlights and advances in this rapidly developing area of science and research. | from microjoules to megajoules and kilobars to gigabars: probing matter at extreme states of deformation |
an interplanetary trajectory optimization of the multi-asteroid rendezvous is investigated for a solar sail-based spacecraft. the optimal control problem is formulated using the calculus of variations and pontryagin's maximum principle and the first order necessary conditions for optimality of the problem are derived. to achieve the optimal solution, the indirect method transforms the optimal control problem into a multiple point boundary value problem and solves it via shooting method, and the pseudospectral method discretizes the state and control variables and solves the nlp problem. in this paper, a numerical method combining the global optimization algorithm and a nonlinear equation solver is proposed to get the optimal solution of the indirect method. additionally, costates are estimated to validate the optimality of the solution obtained via the pseudospectral method. finally, numerical examples are given to verify the effectiveness of the proposed methods. the results indicate that the optimal solutions to the problems with two and three targets can be solved using the proposed indirect method algorithm. as the number of visiting targets is more than three, the direct method will be more efficient. | solar sail trajectory optimization of multi-asteroid rendezvous mission |
in two recent papers published in mnras, namouni and morais claimed evidence for the interstellar origin of some small solar system bodies, including: (i) objects in retrograde co-orbital motion with the giant planets and (ii) the highly inclined centaurs. here, we discuss the flaws of those papers that invalidate the authors' conclusions. numerical simulations backwards in time are not representative of the past evolution of real bodies. instead, these simulations are only useful as a means to quantify the short dynamical lifetime of the considered bodies and the fast decay of their population. in light of this fast decay, if the observed bodies were the survivors of populations of objects captured from interstellar space in the early solar system, these populations should have been implausibly large (e.g. about 10 times the current main asteroid belt population for the retrograde co-orbital of jupiter). more likely, the observed objects are just transient members of a population that is maintained in quasi-steady state by a continuous flux of objects from some parent reservoir in the distant solar system. we identify in the halley-type comets and the oort cloud the most likely sources of retrograde co-orbitals and highly inclined centaurs. | no evidence for interstellar planetesimals trapped in the solar system |
during the planet formation process, billions of comets are created and ejected into interstellar space. the detection and characterization of such interstellar comets (ics) (also known as extra-solar planetesimals or extra-solar comets) would give us in situ information about the efficiency and properties of planet formation throughout the galaxy. however, no ics have ever been detected, despite the fact that their hyperbolic orbits would make them readily identifiable as unrelated to the solar system. moro-martín et al. have made a detailed and reasonable estimate of the properties of the ic population. we extend their estimates of detectability with a numerical model that allows us to consider “close” ics, e.g., those that come within the orbit of jupiter. we include several constraints on a “detectable” object that allow for realistic estimates of the frequency of detections expected from the large synoptic survey telescope (lsst) and other surveys. the influence of several of the assumed model parameters on the frequency of detections is explored in detail. based on the expectation from moro-martín et al., we expect that lsst will detect 0.001-10 ics during its nominal 10 year lifetime, with most of the uncertainty from the unknown number density of small (nuclei of ∼0.1-1 km) ics. both asteroid and comet cases are considered, where the latter includes various empirical prescriptions of brightening. using simulated lsst-like astrometric data, we study the problem of orbit determination for these bodies, finding that lsst could identify their orbits as hyperbolic and determine an ephemeris sufficiently accurate for follow-up in about 4-7 days. we give the hyperbolic orbital parameters of the most detectable ics. taking the results into consideration, we give recommendations to future searches for ics. | realistic detectability of close interstellar comets |
carbonaceous asteroids represent the principal source of water in the inner solar system and might correspond to the main contributors for the delivery of water to earth. hydrogen isotopes in water-bearing primitive meteorites, for example carbonaceous chondrites, constitute a unique tool for deciphering the sources of water reservoirs at the time of asteroid formation. however, fine-scale isotopic measurements are required to unravel the effects of parent-body processes on the pre-accretion isotopic distributions. here, we report in situ micrometre-scale analyses of hydrogen isotopes in six cm-type carbonaceous chondrites, revealing a dominant deuterium-poor water component (δd = -350 ± 40‰) mixed with deuterium-rich organic matter. we suggest that this deuterium-poor water corresponds to a ubiquitous water reservoir in the inner protoplanetary disk. a deuterium-rich water signature has been preserved in the least altered part of the paris chondrite (δdparis ≥ -69 ± 163‰) in hydrated phases possibly present in the cm rock before alteration. the presence of the deuterium-enriched water signature in paris might indicate that transfers of ice from the outer to the inner solar system were significant within the first million years of the history of the solar system. | a dual origin for water in carbonaceous asteroids revealed by cm chondrites |
the mission accessible near-earth object survey (manos) aims to observe and characterize small (mean absolute magnitude h ∼ 25 mag) near-earth objects (neos) that are accessible by spacecraft (mean δv ∼ 5.7 km s-1) and that make close approaches with the earth (mean minimum orbital intersection distance moid ∼ 0.03 au). we present here the first results of the manos visible spectroscopic survey. the spectra were obtained from august 2013 to march 2018 at lowell observatory’s discovery channel 4.3 m telescope, and both gemini north and south facilities. in total, 210 neos have been observed and taxonomically classified. our taxonomic distribution shows significant variations with respect to surveys of larger objects. we suspect these to be due to a dependence of main belt source regions on object size. compared to previous surveys of larger objects, we report a lower fraction of s+q-complex asteroids of 43.8 ± 4.6%. we associate this decrease with a lack of phocaea family members at very small size. we also report higher fractions of x-complex and a-type asteroids of 23.8 ± 3.3% and 3.8 ± 1.3% respectively due to an increase of hungaria family objects at small size. we find a strong correlation between the q/s ratio and perihelion distance. we suggest this correlation is due to planetary close encounters with venus playing a major role in turning asteroids from s to q-type. this hypothesis is supported by a similar correlation between the q/s ratio and venus moid. | visible spectroscopy from the mission accessible near-earth object survey (manos): taxonomic dependence on asteroid size |
the nasa double asteroid redirection test (dart) mission is a planetary defense-driven test of a kinetic impactor on dimorphos, the satellite of the binary asteroid 65803 didymos. dart will intercept dimorphos at a relative speed of ∼ 6 . 5km s-1 , perturbing dimorphos's orbital velocity and changing the binary orbital period. we present three independent methods (one analytic and two numerical) to investigate the post-impact attitude stability of dimorphos as a function of its axial ratios, a / b and b / c (a ≥ b ≥ c), and the momentum transfer efficiency β. the first method uses a novel analytic approach in which we assume a circular orbit and a point-mass primary that identifies four fundamental frequencies of motion corresponding to the secondary's mean motion, libration, precession, and nutation frequencies. at resonance locations among these four frequencies, we find that attitude instabilities are possible. using two independent numerical codes, we recover many of the resonances predicted by the analytic model and indeed show attitude instability. with one code, we use fast lyapunov indicators to show that the secondary's attitude can evolve chaotically near the resonance locations. then, using a high-fidelity numerical model, we find that dimorphos enters a chaotic tumbling state near the resonance locations and is especially prone to unstable rotation about its long axis, which can be confirmed by esa's hera mission arriving at didymos in late 2026. we also show that a fully coupled treatment of the spin and orbital evolution of both bodies is crucial to accurately model the long-term evolution of the secondary's spin state and libration amplitude. finally, we discuss the implications of a post-impact tumbling or rolling state, including the possibility of terminating byorp evolution if dimorphos is no longer in synchronous rotation. | the excited spin state of dimorphos resulting from the dart impact |
dujmović, joret, micek, morin, ueckerdt and wood [j. acm 2020] proved that for every planar graph $g$ there is a graph $h$ with treewidth at most 8 and a path $p$ such that $g\subseteq h\boxtimes p$. we improve this result by replacing "treewidth at most 8" by "simple treewidth at most 6". | an improved planar graph product structure theorem |
this paper summarizes a commercial asteroid mining architecture synthesized by the senior space design class at the university of washington in winter/spring quarters of 2013. the main author was the instructor for that class. these results use design-to-cost development methods and focused infrastructure advancements to identify and characterize a workable space industrialization architecture including space transportation elements, asteroid exploration and mining equipment, and the earth orbit infrastructure needed to make it all work. cost analysis predicts that for an initial investment in time and money equivalent to that for the us north slope oil field, the yearly world supply of platinum group metals could be increased by 50%, roughly 1500 t of lox/lh2 propellant/year would be available in leo, and very low cost solar panels could be assembled at geo using asteroidal materials. the investment also would have a discounted net present value return on investment of 22% over twenty years. | defining a successful commercial asteroid mining program |
