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A gravitational field of an ore-bearing asteroid, which usually has an irregular shape and heterogeneous density, is a prerequisite for asteroid exploration (e.g., space mining) missions. Thus, it is a hot topic in planetary science to model the external gravitational field of the ore-bearing asteroid in an efficient and accurate way. In this study, a computational fluid dynamics (CFD)-based method proposed in our previsous studies is investigated further. Firstly, six types of density distribution in asteroid Bennu are simulated as six experimental cases; then the gravitational fields are derived by using the CFD-based method; and finally, the results are compared to the other two solutions derived from the mascons gravity model method (as a benchmark method) and polyhedron gravity model method, respectively. The CFD-based method shows a superior performance in modelling the gravitational field of an irregularly-shaped asteriod with heterogeneous density in terms of both accuracy and efficiency. For example, the CFD-based method only costs 340 s obtaining 1,650,000 gravitational vectors outside the asteroid with a relative error of 1.27 %, compared to the computation time of 14,520 s and the solution accuracy of 3.84 % for the polyhedron gravity model method on the same testing points. The comparison study demonstrates a good potential application of the CFD-based gravitational field modeling method in asteroid exploration missions. • A CFD method can solve the gravity field of asteroids with various density types. • A CFD method is more efficient in solving gravitational field than other methods. • The mass distribution of an asteroid can be reflected in the external gravity field. • The CFD method exhibits efficient, accurate and stable properties. [ABSTRACT FROM AUTHOR]

Periodic orbit families that exist around the asteroid (101955) Bennu were computed and analyzed to gain insight into the dynamical environment about the asteroid. A constant-density polyhedron model was used to generate the families. The planar direct and retrograde families, and families emanating from equilibria, were computed. Ten distinct families were identified in this set, and many of the orbit structures were similar (e.g., the vertical families emanating from the equilibria behaved in similar ways), and several of these structures were connected to each other. We identified 12 distinct families emanating from bifurcation points in the initial families. These 12 families could be classified into four types. Even though the model of Bennu had no exact symmetry, many nearly symmetric structures were identified. There were also many similarities to structures identified using simplified models like the homogeneous rotating gravitating triaxial ellipsoid. The behavior of the identified families also provided insight into the evolution of the dynamical environment around the asteroid. We expect the qualitative behavior of the families we identified to be similar to the families that would exist around other asteroids that are nearly spherical. [ABSTRACT FROM AUTHOR]

The sample of asteroid (101955) Bennu was collected from the Nightingale sample site by the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer spacecraft and arrived on Earth on 24 September 2023. To better understand Bennu's parent body, we identified boulders over 2 m in diameter around the Nightingale region and analyzed normal albedo, morphology, and surface roughness. We found that boulders can be separated into two groups based on albedo, and four groups using morphology including angularity, texture, and the presence or absence of clasts, layers, and bright spots: Type A is rounded, rugged, and clastic, with the highest root‐mean square deviation roughness; Type B is sub‐angular with intermediate roughness and polygonal surface fractures; Type C is angular, has distinct fractures, and the lowest roughness; and Type D is sub‐angular with intermediate roughness and bright spots. Unsupervised clustering algorithms showed that our Type A‐D classification represents the diversity in the morphology and albedo data. Using documented contacts between boulder groups, we conclude that boulders on Bennu originated on a single, heterogeneous parent body that experienced vertical mixing via impacts prior to or during its disruption. The boulder morphologies on Bennu bear striking resemblance to those on asteroid Ryugu, potentially suggesting a shared origin. Finally, from analyses of sample collection images, we predict that the sample will be heterogeneous in morphology, brightness, and degree of aqueous alteration and dominated by darker Type A and B material. These predictions are supported by initial analyses of the Ryugu sample. Plain Language Summary: Asteroid Bennu is composed of fragments of an ancient, disrupted parent body. The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer spacecraft investigated Bennu and collected a sample, which arrived on Earth on 24 September 2023. We investigated the brightness, appearance, and roughness of boulders near where the sample was collected to better understand the parent body and the diversity of Bennu's surface materials. We found that boulders can be broken into four groups: Type A are dark, rough, and have clasts; Type B are smoother and are similarly bright and rough to Type A; Type C boulders are brighter and very smooth; and Type D boulders have distinct bright spots and similar brightness as Type C. Certain boulders contain more than one morphology, suggesting they formed near each other on the parent body, and impacts onto the parent body mixed rocks from different depths and cemented them into the boulders we observe on Bennu. Bennu boulders resemble those on Ryugu, which may mean they are related. We predict that dark Type A and B particles will be most abundant in the returned sample. Key Points: Boulders in the Nightingale sample region can be divided into four groups that vary in albedo, morphology, and surface roughnessContacts between groups indicate a single heterogeneous parent body modified by impacts and aqueous alteration prior to disruptionWe predict the returned sample will be diverse in reflectance and morphology that reflects the heterogeneity of meter‐scale boulders [ABSTRACT FROM AUTHOR]

