In this work, we study the dynamics of particles around Bennu. The goal is to understand the stability, evolution, and final outcome of the simulated particles around the asteroid. According to the results, the particle sizes can be divided into two main groups depending on their behavior. Particles smaller than a centimeter are quickly removed from the system by solar radiation pressure, while the dynamics of particles larger than a few centimeters is dominated by the gravitational field of Bennu. Because of its shape and spin period, Bennu has eight equilibrium points around it. The structure of the phase space near its equatorial surface is directly connected to these equilibrium points. Therefore, we performed numerical simulations to obtain information about the orbital evolution near the equilibrium points. The results show that most of the particles larger than a few centimeters fall in the equatorial region close to the Kingfisher area or close to the region diametrically opposite to it. In contrast, almost none of these particles fall in the equatorial region close to the Osprey area. In addition, we also performed computational experiments considering a spherical cloud of particles initially orbiting Bennu. Most of the particles in prograde orbits fall on the surface within our integration period, which was limited to 1.14 years. The particles preferentially fall near high-altitude regions at low equatorial latitudes and close to the north pole. The mid-latitudes are those more depleted of falls, as in the Nightingale and Sandpiper areas.
In this work, we investigate the thermophysical properties of OSIRIS-REx target asteroid (101955) Bennu (hereafter, Bennu), where thermal inertia plays an important role in understanding the nature of the asteroid's surface, and will definitely provide substantial information for the sampling return mission. Using a thermophysical model incorporating the recently updated 3D radar-derived shape model \citep{Nolan2013} and mid-infrared observations of Spitzer-PUI, Spitzer-IRAC, Herschel/PACS and ESO VLT/VISIR \citep{Muller2012,Emery2014}, we derive the surface thermophysical properties of Bennu.…
From Jan. 6, 2019 to Feb. 18, 2019, OSIRIS-REx observed asteroid (101955) Bennu ejecting 11 plumes of dust, of which part is escaping and another part is re-captured by the asteroid. The relative magnitudes of the typical forces acting on the emitted dust are quite different from the environments of the planets and other minor planets in the solar system. Here we show that ejected dust grains from the surface of Bennu can be caught in the gravitational field of Bennu. To this end, we calculated numerically the trajectories of dust grains of various sizes, from the 0.1mum to the ten millimeter …
Physical Analysis of Bennu Samples Reveals Regolith Production by Collisional Disruption on Near-Earth Asteroids
Johns Hopkins University Applied Physics Laboratory（约翰斯·霍普金斯大学应用物理实验室）；Lunar and Planetary Laboratory, University of Arizona（亚利桑那大学月球与行星实验室）；Vatican Observatory（梵蒂冈天文台）；Hopkins Extreme Materials Institute, Johns Hopkins University（约翰斯·霍普金斯大学霍普金斯极端材料研究所）；Jacobs-JETS, NASA Johnson Space Center（NASA约翰逊航天中心雅各布斯-JETS）；University of Calgary（卡尔加里大学）；EAPS, Massachusetts Institute of Technology（麻省理工学院地球、大气与行星科学系）；Arizona State University（亚利桑那州立大学）；National Air and Space Museum, Smithsonian Institution（史密森尼学会国家航空和航天博物馆）；Astromaterials Research and Exploration Science (ARES), NASA Johnson Space Center（NASA约翰逊航天中心天体物质研究与探索科学部）；Planetary Science Institute（行星科学研究所）；INAF, Astronomical Observatory of Padova（意大利国家天体物理研究所帕多瓦天文台）；University of Rochester（罗切斯特大学）；Southwest Research Institute（西南研究院）；Department of Geology, Rowan University（罗恩大学地质系）；Department of Earth and Planetary Sciences, American Museum of Natural History（美国自然历史博物馆地球与行星科学系）
We have obtained polarimetric measurements of asteroid (101955) Bennu, a presumably primitive near-Earth object (NEO) that is the target of NASA's sample return mission OSIRIS-REx. During our observing campaign, Bennu was visible from Earth under a wide range of illumination conditions, with phase-angle in the range 16 to 57 degrees. Together with (3200) Phaethon and (152679) 1998 KU2, observed very recently, Bennu is the only existing example of a primitive NEO observed in polarimetric mode over a wide interval of phase angles. Based on our polarimetric data, we propose that Bennu belongs to …
It has been shown that temperature cycles on airless bodies of our Solar System can cause damaging of surface materials. Nevertheless, propagation mechanisms in the case of space objects are still poorly understood. Present work combines a thermoelasticity model together with linear elastic fracture mechanics theory to predict fracture propagation in the presence of thermal gradients generated by diurnal temperature cycling and under conditions similar to those existing on the asteroid Bennu. The crack direction is computed using the maximal strain energy release rate criterion, which is imple…