japan's second asteroid explorer hayabusa2 was successfully launched on dec 3, 2014, to return a sample from asteroid 162173 ryugu by 2020. four xenon ion thrusters based on electron cyclotron resonance discharge propelled the spacecraft for 547 h during its first year in space. hayabusa2 completed an earth gravity assist on dec 3, 2015, followed by 798 and 2593 h of ion thruster operation, called the first and second transfer phases of delta-v, respectively. the third transfer phase of delta-v was conducted from jan 10, 2018, to jun 6, 2018, in which the final 2475-h ion thruster operation was executed before the rendezvous with ryugu. the cumulative operating times for the four ion thrusters are 6,450, 11, 5,193, and 6418 h. this paper summarizes the 6515-h powered flight by the ion engine system, which produced 1015 m/s delta-v, in terms of thruster performance change, roll torques generated by various combinations of ion thrusters, and spacecraft surface erosion history measured by two quartz crystal microbalances located near the thrusters. in parallel with the space flight operation, an engineering model of the microwave discharge neutralizer has been under long-duration testing on the ground since 2012. it has accumulated 55,170 h of diode-mode operation as of mar 15, 2019. | in-flight operation of the hayabusa2 ion engine system on its way to rendezvous with asteroid 162173 ryugu |
the geological map encapsulates basic information that can be crucial in a multitude of fields such as landslide risk assessment, engineering projects, as well as petroleum and mineral resources studies. in addition, it is difficult, expensive and time-consuming to achieve it in complex and inaccessible lands. however, remote sensing data linking and the application of machine learning algorithms (mlas) can be interesting for geological mapping of large areas, especially in arid and semi-arid regions, where remote sensing provides a diversified and detailed spatial database and mlas offer the possibility of effective and efficient classification of remotely sensed images. this article highlights the use of aster spectral data in a comparative approach of the performance of six (mlas) to better produce the geological map of a portion of the aït ahmane region. the results indicated an overall accuracy and a kappa coefficient that exceeded 60% for the different models. prioritizing the regularized discriminant analysis (rda) (kappa = 83.5%) and support vector machines (svm) (kappa = 81%) algorithms, they managed to classify the lithology on aster images of the region. however, the classification of lithology using the rda was slightly more accurate than the one obtained by svm with 2.3%. from the results shown, we can conclude that the ability of rda as a learning algorithm is the best for the geological mapping of our study site. | using remote sensing data for geological mapping in semi-arid environment: a machine learning approach |
this paper presents a study, through the use of a ssdem simulation code, of the possible disruption patterns and mechanisms of self-gravitating aggregates that are spun-up to the point of disruption. we do this survey by systematically changing the angle of friction and tensile stress of the aggregates. it is observed that the amount of deformation that takes place before disruption, as well as its onset, is directly related to the angle of friction. on the other hand, the change in tensile strength allows us to clearly observe a continuous transition from losing surface material to larger scale fission at higher spin rates before disruption, but in no case do we observe surface flow. these results are also compared to other simulation results and the observations of asteroids p/2013 r3, p/2013 p5, 1950 da, 1999 kw4 and geographos. additionally, we propose modifications to previously discussed mechanisms for the formation of binary asteroids and asteroid pairs. | disruption patterns of rotating self-gravitating aggregates: a survey on angle of friction and tensile strength |
context. the populations of small bodies of the solar system (asteroids, comets, and kuiper belt objects) are used to constrain the origin and evolution of the solar system. their orbital distribution and composition distribution are both required to track the dynamical pathway from their formation regions to their current locations.aims: we aim to increase the sample of solar system objects (ssos) that have multifilter photometry and compositional taxonomy.methods: we searched for moving objects in the skymapper southern survey. we used the predicted sso positions to extract photometry and astrometry from the skymapper frames. we then applied a suite of filters to clean the catalog from false-positive detections. we finally used the near-simultaneous photometry to assign a taxonomic class to objects.results: we release a catalog of 880 528 individual observations, consisting of 205 515 known and unique ssos. the catalog completeness is estimated to be about 97% down to v = 18 mag and the purity is higher than 95% for known ssos. the near-simultaneous photometry provides either three, two, or a single color that we use to classify 117 356 ssos with a scheme consistent with the widely used bus-demeo taxonomy.conclusions: the present catalog contributes significantly to the sample of asteroids with known surface properties (about 40% of main-belt asteroids down to an absolute magnitude of 16). we will release more observations of ssos with future skymapper data releases. the catalogs are only available at the cds via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/j/a+a/658/a109 | multifilter photometry of solar system objects from the skymapper southern survey |
over one-quarter of white dwarfs contain observable metallic debris from the break-up of exo-asteroids. understanding the physical and orbital history of this debris would enable us to self-consistently link planetary system formation and fate. one major debris reservoir is generated by yorp-induced rotational fission during the giant branch phases of stellar evolution, where the stellar luminosity can exceed the sun's by four orders of magnitude. here, we determine the efficacy of the giant branch yorp effect for asteroids with non-zero internal strength, and model post-fission evolution by imposing simple analytic fragmentation prescriptions. we find that even the highest realistic internal strengths cannot prevent the widespread fragmentation of asteroids and the production of a debris field over 100 au in size. we compute the number of successive fission events as they occur in progressively smaller time intervals as the star ascends the giant branches, providing a way to generate size distributions of asteroid fragments. the results are highly insensitive to progenitor stellar mass. we also conclude that the ease with which giant branch yorp break-up can generate binary asteroid subsystems is strongly dependent on internal strength. formed binary subsystems in turn could be short-lived due to the resulting luminosity-enhanced binary yorp (byorp) effect. | post-main-sequence debris from rotation-induced yorp break-up of small bodies - ii. multiple fissions, internal strengths, and binary production |
in this paper, four off-earth mining scenarios for water extraction have been designed to supply a h2/lox propellant market in low earth orbit. a non-financial measure for determining feasibility has been devised for the analysis based on the opportunity cost as opposed to the net present value. the first scenario for evaluation is a conventional strip mining operation in a southern lunar crater. an 'in-situ sublimation' method in the same location has been determined to be more competitive than strip mining due to equipment mass requirements. however, neither system can compete with direct launch of water from the earth based on the assumptions in this research. the third and fourth scenarios are based on a near earth dormant comet and also utilize in-situ sublimation techniques. these scenarios focus heavily on transport optimization. a single miner-hauler system was found to be inferior to the multi-craft system. it was also revealed that the dormant comet mining scenarios will not be competitive enough with lunar mining or direct launch from earth to supply a low earth orbit market. | technical evaluation of off-earth ice mining scenarios through an opportunity cost approach |
almahata sitta (ahs), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 tc3). however, correlating properties of the meteorite with those of the asteroid is not straightforward because the ahs stones are diverse types. of those studied prior to this work, 70-80% are ureilites (achondrites) and 20-30% are various types of chondrites. asteroid 2008 tc3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on earth as individual stones and most of its mass lost. we describe ahs 91a and ahs 671, which are the first ahs stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 tc3. ahs 91a and ahs 671 are friable breccias, consisting of a c1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal-sulfide, as well as chondrules ( 130-600 μm) and chondrule fragments. the c1 material consists of fine-grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (fo 28-42), an unidentified ca-rich silicate phase, fe,ni sulfides, and minor ca-phosphate and ilmenite. it has similarities to ci1 but shows evidence of heterogeneous thermal metamorphism. its bulk oxygen isotope composition (δ18o = 13.53‰, δ17o = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several cc-like clasts in typical polymict ureilites. its cr isotope composition is unlike that of any known meteorite. the enclosed clasts and chondrules do not belong to the c1 lithology. the olivine (fo 75-88), pyroxenes (pigeonite of wo 10 and orthopyroxene of wo 4.6), plagioclase, graphite, and some metal-sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. the chondrules are probably derived from type 3 oc and/or cc, based on mineral and oxygen isotope compositions. some of the metal-sulfide clasts are derived from ec. ahs 91a and ahs 671 are plausible representatives of the bulk of the asteroid that was lost. reflectance spectra of ahs 91a are dark (reflectance 0.04-0.05) and relatively featureless in vnir, and have an 2.7 μm absorption band due to oh- in phyllosilicates. spectral modeling, using mixtures of laboratory vnir reflectance spectra of ahs stones to fit the f-type spectrum of the asteroid, suggests that 2008 tc3 consisted mainly of ureilitic and ahs 91a-like materials, with as much as 40-70% of the latter, and <10% of oc, ec, and other meteorite types. the bulk density of ahs 91a (2.35 ± 0.05 g cm-3) is lower than bulk densities of other ahs stones, and closer to estimates for the asteroid ( 1.7-2.2 g cm-3). its porosity (36%) is near the low end of estimates for the asteroid (33-50%), suggesting significant macroporosity. the textures of ahs 91a and ahs 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 tc3 in a regolith environment. ahs 91a and ahs 671 could represent a volume of regolith formed when a cc-like body impacted into already well-gardened ureilitic + impactor-derived debris. ahs 91a bulk samples do not show a solar wind component, so they represent subsurface layers. ahs 91a has a lower cosmic ray exposure (cre) age ( 5-9 ma) than previously studied ahs stones (11-22 ma). the spread in cre ages argues for irradiation in a regolith environment. ahs 91a and ahs 671 show that ureilitic asteroids could have detectable 2.7 μm absorption bands. | the first samples from almahata sitta showing contacts between ureilitic and chondritic lithologies: implications for the structure and composition of asteroid 2008 tc3 |