Bennu, the target of the OSIRIS‐REx mission, is an asteroid with compositions analogous to low petrologic type CI, CM, CR, and/or ungrouped carbonaceous chondrites. Asteroids like Bennu provide information about the building blocks of the early Solar System. Analysis of the mid‐infrared remote sensing data informs mineral quantification. We apply a phyllosilicate specific model, developed by Breitenfeld et al. (2021, https://doi.org/10.1029/2021je007035) that distinguishes between Mg and Fe serpentines, to Baseball Diamond 1 (BBD1), equatorial station 3 (EQ3), and touch‐and‐go OSIRIS‐REx thermal emission spectrometer data. The average total phyllosilicate predictions are 73 (BBD1) and 72 vol% (EQ3). We observe higher Fe‐cronstedtite and lower Mg‐rich serpentine content in the equatorial region of Bennu than average. Mid‐infrared spectral variability may be explained by sorting effects through mass movement. Plain Language Summary: Bennu is an asteroid made of primitive materials that provide information about the early Solar System. Using infrared emission spectra and machine learning modeling, we estimate the amount and composition of phyllosilicate minerals across the surface of Bennu. Our results suggest that Bennu is primarily composed of phyllosilicate minerals as expected from previous studies. Mg‐ and Fe‐rich serpentine abundances are mapped across the asteroid Bennu. Near the equator, our model predicts higher cronstedtite (Fe‐rich serpentine) and lower Mg‐rich serpentine compared to the average composition of the asteroid. The spectral and compositional variability on Bennu can be explained by aqueous processing and sorting effects. Key Points: We estimate Bennu's phyllosilicate content is primarily Mg‐rich serpentine with smaller amounts of cronstedtite and saponiteWe predict higher cronstedtite and lower Mg‐rich serpentine abundances in the equatorial region and vice versa for the middle latitudesWe observe thermal emission spectrometer spectral variability between the equatorial and middle latitude regions of Bennu that may be due to sorting effects [ABSTRACT FROM AUTHOR]

NASA's first asteroid sample return mission, OSIRIS-REx, collected a sample from the surface of near-Earth asteroid Bennu in October 2020 and will deliver it to Earth in September 2023. Selecting a sample collection site on Bennu's surface was challenging due to the surprising lack of large ponded deposits of regolith particles exclusively fine enough (≤ 2 cm diameter) to be ingested by the spacecraft's Touch-and-Go Sample Acquisition Mechanism (TAGSAM). Here we describe the Sampleability Map of Bennu, which was constructed to aid in the selection of candidate sampling sites and to estimate the probability of collecting sufficient sample. "Sampleability" is a numeric score that expresses the compatibility of a given area's surface properties with the sampling mechanism. The algorithm that determines sampleability is a best fit functional form to an extensive suite of laboratory testing outcomes tracking the TAGSAM performance as a function of four observable properties of the target asteroid. The algorithm and testing were designed to measure and subsequently predict TAGSAM collection amounts as a function of the minimum particle size, maximum particle size, particle size frequency distribution, and the tilt of the TAGSAM head off the surface. The sampleability algorithm operated at two general scales, consistent with the resolution and coverage of data collected during the mission. The first scale was global and evaluated nearly the full surface. Due to Bennu's unexpected boulder coverage and lack of ponded regolith deposits, the global sampleability efforts relied heavily on additional strategies to find and characterize regions of interest based on quantifying and avoiding areas heavily covered by material too large to be collected. The second scale was site-specific and used higher-resolution data to predict collected mass at a given contact location. The rigorous sampleability assessments gave the mission confidence to select the best possible sample collection site and directly enabled successful collection of hundreds of grams of material. [ABSTRACT FROM AUTHOR]