The stability and topological structure of equilibrium points in the potential field of the asteroid 101955 Bennu have been investigated with a variable density and rotation period. A dimensionless quantity is introduced for the nondimensionalization of the equations of motion, and this quantity can indicate the effect of both the rotation period and bulk density of the asteroid. Using the polyhedral model of the asteroid Bennu, the number and position of the equilibrium points are calculated and illustrated by a contour plot of the gravitational effective potential field. The topological stru…
In this work, we investigate the thermophysical properties, including thermal inertia, roughness fraction and surface grain size of OSIRIS-REx target asteroid (101955) Bennu by using a thermophysical model with the recently updated 3D radar-derived shape model (\cite[Nolan et al., 2013]{Nolan2013}) and mid-infrared observations (\cite[M$\ddot{u}$ller et al, 2012]{Muller2012}, \cite[Emery et al., 2014]{Emery2014}). We find that the asteroid bears an effective diameter of $510^{+6}_{-40}$ m, a geometric albedo of $0.047^{+0.0083}_{-0.0011}$, a roughness fraction of $0.04^{+0.26}_{-0.04}$, and th…
Exploration of asteroid (101955) Bennu by the OSIRIS-REx mission has provided an in-depth look at this rubble-pile near-Earth asteroid. In particular, the measured gravity field and the detailed shape model of Bennu indicate significant heterogeneities in its interior structure, compatible with a lower density at its center. Here we combine gravity inversion methods with a statistical rubble-pile model to determine the density and size-frequency distribution (SFD) index of the rubble that constitutes Bennu. The best-fitting models indicate that the SFD of the interior is consistent with that o…
A small number of anomalously bright boulders on the near-Earth, rubble-pile asteroid (101955) Bennu were recently identified as eucritic material originating from asteroid (4) Vesta. Building on this discovery, we explored the global presence of exogenic boulders on Bennu. Our analysis focused on boulders larger than 1 m that show the characteristic 1-micron pyroxene absorption band in the four-color MapCam data from the OSIRIS-REx mission. We confirm the presence of exogenic boulders similar to eucrites and find that mixtures of eucrites with carbonaceous material is also a possible composit…
We observed the near-Earth asteroid (101955) Bennu from the ground in 1999 and 2005, and with the Hubble Space Telescope in 2012, to constrain its rotation rate. The data reveal an acceleration of $2.64 \pm 1.05 \times 10^{-6} \mathrm{deg\ day}^{-2}$, which could be due to a change in the moment of inertia of Bennu or to spin up from the YORP effect or other source of angular momentum. The best solution is within 1 sigma of the period determined by Nolan et al. (2013). The OSIRIS-REx mission will determine the rotation state independently in 2019. Those measurements should show whether the cha…
The NASA mission OSIRIS-REx has been observing near-Earth asteroid (101955) Bennu in close proximity since December 2018. In this work, we investigate spectral phase reddening -- that is, the variation of spectral slope with phase angle -- on Bennu using spectra acquired by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) covering a phase angle range of 8-130$^{o}$. We investigate this process at the global scale and for some localized regions of interest (ROIs), including boulders, craters, and the designated sample collection sites of the OSIRIS-REx mission. Bennu has a globally nega…
The Design Reference Asteroid (DRA) is a compilation of all that is known about the OSIRIS-REx mission target, asteroid (101955) Bennu. It contains our best knowledge of the properties of Bennu based on an extensive observational campaign that began shortly after its discovery, and has been used to inform mission plan development and flight system design. The DRA will also be compared with post-encounter science results to determine the accuracy of our Earth-based characterization efforts. The extensive observations of Bennu in 1999 has made it one of the best-characterized near-Earth asteroid…
Asteroids and other small bodies in the solar system tend to have irregular shapes, owing to their low gravity. This irregularity does not only apply to the topology, but also to the underlying geology, potentially containing regions of different densities and materials. The topology can be derived from optical observations, while the mass density distribution of an object is only observable, to some extent, in its gravitational field. In a companion paper, we presented geodesyNets, a neural network approach to infer the mass density distribution of an object from measurements of its gravitati…
In May of 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third mission in the New Frontiers program. The other two New Frontiers missions are New Horizons, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on Jan. 1, 2019, and Juno, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Allian…
The OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) is a point spectrometer covering the spectral range of 0.4 to 4.3 microns (25,000-2300 cm-1). Its primary purpose is to map the surface composition of the asteroid Bennu, the target asteroid of the OSIRIS-REx asteroid sample return mission. The information it returns will help guide the selection of the sample site. It will also provide global context for the sample and high spatial resolution spectra that can be related to spatially unresolved terrestrial observations of asteroids. It is a compact, low-mass (17.8 kg), power efficient (8…
The OSIRIS-REx asteroid sample-return mission is investigating primitive near-Earth asteroid (101955) Bennu. Thousands of images will be acquired by the MapCam instrument onboard the spacecraft, an imager with four color filters based on the Eight-Color Asteroid Survey (ECAS): $b$' (473 nm), $v$ (550 nm), $w$ (698 nm), and $x$ (847 nm). This set of filters will allow identification and characterization of the absorption band centered at 700 nm and associated with hydrated silicates. In this work, we present and validate a spectral clustering methodology for application to the upcoming MapCam i…
The target asteroid of the OSIRIS-REx asteroid sample return mission, (101955) Bennu (formerly 1999 RQ$_{36}$), is a half-kilometer near-Earth asteroid with an extraordinarily well constrained orbit. An extensive data set of optical astrometry from 1999--2013 and high-quality radar delay measurements to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect, with a mean semimajor axis drift rate $da/dt = (-19.0 \pm 0.1)\times 10^{-4}$ au/Myr or $284\pm 1.5\;\rm{m/yr}$. The accuracy of this result depends critically on the fidelity of the observational and dynamical model. As a…
We consider an impact on an asteroid that is energetic enough to cause resurfacing by seismic reverberation and just below the catastrophic disruption threshold, assuming that seismic waves are not rapidly attenuated. In asteroids with diameter less than 1 km we identify a regime where rare energetic impactors can excite seismic waves with frequencies near those of the asteroid's slowest normal modes. In this regime, the distribution of seismic reverberation is not evenly distributed across the body surface. With mass-spring model elastic simulations, we model impact excitation of seismic wave…
Using the multi-band imager MapCam onboard the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft, we identified 77 instances of proposed exogenic materials distributed globally on the surface of the B-type asteroid (101955) Bennu. We identified materials as exogenic on the basis of an absorption near 1 um that is indicative of anhydrous silicates. The exogenic materials are spatially resolved by the telescopic camera PolyCam. All such materials are brighter than their surroundings, and they are expressed in a variety of morphologi…
NASA's OSIRIS-REx and JAXA's Hayabusa2 sample-return missions are currently on their way to encounter primitive near-Earth asteroids (101955) Bennu and (162173) Ryugu, respectively. Spectral and dynamical evidence indicates that these near-Earth asteroids originated in the inner part of the main belt. There are several primitive collisional families in this region, and both these asteroids are most likely to have originated in the Polana-Eulalia family complex. We present the expected spectral characteristics of both targets based on our studies of our primitive collisional families in the inn…
Asteroid Ryugu and asteroid Bennu, which were recently visited by spacecraft Hayabusa2 and OSIRIS-REx, respectively, are spinning top-shaped rubble piles. Other axisymmetric top-shaped near-Earth asteroids have been observed with ground-based radar, most of which rotate near breakup rotation periods of ~ 3 hours. This suggests that rotation-induced deformation of asteroids through rotational spinup produces top shapes. Although some previous simulations using the Discrete Element Method showed that spinup of rubble piles may produce oblate top shapes, it is still unclear what kinds of conditio…
Proximity observations by OSIRIS-REx and Hayabusa2 provided clues on the shape evolution processes of the target asteroids, (101955) Bennu and (162173) Ryugu. Their oblate shapes with equatorial ridges, or the so-called top shapes, may have evolved due to their rotational conditions at present and in the past. Different shape evolution scenarios were previously proposed; Bennu's top shape may have been driven by surface processing, while Ryugu's may have been developed due to large deformation. These two scenarios seem to be inconsistent. Here, we revisit the structural analyses in earlier wor…