the otherwise homogeneous surface of ceres is dotted with hundreds of anomalously bright, predominantly carbonate-bearing areas, termed "faculae," with bond albedos ranging from ∼0.02 to >0.5. here, we classify and map faculae globally to characterize their geological setting, assess potential mechanisms for their formation and destruction, and gain insight into the processes affecting the ceres surface and near-surface. faculae were found to occur in four distinct geological settings, associated predominantly with impact craters: (1) crater pits, peaks, or floor fractures (floor faculae), (2) crater rims or walls (rim/wall faculae), (3) bright ejecta blankets, and (4) the mountain ahuna mons. floor faculae were identified in eight large, deep, and geologically young (asteroid-derived model (adm) ages of <420 ± 60 ma) craters: occator, haulani, dantu, ikapati, urvara, gaue, ernutet, and azacca. the geometry and geomorphic features of the eight craters with floor faculae are consistent with facula formation via impact-induced heating and upwelling of volatile-rich materials, upwelling/excavation of heterogeneously distributed subsurface brines or their precipitation products, or a combination of both processes. rim/wall faculae and bright ejecta occur in and around hundreds of relatively young craters of all sizes, and the geometry of exposures is consistent with facula formation via the excavation of subsurface bright material, possibly from floor faculae that were previously emplaced and buried. a negative correlation between rim/wall facula albedo and crater age indicates that faculae darken over time. models using the ceres crater production function suggest initial production or exposure of faculae by large impacts, subsequent dissemination of facula materials to form additional small faculae, and then burial by impact-induced lateral mixing, which destroys faculae over timescales of less than 1.25 gyr. cumulatively, these models and the observation of faculae limited to geologically young craters indicate relatively modern formation or exposure of faculae, indicating that ceres' surface remains active and that the near surface may support brines in the present day. | the formation and evolution of bright spots on ceres |
photosynthesis (gross primary production, gpp) and evapotranspiration (et) are ecosystem processes with global significance for climate, the global carbon and hydrological cycles and a range of ecosystem services. the mechanisms governing these processes are complex but well understood. there is strong coupling between these processes, mediated directly by stomatal conductance and indirectly by root zone soil moisture content and its accessibility. this coupling must be effectively modelled for robust predictions of earth system responses to global change. yet, it is highly demanding to model leaf and cellular processes, like stomatal conductance or electron transport, with response times of minutes, over decadal and global domains. computational demand means models resolving this level of complexity cannot be easily evaluated for their parameter sensitivity nor calibrated using earth observation information through data assimilation approaches requiring large ensembles. to overcome these challenges, here we describe a coupled photosynthesis evapotranspiration model of intermediate complexity. the model reduces computational load and parameter numbers by operating at canopy scale and daily time step. through the inclusion of simplified representation of key process interactions, it retains sensitivity to variation in climate, leaf traits, soil states and atmospheric co2. the new model is calibrated to match the biophysical responses of a complex terrestrial ecosystem model (tem) of gpp and et through a bayesian model-data fusion framework. the calibrated acm-gpp-et generates unbiased estimates of tem gpp and et and captures 80 %-95 % of the sensitivity of carbon and water fluxes by the complex tem. the acm-gpp-et model operates 3 orders faster than the complex tem. independent evaluation of acm-gpp-et at fluxnet sites, using a single global parameterisation, shows good agreement, with typical r2∼0.60 for both gpp and et. this intermediate complexity modelling approach allows full monte carlo-based quantification of model parameter and structural uncertainties and global-scale sensitivity analyses for these processes and is fast enough for use within terrestrial ecosystem model-data fusion frameworks requiring large ensembles. | description and validation of an intermediate complexity model for ecosystem photosynthesis and evapotranspiration: acm-gpp-etv1 |
of the potential volatile sources for the terrestrial planets, the ci and cm carbonaceous chondrites are closest to the planets' bulk h and n isotopic compositions. for the earth, the addition of approximately 2-4 wt% of ci/cm material to a volatile-depleted proto-earth can explain the abundances of many of the most volatile elements, although some solar-like material is also required. two dynamical models of terrestrial planet formation predict that the carbonaceous chondrites formed either in the asteroid belt (`classical' model) or in the outer solar system (5-15 au in the grand tack model). to test these models, at present the h isotopes of water are the most promising indicators of formation location because they should have become increasingly d-rich with distance from the sun. the estimated initial h isotopic compositions of water accreted by the ci, cm, cr and tagish lake carbonaceous chondrites were much more d-poor than measured outer solar system objects. a similar pattern is seen for n isotopes. the d-poor compositions reflect incomplete re-equilibration with h2 in the inner solar system, which is also consistent with the o isotopes of chondritic water. on balance, it seems that the carbonaceous chondrites and their water did not form very far out in the disc, almost certainly not beyond the orbit of saturn when its moons formed (approx. 3-7 au in the grand tack model) and possibly close to where they are found today. this article is part of the themed issue 'the origin, history and role of water in the evolution of the inner solar system'. | the origin of inner solar system water |
the zag meteorite which is a thermally-metamorphosed h ordinary chondrite contains a primitive xenolithic clast that was accreted to the parent asteroid after metamorphism. the cm-sized clast contains abundant large organic grains or aggregates up to 20 μm in phyllosilicate-rich matrix. here we report organic and isotope analyses of a large ( 10 μm) om aggregate in the zag clast. the x-ray micro-spectroscopic technique revealed that the om aggregate has sp2 dominated hydrocarbon networks with a lower abundance of heteroatoms than in iom from primitive (ci,cm,cr) carbonaceous chondrites, and thus it is distinguished from most of the om in carbonaceous meteorites. the om aggregate has high d/h and 15n/14n ratios (δd = 2,370 ± 74‰ and δ15n = 696 ± 100‰), suggesting that it originated in a very cold environment such as the interstellar medium or outer region of the solar nebula, while the om is embedded in carbonate-bearing matrix resulting from aqueous activities. thus, the high d/h ratio must have been preserved during the extensive late-stage aqueous processing. it indicates that both the om precursors and the water had high d/h ratios. combined with 16o-poor nature of the clast, the om aggregate and the clast are unique among known chondrite groups. we further propose that the clast possibly originated from d/p type asteroids or trans-neptunian objects. | a novel organic-rich meteoritic clast from the outer solar system |
asteroids are fascinating worlds. considered the building blocks of our planets, many of the authors of this book have devoted their scientific careers to exploring them with the tools of our trade: ground- and spacebased observations, in situ space missions, and studies that run the gamut from theoretical modeling efforts to laboratory work. like fossils for paleontologists, or dna for geneticists, they allow us to construct a veritable time machine and provide us with tantalizing glimpses of the earliest nature of our solar system. by investigating them, we can probe what our home system was like before life or even the planets existed. the origin and evolution of life on our planet is also intertwined with asteroids in a different way. it is believed that impacts on the primordial earth may have delivered the basic components for life, with biology favoring attributes that could more easily survive the aftermath of such energetic events. in this fashion, asteroids may have banished many probable avenues for life to relative obscurity. similarly, they may have also prevented our biosphere from becoming more complex until more recent eras. the full tale of asteroid impacts on the history of our world, and how human life managed to emerge from myriad possibilities, has yet to be fully told. the hazard posed by asteroid impacts to our civilization is low but singular. the design of efficient mitigation strategies strongly relies on asteroid detection by our ground- and spacebased surveys as well as knowledge of their physical properties. a more positive motivation for asteroid discovery is that the proximity of some asteroids to earth may allow future astronauts to harvest their water and rare mineral resources for use in exploration. a key goal of asteroid science is therefore to learn how humans and robotic probes can interact with asteroids (and extract their materials) in an efficient way. we expect that these adventures may be commonplace in the future. asteroids, like planets, are driven by a great variety of both dynamical and