We develop a continuum framework of regolith flow on asteroids. We focus on top-shaped asteroids that may be taken as consisting of regolith lying on a solid core. Depth-averaging is employed to model the regolith flow, and effects due to the asteroid's rotation and its complex gravity field are retained. Angular momentum conservation is invoked to couple regolith flow to the asteroid's changing shape and spin. This framework is first used to explore the equilibrium of regolith as a function of its friction, and the asteroid's shape and spin rate. Next, we study regolith flow on top-shaped spinning asteroids and find conditions for the regolith's shedding or deposition. We also discuss how the regolith's flow and the asteroid's spin influence each other. Finally, as an application, we propose and investigate the following evolution history of Bennu: a fast spinning Bennu was slowed down by multiple, impact-induced global landslides to its present spin state. Regolith was shed if the spin was higher than a critical rate. Once the spin rate fell below this critical value, regolith flow from higher latitudes began depositing regolith at its equator, giving Bennu its distinctive shape. [ABSTRACT FROM AUTHOR]

The OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft sampled asteroid (101955) Bennu on 20 October 2020 and will return the collected regolith to Earth in 2023. Before sample collection, spectral observations of four regions of interest on Bennu's surface were acquired at high spatial resolution (2–9 m per spectrometer spot) to identify the most suitable site for sampling and provide contextual information for the returned sample. In this study, we investigate thermal‐infrared (6–50 μm) observations of these four regions, including the site that OSIRIS‐REx ultimately sampled, using the Advanced Thermophysical Model with input digital terrain models derived from laser altimetry. From model‐to‐measurement comparisons, we find that the observed brightness temperatures depend strongly on small‐scale topography, local variations in thermal inertia, and the observation phase angle. Thermal inertia mapping reveals spatial variations that distinguish the different boulder types found on Bennu. A boulder bearing carbonate veins has higher thermal inertia than average, suggesting that cementation processes reduced its porosity. The thermal inertia of the site sampled is 190 ± 30 J m−2 K−1 s−1/2, which is consistent with observations of a fine‐grained regolith mixed with porous rocks. Thermophysical modeling of the site sampled predicts that the maximum temperatures experienced by the collected sample while on Bennu were 357 ± 3 and 261 ± 3 K for the surface and 50 cm depth, respectively. We predict that OSIRIS‐REx will return a sample with thermophysical properties unique from those of meteorites. Plain Language Summary: Thermal inertia is a physical characteristic of materials that, along with topography, controls how the surface temperature of a planetary body changes from day to night. On asteroids like Bennu, the target of the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS‐REx) sample return mission, thermal inertia is dictated by several properties of the surface, such as the porosity and abundance of boulders and the size of smaller particles. Thermal inertia is quantified by running numerical simulations of asteroid surface temperatures and comparing the results to the observed temperatures collected by a thermal‐infrared instrument onboard the OSIRIS‐REx spacecraft. Here, we ran a very high‐resolution temperature model, using topographic data from an onboard laser altimeter, for each site on Bennu that the OSIRIS‐REx mission considered sampling. The resulting local maps of thermal inertia distinguish boulders with different surface textures and porosities. The thermal inertia of the site that OSIRIS‐REx sampled is consistent with the observed presence of both small particles and porous boulders. The predicted maximum temperatures experienced by the collected samples while on Bennu imply that any ancient organic compounds will have undergone only minimal alteration by heating from the Sun. We expect the returned samples to be unique among meteorites, based on their distinct thermal properties. Key Points: Brightness temperatures of select sites on Bennu acquired at 2–9 m spatial scales depend on topography, thermal inertia, and phase angleThermophysical modeling using digital terrain models of these sites finds diversity in thermal inertia among bouldersThe site where regolith was sampled has a thermal inertia of 190 ± 30 J m–2 K–1 s–1/2, consistent with fine grains mixed with porous rocks [ABSTRACT FROM AUTHOR]