The REgolith X-ray Imaging Spectrometer (REXIS) instrument on board NASA's OSIRIS-REx mission to the asteroid Bennu is a Class-D student collaboration experiment designed to detect fluoresced X-rays from the asteroid's surface to measure elemental abundances. In July and November 2019 REXIS collected ~615 hours of integrated exposure time of Bennu's sun-illuminated surface from terminator orbits. As reported in Hoak et al. (2021), the REXIS data do not contain a clear signal of X-ray fluorescence from the asteroid, in part due to the low incident solar X-ray flux during periods of observation.…
The dark asteroid (101955) Bennu studied by NASA\textquoteright s OSIRIS-REx mission has a boulder-rich and apparently dust-poor surface, providing a natural laboratory to investigate the role of single-scattering processes in rough particulate media. Our goal is to define optical roughness and other scattering parameters that may be useful for the laboratory preparation of sample analogs, interpretation of imaging data, and analysis of the sample that will be returned to Earth. We rely on a semi-numerical statistical model aided by digital terrain model (DTM) shadow ray-tracing to obtain scat…
The Polana-Eulalia Complex with JWST NIRSpec: Connections to Bennu, Ryugu, and Outer Solar System Bodies
Northern Arizona University（北亚利桑那大学）；Space Science Institute（太空科学研究所）；Instituto de Astrofísica de Canarias（加那利天体物理学研究所）；Universidad de La Laguna（拉古纳大学）；Observatório Nacional（国家天文台）；University of Central Florida（中佛罗里达大学）；Space Telescope Science Institute（空间望远镜科学研究所）；IPAC, Caltech（加州理工学院红外处理与分析中心）；Instituto de Ciencias y Tecnologías Espaciales de Asturias (ICTEA)（阿斯图里亚斯空间科学与技术研究所）；Universidad de Oviedo（奥维耶多大学）
The OSIRIS-REx Asteroid Sample Return Mission is the third mission in NASA's New Frontiers Program and is the first U.S. mission to return samples from an asteroid to Earth. The most important decision ahead of the OSIRIS-REx team is the selection of a prime sample-site on the surface of asteroid (101955) Bennu. Mission success hinges on identifying a site that is safe and has regolith that can readily be ingested by the spacecraft's sampling mechanism. To inform this mission-critical decision, the surface of Bennu is mapped using the OSIRIS-REx Camera Suite and the images are used to develop …
We are interested in the periodic motion and bifurcations near the surface of an asteroid. The gravity field of an irregular asteroid and the equation of motion of a particle near the surface of an asteroid are studied. The periodic motions around the major body of triple asteroid 216 Kleopatra and the OSIRIS REx mission target asteroid 101955 Bennu are discussed. We find that motion near the surface of an irregular asteroid is quite different from the motion near the surface of a homoplastically spheroidal celestial body. The periodic motions around the asteroid 101955 Bennu and 216 Kleopatra…
Top-shaped asteroids have been observed among near-Earth asteroids. About half of them are reported to have moons (on the order of $\sim 1$wt.\% of the top-shaped primary) and many of them have an equatorial ridge. A recent study has shown that the enigmatic top-shaped figure of asteroids (e.g., Ryugu, Bennu, and Didymos) could result from an axisymmetric landslide of the primary during a fast spin-up near the breakup rotation period. Such a landslide would inevitably form a particulate disk around an asteroid with a short timescale ($\sim 3$ hours). However, the long-term full dynamical evolu…
The OSIRIS-REx spacecraft encountered the asteroid (101955) Bennu on December 3, 2018, and has since acquired extensive data from the payload of scientific instruments on board. In 2019, the OSIRIS-REx team selected primary and backup sample collection sites, called Nightingale and Osprey, respectively. On October 20, 2020, OSIRIS-REx successfully collected material from Nightingale. In this work, we apply an unsupervised machine learning classification through the K-Means algorithm to spectrophotometrically characterize the surface of Bennu, and in particular Nightingale and Osprey. We first …
The exploration of small body asteroids provides direct access to the primitive building blocks of our solar system. Sample and return missions enable chemical and radioisotope studies which not only provide evidence for the formation of the solar system, but also a basic understanding of where resources might be found for future exploitation. The touch-down and sample techniques established by Hayabusa-2 and OSIRIS-REx accomplish this mission by physically touching down on the asteroid and collecting samples into a basket extended via a probe from the bottom of the spacecraft. This technique …
The Hayabusa2 mission impact experiment on asteroid Ryugu created an unexpectedly large crater. The associated regime of low-gravity, low-strength cratering remained largely unexplored so far, because these impact conditions cannot be re-created in laboratory experiments on Earth. Here we show that the target cohesion may be very low and the impact probably occurred in the transitional cratering regime, between strength and gravity. For such conditions, our numerical simulations are able to reproduce the outcome of the impact on Ryugu, including the effects of boulders originally located near …