physical mechanisms. in fact, images sent back by space missions show a collection of small worlds whose characteristics seem designed to overthrow our preconceived notions. given their wide range of sizes and surface compositions, it is clear that many formed in very different places and at different times within the solar nebula. these characteristics make them an exciting challenge for researchers who crave complex problems. the return of samples from these bodies may ultimately be needed to provide us with solutions. in the book asteroids iv, the editors and authors have taken major strides in the long journey toward a much deeper understanding of our fascinating planetary ancestors. this book reviews major advances in 43 chapters that have been written and reviewed by a team of more than 200 international authorities in asteroids. it is aimed to be as comprehensive as possible while also remaining accessible to students and researchers who are interested in learning about these small but nonetheless important worlds. we hope this volume will serve as a leading reference on the topic of asteroids for the decade to come. we are deeply indebted to the many authors and referees for their tremendous efforts in helping us create asteroids iv. we also thank the members of the asteroids iv scientific organizing committee for helping us shape the structure and content of the book. the conference associated with the book, "asteroids comets meteors 2014" held june 30-july 4, 2014, in helsinki, finland, did an outstanding job of demonstrating how much progress we have made in the field over the last decade. we are extremely grateful to our host karri muinonnen and his team. the editors are also grateful to the asteroids iv production staff, namely renée dotson and her colleagues at the lunar and planetary institute, for their efforts, their invaluable assistance, and their enthusiasm; they made life as easy and pleasant as possible for the editors, authors, and referees. they also thank richard binzel, the general editor of the space science series, for his strong support and advice during this process, as well as the staff at the university of arizona press. finally, editor patrick michel would like to thank his wife delphine, who married him on june 14, 2013, almost at the birth of the book process. he is grateful that she was willing to put up with him as he spent many of his nights and weekends working on the book. thanks to her support, their trajectories are as bounded as a perfectly stable asteroid binary system, and this was probably the best way to experience from the start what her life would be like with a researcher! co-editor bottke would also like to thank his wife veronica and his children kristina-marie, laura, and julie, who make up his own favorite asteroid family. since asteroids iii, the size distribution of the family members has been steadily changing, and who knows how many tiny new members it will contain by asteroids v! co-editor demeo would like to thank her husband alfredo for his support and encouragement throughout the process of creating this book. they met at the beginning of her career in research, becoming an asteroid pair and now continuing on the same orbit in life. | asteroids iv |
context. the sample of solar system objects has dramatically increased over the last decade. the number of measured properties (e.g., diameter, taxonomy, rotation period, thermal inertia, etc.) has expanded even more quickly. however, this wealth of information is spread over a myriad of studies, with different designations reported per object.aims: we provide a solution to the identification of solar system objects based on any of their multiple names or designations. we also compile and rationalize their properties to provide an easy access to them. we aim to continuously update the database as new measurements become available.methods: we built a web service, ssodnet, which offers four access points, each corresponding to an identified necessity in the community: name resolution (quaero), compilation of a large corpus of properties (datacloud), determination of the best estimate among compiled values (ssocard), and a statistical description of the population (ssobft).results: the ssodnet interfaces are fully operational and freely accessible to everyone. the name resolver quaero translates any of the ~5.3 million designations of objects into their current and official designation. the datacloud includes about 105 million parameters (osculating and proper elements, pair and family membership, diameter, albedo, mass, density, rotation period, spin coordinates, phase function parameters, colors, taxonomy, thermal inertia, and yarkovsky drift) from over 3000 articles (updated continuously). for each of the known asteroids and dwarf planets (~1.2 million), a ssocard that provides a single best-estimate for each parameter is available. the ssodnet service provides these resources in a fraction of second upon query. finally, the extensive ssobft table compiles all the best estimates in a single table for population-wide studies. | ssodnet: solar system open database network |
the migration history of jupiter in the sun's natal disk remains poorly constrained. here we consider how jupiter's migration affects small-body reservoirs and how this constrains its original orbital distance from the sun. we study the implications of large-scale and inward radial migration of jupiter for the inner solar system while considering the effects of collisional evolution of planetesimals. we use analytical prescriptions to simulate the growth and migration of jupiter in the gas disk. we assume the existence of a planetesimal disk inside jupiter's initial orbit. this planetesimal disk received an initial total mass and size-frequency distribution (sfd). planetesimals feel the effects of aerodynamic gas drag and collide with one another, mostly while shepherded by the migrating jupiter. our main goal is to measure the amount of mass in planetesimals implanted into the main asteroid belt (mab) and the sfd of the implanted population. we also monitor the amount of dust produced during planetesimal collisions. we find that the sfd of the planetesimal population implanted into the mab tends to resemble that of the original planetesimal population interior to jupiter. we also find that unless very little or no mass existed between 5 au and jupiter's original orbit, it would be difficult to reconcile the current low mass of the mab with the possibility that jupiter migrated from distances beyond 15 au. this is because the fraction of the original disk mass that gets implanted into the mab is very large. finally, we discuss the implications of our results in terms of dust production to the so-called nc-cc isotopic dichotomy. | implications of jupiter inward gas-driven migration for the inner solar system |
we report on the mass-independent cr isotope compositions of 11 main group ureilites and an ureilitic trachyandesite (alm-a). the 54cr/52cr ratios for main group ureilites vary from -1.06 ± 0.04 to -0.78 ± 0.05 and averaged at -0.91 ± 0.15 (2sd, n = 18) including the data from literature. we argue that this variation reflects primitive mantle heterogeneities within the ureilite parent body (upb). as such, this body did not experience a global-scale magma ocean, which is consistent with heterogeneous o isotope in ureilites. furthermore, the ɛ54cr values, mn/cr ratios, c isotope ratios, mg# values, and fe/mn ratios in the olivine cores of ureilites are correlated with each other, which suggests the mixing of ureilite precursors from at least two reservoirs, rather than a smelting process or the oxidation from ice melting. all the ureilite samples (including the alm-a) fall on a well-defined 53mn-53cr isochron corresponding to a 53mn/55mn ratio of (6.02 ± 1.59) × 10-6, which translates to an age of 4566.7 ± 1.5 ma (within 2 ma after calcium-aluminum-rich inclusions; cais) when anchored to the u-corrected pb-pb age for the d'orbigny angrite. this old age indicates early partial melting on the upb, consistent with the early accretion of the upb (within 1 ma after cais) predicted by thermal modeling. furthermore, there is a 4∼5 ma age difference between the external isochron in this study and internal isochron ages for the feldspathic clasts in polymict ureilites, which likely reflects an impact history during the early evolution of the upb. | chromium isotopic constraints on the origin of the ureilite parent body |
both physical and dynamical properties must be considered to constrain the origins of the dynamically excited distant solar system populations. we present high-precision (g-r) colors for 25 small (h r > 5) dynamically excited trans-neptunian objects (tnos) and centaurs acquired as part of the colours of the outer solar system origins survey. we combine our data set with previously published measurements and consider a set of 229 colors of outer solar system objects on dynamically excited orbits. the overall color distribution is bimodal and can be decomposed into two distinct classes, termed gray and red, that each has a normal color distribution. the two color classes have different inclination distributions: red objects have lower inclinations than the gray ones. this trend holds for all dynamically excited tno populations. even in the worst-case scenario, biases in the discovery surveys cannot account for this trend; it is intrinsic to the tno population. considering that tnos are the precursors of centaurs, and that their inclinations are roughly preserved as they become centaurs, our finding solves the conundrum of centaurs being the only outer solar system population identified so far to exhibit this property. the different orbital distributions of the gray and red dynamically excited tnos provide strong evidence that their colors are due to different formation locations in a disk of planetesimals with a compositional gradient. | col-ossos: color and inclination are correlated throughout the kuiper belt |
we present archival observations demonstrating that main belt asteroid (6478) gault has an extensive history of comet-like activity. outbursts have taken place during multiple epochs since 2013 and at distances extending as far as 2.68 au, nearly aphelion. (6478) gault is a member of the predominately s-type (i.e., volatile-poor) phocaea family; no other main belt object of this type has ever shown more than a single activity outburst. furthermore, our data suggest that this is the longest duration of activity caused by a body spinning near the rotational breakup barrier. if activity is indeed unrelated to volatiles, as appears to be the case, (6478) gault represents a new class of object, perpetually active due to rotational spin-up. | six years of sustained activity in (6478) gault |