The surface of the rubble‐pile asteroid (101955) Bennu has been characterized in detail by the OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission. By examining global and local digital terrain models, we observed that Bennu possesses terraces, that is, a series of roughly latitude‐parallel, step‐like slope breaks. These partially circumscribe the poles and extend east‐west over several longitudinal quadrants at mid‐ to high (≥30°) latitudes. The terraces are subtle in amplitude, with heights ranging from 1 to 5 m. They often exhibit back‐wasting that results in V‐shaped scarps that open downslope in some locations. When boulders >5–10 m are absent at or near a terrace, the steeper portion (the drop) of the terrace lacks rocks, whereas the flatter portion (the bench) of the terrace has accumulations of rocks at its crest. When boulders >5–10 m are present, their steep downslope faces often make up the drop from the terrace crest, and they retain debris upslope, thereby enhancing the terrace structure. A geotechnical stability analysis indicates that Bennu's surface is likely unstable and that surface cohesion is <0.6 Pa. Bennu's terraces strongly resemble scarps generated in laboratory and numerical simulations of a cohesionless granular bed as the slope of the bed increases quasi‐statically. We conclude that terraces are probably actively forming on Bennu as its surface slowly fails owing to creep induced by spin acceleration. Plain Language Summary: Analyzing the landscape of a planetary body can shed light on the processes that shape its surface over time. We used data acquired by the OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission to explore the rubble‐covered landscape of the near‐Earth asteroid Bennu. We observed subtle step‐like features, or terraces, no more than about 5 m in relief (about half the height of a telephone pole). These terraces are approximately parallel to lines of latitude (i.e., they run east–west) and are located in both the northern and southern hemispheres of Bennu, but not near the equator. Accumulations of rocks tend to be present at the terrace crests, and the terrace faces are usually smooth. Boulders 5–10 m in size form some terraces and enhance the terrace structure by retaining loose material upslope. Laboratory experiments and numerical simulations show that subtle terraces can form in Bennu‐like conditions if the slope of the surface increases. Thus, these landforms are evidence for the ongoing "creep" of loose material on Bennu as surface slopes change in response to the gradual acceleration in the asteroid's spin rate. Key Points: Bennu exhibits subtle latitudinal terracing at mid‐ to high latitudesTerrace surface expressions follow expectations from laboratory and numerical investigationsTerraces are an expression of Bennu's current surface instability [ABSTRACT FROM AUTHOR]

Planetary surfaces can be complex mixtures of coarse and fine particles that exhibit linear and nonlinear mixing behaviors at mid‐infrared (MIR) wavelengths. Machine learning multivariate analysis can estimate modal mineralogy of mixtures and is favorable because it does not assume linear mixing across wavelengths. We used partial least squares (PLS) and least absolute shrinkage and selection operator (lasso), two types of machine learning, to build MIR spectral models to determine the surface mineralogy of the asteroid (101955) Bennu using OSIRIS‐REx Thermal Emission Spectrometer (OTES) data. We find that PLS models outperform lasso models. The cross‐validated root‐mean‐square error of our final PLS models (consisting of 317 unique spectra of samples derived from 13 analog mineral samples and eight meteorites) range from ∼4 to 13 vol% depending on the mineral group. PLS predictions in vol% of Bennu's average global composition are 78% phyllosilicate, 9% olivine, 11% carbonates, and 6% magnetite. Pyroxene is not predicted for the global average spectrum, though it has been detected in small amounts on Bennu. These mineral abundances confirm previous findings that the composition of Bennu is consistent with CI/CM chondrites with high degrees of aqueous alteration. The predicted mineralogy of two previously identified OTES spectral types vary minimally from the global average. In agreement with previous work, we interpret OTES spectral differences as primarily caused by relative abundances of fine particulates rather than major compositional variations. Plain Language Summary: The OTES instrument onboard the OSIRIS‐REx spacecraft collects infrared emission spectra that can, in principle, be used to determine the mineralogy of Bennu, the target asteroid of the OSIRIS‐REx mission. However, predicting mineral abundances on remote planetary bodies from infrared spectra is particularly complex when there are fine particles (<∼100 μm) on the surface. To circumvent this problem, we created a training set of mineral mixture spectra acquired under asteroid (vacuum) conditions and used machine learning to create models for mineral abundance predictions on asteroids like Bennu. Our results support previous findings that Bennu has a composition consistent with carbonaceous chondrites, the most primitive meteorites. Key Points: Machine learning models were constructed to predict phyllosilicate, olivine, carbonate, pyroxene, and magnetite abundances using mid‐infrared spectraMineral abundance predictions of Bennu indicate the composition is consistent with CI/CM chondrites with high degrees of aqueous alterationThe predicted mineralogy of two previously identified OSIRIS‐REx Thermal Emission Spectrometer spectral types vary minimally from the global average [ABSTRACT FROM AUTHOR]

Abstract 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.