The thermal inertia of an asteroid's surface can provide insight into regolith properties, such as the presence of a layer of fine dust, the density and thermal conductivity of a rocky surface, and, together with other observational data, mineralogy. Knowledge of the surface characteristics of asteroids is important for planetary defense initiatives and the extraction of resources ("asteroid mining"). A simple means of estimating asteroid thermal inertia has been proposed by Harris & Drube, which is suitable for application to large sets of thermal-infrared observational data, such as those ob…
The surfaces of airless bodies such as asteroids are exposed to many phenomena that can alter their physical properties. Bennu, the target of the OSIRIS-REx mission, has demonstrated how complex the surface of a small body can be. In 2019 November, the potentially hazardous asteroid 2015 JD1 experienced a close approach of 0.0331 au from the Earth. We present results of the physical characterization of 2015 JD1 based on ground-based radar, spectroscopy, and photometric observations acquired during 2019 November. Radar polarimetry measurements from the Arecibo Observatory indicate a morphologic…
Near-Earth small-body mission targets 162173 Ryugu, 101955 Bennu, and 25143 Itokawa produce gravity fields around 4 orders of magnitude below that of Earth and their irregular shapes, combined with rotational effects produce varying surface potentials. Still, we observe familiar geologic textures and landforms that are the result of the granular physical processes that take place on their surfaces. The nature of these landforms, however, their origins, and how these surfaces react to interrogation by probes, landers, rovers, and penetrators, remain largely unknown, and therefore landing on an …
Understanding the geological modification processes on asteroids can provide information concerning their surface history. Images of small asteroids from spacecraft show a depletion in terms of smaller craters. Seismic shaking was considered to be responsible for crater erasure and the main driver modifying the geology of asteroids via regolith convection or the Brazil nut effect. However, a recent artificial impact experiment on the asteroid Ryugu by the Japanese Hayabusa2 mission revealed minimal seismic activity. To investigate whether a seismic shaking model can reproduce the observed crat…
Institut für Planetologie, Westfäelische Wilhelm Universität Münster（明斯特大学行星学研究所）；Center for Space and Habitability, University of Bern（伯尔尼大学空间与宜居性中心）；ETH Zurich（苏黎世联邦理工学院）
It has been recently recognized that the surface of sub-km asteroids in contact with the space environment is not fine-grained regolith but consists of centimeter to meter-scale rocks. Here we aim to understand how the rocky morphology of minor bodies react to the well known space erosion agents on the Moon. We deploy a neural network and map a total of ~130,000 fragmented boulders scattered across the lunar surface and visually identify a dozen different desintegration morphologies corresponding to different failure modes. We find that several fragmented boulder morphologies are equivalent to…
We consider how an impact generated seismic pulse affects the surface of an asteroid distant from the impact site. With laboratory experiments on dry polydisperse gravel mixtures, we track the trajectories of particles ejected from the surface by a single strong upward propagating pressure pulse. High speed video images show that ejecta trajectories are independent of particle size, and collisions primarily take place upon landing. When they land particles are ballistically sorted, as proposed by Shinbrot et al. (2017), leaving larger particles on the surface and smaller particles more widely …
The (153591) 2001 SN263 asteroid system, target of the first Brazilian interplanetary space mission, is one of the known three triple systems within the population of NEAs. One of the mission objectives is to collect data about the formation of this system. The analysis of these data will help in the investigation of the physical and dynamical structures of the components (Alpha, Beta and Gamma) of this system, in order to find vestiges related to its origin. In this work, we assume the irregular shape of the 2001 SN263 system components as uniform density polyhedra and computationally investi…
In this paper, we investigate thermophysical characteristics of near-Earth asteroid (341843) 2008 EV5, based on our improved Advanced Thermal Physical Model (ATPM) by considering the contribution of sunlight-reflection for rough surface, along with four wavebands observations from Wide-field Infrared Survey Explorer (WISE) and the radar-derived shape model. Here we derive that 2008 EV5 has a relatively low thermal inertia of $Γ=110 _{-12}^{+40}\rm~J m^{-2} s^{-1/2} K^{-1}$ but a high roughness fraction. The geometric albedo and effective diameter are then constrained to be $p_v=0.095_{-0.003}^…