climate simulations that consider injection into the atmosphere of 15,000 tg of soot, the amount estimated to be present at the cretaceous-paleogene boundary, produce what might have been one of the largest episodes of transient climate change in earth history. the observed soot is believed to originate from global wildfires ignited after the impact of a 10-km-diameter asteroid on the yucatán peninsula 66 million y ago. following injection into the atmosphere, the soot is heated by sunlight and lofted to great heights, resulting in a worldwide soot aerosol layer that lasts several years. as a result, little or no sunlight reaches the surface for over a year, such that photosynthesis is impossible and continents and oceans cool by as much as 28 °c and 11 °c, respectively. the absorption of light by the soot heats the upper atmosphere by hundreds of degrees. these high temperatures, together with a massive injection of water, which is a source of odd-hydrogen radicals, destroy the stratospheric ozone layer, such that earth’s surface receives high doses of uv radiation for about a year once the soot clears, five years after the impact. temperatures remain above freezing in the oceans, coastal areas, and parts of the tropics, but photosynthesis is severely inhibited for the first 1 y to 2 y, and freezing temperatures persist at middle latitudes for 3 y to 4 y. refugia from these effects would have been very limited. the transient climate perturbation ends abruptly as the stratosphere cools and becomes supersaturated, causing rapid dehydration that removes all remaining soot via wet deposition. | on transient climate change at the cretaceous-paleogene boundary due to atmospheric soot injections |
leading up to the 2015 iau general assembly, the international astronomical union's working list of meteor showers included 486 unconfirmed showers, showers that are not certain to exist. if confirmed, each shower would provide a record of past comet or asteroid activity. now, we report that 41 of these are detected in the cameras for allsky meteor surveillance (cams) video-based meteor shower survey. they manifest as meteoroids arriving at earth from a similar direction and orbit, after removing the daily radiant drift due to earth's motion around the sun. these showers do exist and, therefore, can be moved to the iau list of established meteor showers. this adds to 31 previously confirmed showers from cams data. for each shower, finding charts are presented based on 230,000 meteors observed up to march of 2015, calculated by re-projecting the drift-corrected sun-centered ecliptic coordinates into more familiar equatorial coordinates. showers that are not detected, but should have, and duplicate showers that project to the same sun-centered ecliptic coordinates, are recommended for removal from the working list. | cams confirmation of previously reported meteor showers |
we have observed c/2014 s3 (panstarrs), a recently discovered object on a cometary orbit coming from the oort cloud that is physically similar to an inner main belt rocky s-type asteroid. recent dynamical models successfully reproduce the key characteristics of our current solar system; some of these models require significant migration of the giant planets, whereas others do not. these models provide different predictions on the presence of rocky material expelled from the inner solar system in the oort cloud. c/2014 s3 could be the key to verifying these predictions of the migration-based dynamical models. furthermore, this object displays a very faint, weak level of comet-like activity, five to six orders of magnitude less than that of typical ice-rich comets on similar orbits coming from the oort cloud. for the nearly tailless appearance, we are calling c/2014 s3 a manx object. various arguments convince us that this activity is produced by sublimation of volatile ice, that is, normal cometary activity. the activity implies that c/2014 s3 has retained a tiny fraction of the water that is expected to be present at its formation distance in the inner solar system. we may be looking at fresh inner solar system earth-forming material that was ejected from the inner solar system and preserved for billions of years in the oort cloud. | inner solar system material discovered in the oort cloud |
spectroscopy in planetary science often provides the only information regarding the compositional and mineralogical make up of planetary surfaces. the methods employed when curve fitting and modelling spectra can be confusing and difficult to visualize and comprehend. researchers who are new to working with spectra may find inadequate help or documentation in the scientific literature or in the software packages available for curve fitting. this problem also extends to the parameterization of spectra and the dissemination of derived metrics. often, when derived metrics are reported, such as band centres, the discussion of exactly how the metrics were derived, or if there was any systematic curve fitting performed, is not included. herein we provide both recommendations and methods for curve fitting and explanations of the terms and methods used. techniques to curve fit spectral data of various types are demonstrated using simple-to-understand mathematics and equations written to be used in microsoft excel® software, free of macros, in a cut-and-paste fashion that allows one to curve fit spectra in a reasonably user-friendly manner. the procedures use empirical curve fitting, include visualizations, and ameliorates many of the unknowns one may encounter when using black-box commercial software. the provided framework is a comprehensive record of the curve fitting parameters used, the derived metrics, and is intended to be an example of a format for dissemination when curve fitting data. | fitting the curve in excel®: systematic curve fitting of laboratory and remotely sensed planetary spectra |
this paper presents a study, through the use of a soft-sphere discrete element method, of possible deformation and disruption patterns of spinning, self-gravitating spherical aggregates with cores of variable strengths. we present this study as a complement to our previous study about aggregates with strong cores that also provided some insight into the occurrence of surface shedding. it is observed that the inclusion of a weak core produces a very symmetric deformation pattern and even a shape that resembles that of asteroid 25143 itokawa though this only happens for the weakest of the tested cores. at this level of shell strength, and in agreement with our previous studies, most aggregates fission off coherent pieces of the original bodies, which could potentially form binary systems. additionally, we try to understand how strong, relative to the shell, the core has to be to prevent it from failing before the shell does. | rotational evolution of self-gravitating aggregates with cores of variable strength |
the moon is the only planetary body other than the earth for which samples have been collected in situ by humans and robotic missions and returned to earth. scientific investigations of the first lunar samples returned by the apollo 11 astronauts 50 years ago transformed the way we think most planetary bodies form and evolve. identification of anorthositic clasts in apollo 11 samples led to the formulation of the magma ocean concept, and by extension the idea that the moon experienced large-scale melting and differentiation. this concept of magma oceans would soon be applied to other terrestrial planets and large asteroidal bodies. dating of basaltic fragments returned from the moon also showed that a relatively small planetary body could sustain volcanic activity for more than a billion years after its formation. finally, studies of the lunar regolith showed that in addition to containing a treasure trove of the moon's history, it also provided us with a rich archive of the past 4.5 billion years of evolution of the inner solar system. further investigations of samples returned from the moon over the past five decades led to many additional discoveries, but also raised new and fundamental questions that are difficult to address with currently available samples, such as those related to the age of the moon, duration of lunar volcanism, the lunar paleomagnetic field and its intensity, and the record on the moon of the bombardment history during the first billion years of evolution of the solar system. in this contribution, we review the information we currently have on some of the key science questions related to the moon and discuss how future sample-return missions could help address important knowledge gaps. | constraining the evolutionary history of the moon and the inner solar system: a case for new returned lunar samples |
geochemical models describing the behaviour of niobium during earth’s growth rely on the general paradigm that niobium was delivered by earth’s asteroidal building blocks at chondritic abundances. this paradigm is based on the observation that niobium is traditionally regarded as a refractory and strongly lithophile element, and thus stored in the silicate portions of earth and differentiated asteroids. however, earth’s silicate mantle is instead selectively depleted in niobium, in marked contrast to the silicate mantles of many asteroids and smaller planets that apparently lack any significant depletion in niobium. here we present results of high-precision measurements for niobium and other lithophile elements in representative meteorites from various small differentiated asteroids. our data, along with the results of low-pressure experiments, show that in more reduced asteroids--such as earth’s first building blocks--niobium is moderately chalcophile and more so than its geochemical twin tantalum by an order of magnitude. accordingly, niobium can be sequestered into the cores of more reduced asteroids during differentiation via the segregation of sulfide melts in a carbon-saturated environment. we suggest that the niobium deficit in earth’s silicate mantle may be explained by the earth’s silicate mantle preferentially accreting the silicate portions of reduced asteroidal building blocks. | silicate earth’s missing niobium may have been sequestered into asteroidal cores |
a fundamental aspect of the three-body problem is its stability. most stability studies have focused on the co-planar three-body problem, deriving analytic criteria for the dynamical stability of such pro/retrograde systems. numerical studies of inclined systems phenomenologically mapped their stability regions, but neither complement it by theoretical framework, nor provided satisfactory fit for their dependence on mutual inclinations. here we present a novel approach to study the stability of hierarchical three-body systems at arbitrary inclinations, which accounts not only for the instantaneous stability of such systems, but also for the secular stability and evolution through lidov-kozai cycles and evection. we generalize the hill-stability criteria to arbitrarily inclined triple systems, explain the existence of quasi-stable regimes and characterize the inclination dependence of their stability. we complement the analytic treatment with an extensive numerical study, to test our analytic results. we find excellent correspondence up to high inclinations (∼120°), beyond which the agreement is marginal. at such high inclinations, the stability radius is larger, the ratio between the outer and inner periods becomes comparable and our secular averaging approach is no longer strictly valid. we therefore combine our analytic results with polynomial fits to the numerical results to obtain a generalized stability formula for triple systems at arbitrary inclinations. besides providing a generalized secular-based physical explanation for the stability of non-co-planar systems, our results have direct implications for any triple systems and, in particular, binary planets and moon/satellite systems; we briefly discuss the latter as a test case for our models. | generalized hill-stability criteria for hierarchical three-body systems at arbitrary inclinations |
small bodies such as the near-earth asteroid bennu drift in their orbit due to thermal radiation forces (the yarkovsky effect). ground-based observations have indicated a nonzero probability of bennu impacting earth, depending on how its orbit evolves. thus, among the goals of the osiris-rex (origins, spectral interpretation, resource identification, and security-regolith explorer) mission to bennu were to precisely measure the yarkovsky effect and refine the impact hazard assessment for this body. here we address these objectives. using osiris-rex spacecraft tracking data, we derive meter-level constraints on the distance between earth and bennu from january 2019 to october 2020. while these data greatly improve the knowledge of the trajectory of bennu, they also require an unprecedented fidelity for the modeling of an asteroid's trajectory. in particular, special care is needed to take into account the contribution of 343 small-body perturbers and the uncertainty in their masses. radiation effects such as the poynting-robertson drag, so far only considered for interplanetary dust dynamics, now become a consideration for modeling the trajectory of a 500-m asteroid such as bennu. by employing a thermophysical model based on osiris-rex's characterization of bennu, we estimate a semimajor axis drift of - 284 . 6 ± 0 . 2 m/yr (signal-to-noise ratio ∼1400) at epoch 2011 january 1 caused by the yarkovsky effect. the largest source of modeling error is solar wind drag, which may lower the magnitude of the semimajor axis drift from the yarkovsky effect by up to 0.16 m/yr. the yarkovsky-related semimajor axis drift varies by roughly ± 1 m/yr as the orbit of bennu evolves due to planetary perturbations from 1900 to 2135. the yarkovsky thermophysical model proves to be extremely accurate by predicting a bulk density estimate within 0.1% of that estimated through gravity science analysis. compared to the information available before the osiris-rex mission, the knowledge of the circumstances of the scattering earth encounter that will occur in 2135 improves by a factor of 20, thus allowing us to rule out many previously possible impact trajectories. however, there remain some impact trajectories compatible with the data. prior to the spacecraft encounter, the overall impact probability through 2200 was 3 . 7 × 10-4 (1 in 2700). as a result of our analysis, the cumulative impact probability through 2300 becomes 5 . 7 × 10-4 (1 in 1750) and the most significant individual impact solution is for september 2182, with an impact probability of 3 . 7 × 10-4 (1 in 2700). both bennu and (29075) 1950 da have a palermo scale value of - 1 . 42 and share the distinction as the currently most hazardous object in the asteroid catalog. | ephemeris and hazard assessment for near-earth asteroid (101955) bennu based on osiris-rex data |
we report radar observations of apophis obtained during the 2012-2013 apparition. we observed apophis on fourteen days at goldstone (8560 mhz, 3.5 cm) and on five days at arecibo (2380 mhz, 12.3 cm) between 2012 december 21 to 2013 march 16. closest approach occurred on january 9 at a distance of 0.097 au. we obtained relatively weak echo power spectra and delay-doppler images. the highest range resolution was achieved at goldstone, 0.125 μs or ∼20 m/px. the data suggest that apophis is an elongated, asymmetric, and possibly bifurcated object. the images place a lower bound on the long axis of 450 m. we used the pravec et al. (2014) lightcurve-derived shape and spin state model of apophis to test for short axis mode (sam) non-principal axis rotation (npa) and to estimate the asteroid's dimensions. the radar data are consistent with the npa spin state and they constrain the equivalent diameter to be d = 0.34 ± 0.04 km (1σ bound). this is slightly smaller than the most recent ir observation estimates of 375(-10)(+14) m and 380-393 m, reported by müller et al. (2014) and licandro et al. (2016) respectively. we estimated a radar albedo of 0.25 ± 0.11 based on goldstone data, and an optical albedo, pv, of 0.35 ± 0.10. licandro et al. (2016) reported pv in the range of 0.24-0.33. the radar astrometry has been updated using a 3-d shape model. the yarkovsky acceleration has not been detected in the current orbital fit, but if the position error during the 2021 encounter exceeds 8-12 km, this could signal a detection of the yarkovsky effect. | goldstone and arecibo radar observations of (99942) apophis in 2012-2013 |
ceres, a dwarf planet located in the main asteroid belt, has a low bulk density1, and models predict that a substantial amount of water ice is present in its mantle and outer shell2-4. the herschel telescope and the dawn spacecraft5 have observed the release of water vapour from ceres6,7, and exposed water ice has been detected by dawn on its surface at mid-latitudes8. water molecules from endogenic and exogenic sources can also be cold-trapped in permanent shadows at high latitudes9-11, as happens on the moon12,13 and mercury14,15. here we present the first image-based survey of ceres's northern permanent shadows and report the discovery of bright deposits in cold traps. we identify a minimum of 634 permanently shadowed craters. bright deposits are detected on the floors of just 10 of these craters in multi-scattered light. we spectroscopically identify one of the bright deposits as water ice. this detection strengthens the evidence that permanently shadowed areas have preserved water ice on airless planetary bodies. | surface water-ice deposits in the northern shadowed regions of ceres |
artificial manipulation of acoustic waves is a highly active research area, in which negative reflection is an exotic phenomenon. emerging acoustic coding metasurfaces (acms) show great power in controlling acoustic waves. here, we propose an acm, where the state "0" or "1" is achieved by pushing up or pressing down a rigid rectangular strip. the acm with a tunable coding sequence can adjust the direction of negative reflection as well as suppress the specular reflection. the feasibility of the acm is demonstrated by totally reflecting the incident acoustic waves from 22 ° to the directions of - 39 °, - 47 °, and - 61 °. furthermore, the acm can spread the incident white noise into its acoustic spectral components like rainbows. this work may open diverse routes toward reconfigurable acoustic metasurfaces that are critical for practical functional devices, which are promising in potential applications including acoustic beam steering, retro-reflection, and surface wave sensing. | tunable perfect negative reflection based on an acoustic coding metasurface |
context. pebble accretion is a newly discovered mechanism to quickly grow the cores of planets. in pebble accretion, gravity and gas drag conspire to yield large collisional cross sections for small particles in protoplanetary disks. however, before pebble accretion commences, aerodynamic deflection may act to prevent planetesimals from becoming large, because particles tend to follow gas streamlines.aims: we derive the planetesimal radius where pebble accretion is initiated and determine the growth timescales of planetesimals by sweep-up of small particles.methods: the equation of motion for a pebble, including gas drag and gravitational interactions, was integrated in three dimensions at distances of 1, 3, and 10 au from the star. we obtained the collision efficiency factor as the ratio of the numerically obtained collisional cross section to the planetesimal surface area, from which we obtained the growth timescales. integrations were conducted in the potential flow limit (steady, inviscid) and in the stokes flow regime (steady, viscid).results: only particles of stopping time ts ≪ tx where tx ≈ 103 s experience aerodynamic deflection. even in this case, the planetesimal's gravity always ensures positive collision factors. the planetesimal radius where growth proceeds slowest is ≈ 100 km (less for colder disks) corresponding to interactions shifting from the geometric to the safronov focusing regime. for particles ts ≫ tx pebble accretion only commences after this phase and is characterized by a steep drop in growth timescales. at 1 au, growth timescales are shorter than the disk lifetime for pebbles larger than 0.03 cm. the planetesimal radius rpa where pebble accretion commences increases with disk orbital radius. at distances beyond ~ 10 au, sweep-up growth times are always longer than 10 myr, while in the inner disk (≲3 au) the viability of the sweep-up scenario is determined by the outcome of pebble-planetesimal collisions in the geometric regime. we present analytical fits for the collision efficiency factors and the minimum planetesimal radius rpa required for pebble accretion. | on the growth of pebble-accreting planetesimals |
context. asteroid modeling efforts in the last decade resulted in a comprehensive dataset of almost 400 convex shape models and their rotation states. these efforts already provided deep insight into physical properties of main-belt asteroids or large collisional families. going into finer detail (e.g., smaller collisional families, asteroids with sizes ≲20 km) requires knowledge of physical parameters of more objects.aims: we aim to increase the number of asteroid shape models and rotation states. such results provide important input for further studies, such as analysis of asteroid physical properties in different populations, including smaller collisional families, thermophysical modeling, and scaling shape models by disk-resolved images, or stellar occultation data. this provides bulk density estimates in combination with known masses, but also constrains theoretical collisional and evolutional models of the solar system.methods: we use all available disk-integrated optical data (i.e., classical dense-in-time photometry obtained from public databases and through a large collaboration network as well as sparse-in-time individual measurements from a few sky surveys) as input for the convex inversion method, and derive 3d shape models of asteroids together with their rotation periods and orientations of rotation axes. the key ingredient is the support of more that 100 observers who submit their optical data to publicly available databases.results: we present updated shape models for 36 asteroids, for which mass estimates are currently available in the literature, or for which masses will most likely be determined from their gravitational influence on smaller bodies whose orbital deflections will be observed by the esa gaia astrometric mission. moreover, we also present new shape model determinations for 250 asteroids, including 13 hungarias and three near-earth asteroids. the shape model revisions and determinations were enabled by using additional optical data from recent apparitions for shape optimization. | new and updated convex shape models of asteroids based on optical data from a large collaboration network |
we conducted a two-night photometric survey of small kuiper belt objects (kbos) near opposition using the wide-field hyper suprime-cam instrument on the 8.2 m subaru telescope. the survey covered about 90 deg2 of sky, with each field imaged in the g and i bands. we detected 356 kbos, ranging in absolute magnitude from 6.5 to 10.4. filtering for high-inclination objects within the hot kbo population, we show that the g - i color distribution is strongly bimodal, indicative of two color classes—the red and very red subpopulations. after categorizing objects into the two subpopulations by color, we present the first dedicated analysis of the magnitude distributions of the individual color subpopulations and demonstrate that the two distributions are roughly identical in shape throughout the entire size range covered by our survey. comparing the color distribution of small hot kbos with that of centaurs, we find that they have similar bimodal shapes, thereby providing strong confirmation of previous explanations for the attested bimodality of centaurs. we also show that the magnitude distributions of the two kbo color subpopulations and the two color subpopulations observed in the jupiter trojans are statistically indistinguishable. finally, we discuss a hypothesis describing the origin of the kbo color bimodality based on our survey results. based on data collected at subaru telescope, which is operated by the national astronomical observatory of japan. | the bimodal color distribution of small kuiper belt objects |
solar system bodies with surface and subsurface volatiles will show observational evidence of activity when they reach a temperature where those volatiles change from solid to gas and are released. this is most frequently seen in comets, where activity is driven by the sublimation of water, carbon dioxide, or carbon monoxide ices. however, some bodies (notably the asteroid (3200) phaethon) show initiation of activity at very small heliocentric distances, long after they have reached the sublimation temperatures of these ices. we investigate whether the sodium present in the mineral matrix could act as the volatile element responsible for this activity. we conduct theoretical modeling which indicates that sodium has the potential to sublimate in the conditions that phaethon experiences, depending on the mineral phase it is held in. to test this, we then exposed samples of the carbonaceous chondrite allende to varying heating events, similar to what would be experienced by low-perihelion asteroids. we measured the change in sodium present in each sample and find that the highest temperature samples show a significant loss of sodium from specific mineral phases over a single heating event, comparable to a day on the surface of phaethon. under specific thermal histories possible for phaethon, this outgassing could be sufficient to explain this object's observed activity. this effect would also be expected to be observed for other low-perihelion asteroids as well and may act as a critical step in the process of disrupting small low-albedo asteroids. | volatility of sodium in carbonaceous chondrites at temperatures consistent with low-perihelion asteroids |
many asteroids are rubble piles with irregular shapes. while the irregular shapes of large asteroids may be attributed to collisional events, those of small asteroids may result from not only impact events but also rotationally induced failure, a long-term consequence of small torques caused by, for example, solar radiation pressure. a better understanding of shape deformation induced by such small torques will allow us to give constraints on the evolution process of an asteroid and its structure. however, no quantitative study has been reported to provide the relationship between an asteroid's shape and its failure mode due to its fast rotation. here, we use a finite element model (fem) technique to analyze the failure modes and conditions of 24 asteroids with diameters less than 30-40 km, which were observed at high resolution by ground radar or asteroid exploration missions. assuming that the material distribution is uniform, we investigate how these asteroids fail structurally at different spin rates. our fem simulations describe the detailed deformation mode of each irregularly shaped asteroid at fast spin. the failed regions depend on the original shape. spheroidal objects structurally fail from the interior, while elongated objects experience structural failure on planes perpendicular to the minimum moment of inertia axes in the middle of their structure. contact binary objects have structural failure across their most sensitive cross-sections. we further investigate if our fem analysis is consistent with earlier works that theoretically explored a uniformly rotating triaxial ellipsoid. the results show that global shape variations may significantly change the failure condition of an asteroid. our work suggests that it is critical to take into account the actual shapes of asteroids to explore their failure modes in detail. | rotationally induced failure of irregularly shaped asteroids |
the appearance of interstellar objects (isos) in the solar system-and specifically the arrival of 1i/'oumuamua-points to a significant number density of free-floating bodies in the solar neighborhood. we review the details of 'oumuamua's pre-encounter galactic orbit, which intersected the solar system at very nearly its maximum vertical and radial excursion relative to the galactic plane. these kinematic features are strongly emblematic of nearby young stellar associations. we obtain an a priori order-of-magnitude age estimate for 'oumuamua by comparing its orbit to the orbits of 50,899 f-type stars drawn from gaia dr2; a diffusion model then suggests a ~35 myr dynamical age. we compare 'oumuamua's orbit with the trajectories of individual nearby moving groups, confirming that its motion is fully consistent with membership in the carina (car) moving group with an age of ~30 myr. we conduct monte carlo simulations that trace the orbits of test particles ejected from the stars in the car association. the simulations indicate that in order to uniformly populate the ~106 pc3 volume occupied by car members with the inferred number density, n = 0.2 au-3, of isos implied by pan-starrs's detection of 'oumuamua, the required ejection mass is m ~ 500 mjup per known star within the car association. this suggests that the pan-starrs observation is in significant tension with scenarios that posit 'oumuamua's formation and ejection from a protostellar disk. | evidence suggesting that 'oumuamua is the30 myr old product of a molecular cloud |
two meteorite pieces have been recovered in italy, near the town of cavezzo (modena), on 2020 january 4th. the associated fireball was observed on the evening of new year's day 2020 by eight all-sky cameras of the prisma fireball network, a partner of fripon. the computed trajectory had an inclination angle of approximately 68° and a velocity at infinity of 12.8 km s-1. together with the relatively low terminal height, estimated as 21.5 km, those values were indicating the significant possibility of a meteorite dropping event, as additionally confirmed by the non-zero residual total mass. the strewn-field was computed taking into account the presence of two bright light flashes, revealing that the meteoroid had been very likely subject to fragmentation. three days after the event, two samples, weighing 3.1 and 52.2 g, were collected as a result of a dedicated field search and thanks to the involvement of the local people. the two pieces were immediately recognized as freshly fallen fragments of meteorite. the computed orbital elements, compared with the ones of known near-earth asteroids from the neodys database, are compatible with one asteroid only; 2013 vc10. the estimated original mass of the meteoroid, 3.5 kg, and size, approximately 13 cm, is so far the smallest among the current 35 cases in which meteorites were recovered from precise strewn-field computation thanks to observational data. this result demonstrates the effectiveness of accurate processing of fireball network data even on challenging events generated by small size meteoroids. | cavezzo, the first italian meteorite recovered by the prisma fireball network. orbit, trajectory, and strewn-field |
the relative motion about 4179 toutatis is studied in order to investigate the feasibility of formation flying as an alternative concept for future asteroid exploration missions. in particular, the existence of quasi-frozen orbits about slowly rotating bodies allows us to compute families of periodic orbits in the body-fixed frame of the asteroid. since these periodic orbits are of the center×center type, quasi-periodic invariant tori are calculated via fully numerical procedures and used to initialize spacecraft formations about the central body. numerical simulations show that the resulting in-plane and out-of-plane relative trajectories remain bounded over long time spans; i.e., more than 30 days. | bounded relative orbits about asteroids for formation flying and applications |
polluted white dwarfs that have accreted planetary material provide a unique opportunity to probe the geology of exoplanetary systems. however, the nature of the bodies that pollute white dwarfs is not well understood: are they small asteroids, minor planets, or even terrestrial planets? we present a novel method to infer pollutant masses from detections of ni, cr, and si. during core-mantle differentiation, these elements exhibit variable preference for metal and silicate at different pressures (i.e. object masses), affecting their abundances in the core and mantle. we model core-mantle differentiation self-consistently using data from metal-silicate partitioning experiments. we place statistical constraints on the differentiation pressures, and hence masses, of bodies which pollute white dwarfs by incorporating this calculation into a bayesian framework. we show that ni observations are best suited to constraining pressure when pollution is mantle-like, while cr and si are better for core-like pollution. we find three systems (wd0449-259, wd1350-162, and wd2105-820) whose abundances are best explained by the accretion of fragments of small parent bodies (<0.2 m⊕). for two systems (gd61 and wd0446-255), the best model suggests the accretion of fragments of earth-sized bodies, although the observed abundances remain consistent (<3σ) with the accretion of undifferentiated material. this suggests that polluted white dwarfs potentially accrete planetary bodies of a range of masses. however, our results are subject to inevitable degeneracies and limitations given current data. to constrain pressure more confidently, we require serendipitous observation of (nearly) pure core and/or mantle material. | planets or asteroids? a geochemical method to constrain the masses of white dwarf pollutants |
a cluster of gridded microwave discharge ion thrusters μ10 contributed to the success of the deep space exploration missions of the japanese asteroid explorers hayabusa and hayabusa2. to meet the demands of the next mission destiny+, the maximum ion beam current was increased to 200 ma from the 170 ma used in hayabusa2 by redesigning the magnetic field and grid configuration. however, this thruster exhibited the plasma hysteresis caused by a high-voltage breakdown. specifically, the ion beam current (i.e. thrust) has two modes at a given propellant flow rate, namely high- and low-current modes. in nominal operation, the thruster operates in high-current mode. however, once a high-voltage breakdown occurs, the ion beam current transitions to and remains in low-current mode, which significantly decreases thrust efficiency. in this paper, based on an investigation of the physical mechanism that causes the hysteresis, we proposed a recovery method to return the thruster operation to high-current mode. in the investigation, we focused on the plasma mode transition, where plasma is generated inside the waveguide. we experimentally investigated the hysteresis by varying the grid configuration and the waveguide geometries while maintaining the magnetic field. the results show that the neutral density and plasma conditions inside the waveguide are strongly related to the hysteresis, which indicates that the main cause of the hysteresis is plasma generation inside the waveguide. | plasma hysteresis caused by high-voltage breakdown in gridded microwave discharge ion thruster μ10 |
the inner solar system’s biggest and most recent known collision was the moon-forming giant impact between a large protoplanet and proto-earth. not only did it create a disk near earth that formed the moon, it also ejected several percent of an earth mass out of the earth-moon system. here, we argue that numerous kilometer-sized ejecta fragments from that event struck main-belt asteroids at velocities exceeding 10 kilometers per second, enough to heat and degas target rock. such impacts produce ~1000 times more highly heated material by volume than do typical main belt collisions at ~5 kilometers per second. by modeling their temporal evolution, and fitting the results to ancient impact heating signatures in stony meteorites, we infer that the moon formed ~4.47 billion years ago, which is in agreement with previous estimates. | dating the moon-forming impact event with asteroidal meteorites |
exoplanet discoveries since the mid-1990's have revealed an astounding diversity of planetary systems. studying these systems is essential to understanding planetary formation processes, as well as the development of life in the universe. unfortunately, humanity can only observe limited aspects of exoplanetary systems by telescope, and the significant distances between stars presents a barrier to in situ exploration. in this study, we propose an alternative path to gain insight into exoplanetary systems: bridge, a mission concept design to fly by an interstellar object as it passes through our solar system. designed as a new frontiers-class mission during the national aeronautics and space administration (nasa) planetary science summer seminar, bridge would provide a unique opportunity to gain insight into potential physical, chemical, and biological differences between solar systems as well as the possible exchange of planetary materials between them. bridge employs ultraviolet/visible, near-infrared, and mid-infrared point spectrometers, a visible camera, and a guided impactor. we also provide a quantitative monte carlo analysis that estimates wait times for a suitable target, and examines key trades between ground storage and a parking orbit, power sources, inner versus outer solar system encounters, and launch criteria. due to the fleeting nature of interstellar objects, reaching an interstellar object may require an extended ground storage phase for the spacecraft until a suitable target is discovered, followed by a rapid response launch strategy. to enable rapid response missions designed to intercept such unique targets, language would need to be added to future nasa announcements of opportunity such that ground storage and rapid response would be allowable components of a proposed mission. | bridge to the stars: a mission concept to an interstellar object |
tropical forests act as a major sink of atmospheric carbon dioxide, and store large amounts of carbon in biomass. india is a tropical country with regions of dense vegetation and high biodiversity. however due to the paucity of observations, the carbon sequestration potential of these forests could not be assessed in detail so far. to address this gap, several flux towers were erected over different ecosystems in india by indian institute of tropical meteorology as part of the metflux india project funded by moes (ministry of earth sciences, government of india). a 50 m tall tower was set up over a semi-evergreen moist deciduous forest named kaziranga national park in north-eastern part of india which houses a significant stretch of local forest cover. climatically this region is identified to be humid sub-tropical. here we report first generation of the in situ meteorological observations and leaf area index (lai) measurements from this site. lai obtained from nasa's moderate resolution imaging spectroradiometer (modis) is compared with the in situ measured lai. we use these in situ measurements to calculate the total gross photosynthesis (or gross primary productivity, gpp) of the forest using a calibrated model. lai and gpp show prominent seasonal variation. lai ranges between 0.75 in winter to 3.25 in summer. annual gpp is estimated to be 2.11 kg c m^{-2} year^{-1}. | estimating gross primary productivity of a tropical forest ecosystem over north-east india using lai and meteorological variables |
we observed the near-earth asteroid (101955) bennu from the ground in 1999 and 2005, and with the hubble space telescope (hst) in 2012, to constrain its rotation rate. the data reveal an acceleration of 2.64 ± 1.05 × 10-6 deg/day2, which could be due to a change in the moment of inertia of bennu or to spin up from the yarkovsky-o'keefe-radzievskii-paddack effect or other source of angular momentum. the best solution is within 1 σ of the period determined by nolan et al. (2013, https://doi.org/10.1016/j.icarus.2013.05.028). the origins, spectral interpretation, resource identification, and security-regolith explorer (osiris-rex) mission will determine the rotation state independently in 2019. those measurements should show whether the change in rotation rate is a steady increase (due, e.g., to the yarkovsky-o'keefe-radzievskii-paddack effect) or some other phenomenon. the precise shape and surface properties measured by the osiris-rex science team will allow for a better understanding of variations in rotation rate of small asteroids. | detection of rotational acceleration of bennu using hst light curve observations |
the main asteroid belt (mb) is low in mass but dynamically excited. here we propose a new mechanism to excite the mb during the giant planet (the “nice model”) instability, which is expected to feature repeated close encounters between jupiter and one or more ice giants (“jumping jupiter” or jj). we show that, when jupiter temporarily reaches a high-enough level of excitation, both in eccentricity and inclination, it induces strong forced vectors of eccentricity and inclination across the mb region. because during the jj instability jupiter’s orbit “jumps” around, the forced vectors keep changing both in magnitude and phase throughout the whole mb region. the entire cold primordial mb is thus excited as a natural outcome of the jj instability. the level of such an excitation, however, is typically larger than the current orbital excitation observed in the mb. we show that the subsequent evolution of the solar system is capable of reshaping the resultant overexcited mb to its present-day orbital state, and that a strong mass depletion (∼90%) is associated with the jj instability phase and its subsequent evolution throughout the age of the solar system. | excitation of a primordial cold asteroid belt as an outcome of planetary instability |
boulders are ubiquitously found on the surfaces of small rocky bodies in the inner solar system and their spatial and size distributions give insight into the geological evolution and collisional history of the parent bodies. using images acquired by the chang’e-2 spacecraft, more than 200 boulders have been identified over the imaged area of the near-earth asteroid toutatis. the cumulative boulder size frequency distribution (sfd) shows a steep slope of -4.4 ± 0.1, which is indicative of a high degree of fragmentation. similar to itokawa, toutatis probably has a rubble-pile structure, as most boulders on its surface cannot solely be explained by impact cratering. the significantly steeper slope for toutatis’ boulder sfd compared to itokawa may imply a different preservation state or diverse formation scenarios. in addition, the cumulative crater sfd has been used to estimate a surface crater retention age of approximately 1.6 ± 0.3 gyr. | boulders on asteroid toutatis as observed by chang’e-2 |
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