Abstract We searched for an optimized protocol for mapping observations from a point spectrometer onto a shape model composed of triangular facets, in the context of NASA's asteroid sample return mission, OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer). Our study was conducted before the spacecraft arrived at the mission target asteroid (101955) Bennu, and we used observational sequence plans of the OSIRIS‐REx Visible and InfraRed Spectrometer (OVIRS). We explored six methods of mapping data to shape model facets, using three spatial resolutions. We attempted to boost map fidelity by increasing the observational coverage of the surface. We find that increasing shape model resolution improves mapping quality. However, once the shape model's mean facet edge length is smaller than two‐fifths of the diameter of the instrument's field of view (FOV), the increase in quality tapers off. The six mapping methods can be broken into two categories: facets that (1) select or (2) combine (average) data from observations. The quality differences between similar averaging methods (clipped average, weighted average, etc.) are insignificant. Selecting the nearest observation to a facet best preserves an enclosed outcrop's shape and signal, but averaging spots are more conservative against errors in photometric modeling. A completely enclosed outcrop border expands into the surrounding region by 0.8–1.5 radii of the instrument's FOV. Regions smaller than the instrument's FOV are present in resulting maps; however, their signal strength is reduced as a function of their size relative to the instrument FOV.

The CM chondrites are samples of primitive water-rich asteroids formed during the early solar system. They record significant interaction between liquid water and silicate rock, resulting in a mineralogy dominated by hydrated secondary phases. Their similarity to the near-Earth asteroids Bennu and Ryugu – targets of current sample return space missions – makes the analysis of CM chondrites essential to the interpretation of these enigmatic bodies. Here, we review the aqueous alteration history of the CM chondrite group. Initially, amorphous silicate, metal and sulphides within the matrix were converted into Fe-cronstedtite and tochilinite. Later, the serpentinization of refractory coarse-grained inclusions led to the addition of Mg to the fluid phase. This is reflected in the cation composition of secondary phases which evolved from Fe-rich to Mg-rich. Although most CM meteorites are classified as CM2 chondrites and retain some unaltered anhydrous silicates, a few completely altered CM1s exist (∼4.2% [Meteoritical Bulletin, 2021]). The extent of aqueous alteration can be quantified through various techniques, all of which trace the progression of secondary mineralization. Early attempts employed petrographic criteria to assign subtypes – most notably the Browning and Rubin scales have been widely adopted. Alternatively, bulk techniques evaluate alteration either by measuring the ratio of phyllosilicate to anhydrous silicate (this can be with X-ray diffraction [XRD] or infrared spectroscopy [IR]) or by measuring the combined H abundance/δD compositions. The degree of aqueous alteration appears to correlate with petrofabric strength (most likely arising due to shock deformation). This indicates that aqueous alteration may have been driven primarily by impact rather than by radiogenic heating. Alteration extent and bulk O-isotope compositions show a complex relationship. Among CM2 chondrites higher initial water contents correspond to more advanced alteration. However, the CM1s have lighter-than-expected bulk compositions. Although further analyses are needed these findings could suggest either differences in alteration conditions or initial isotopic compositions – the latter scenario implies that the CM1 chondrites formed on a separate asteroid from the CM2 chondrites. Secondary phases (primarily calcite) act as proxies for the conditions of aqueous alteration and demonstrate that alteration was prograde, with an early period at low temperatures (<70 °C), while later alteration operated at higher temperatures of 100–250 °C. Estimates for the initial water-to-rock ratios (W/R) vary between 0.2–0.7. They are based either on isotopic mass balance or mineral stoichiometry calculations – variability reflects uncertainties in the primordial water and protolith compositions and whether alteration was open or closed system. Some CM chondrites (<36%) experienced a later episode of post-hydration thermal metamorphism, enduring peak temperatures <900 °C and resulting in a dehydrated mineralogy and depleted volatile element abundances. Heating was likely short-duration and caused by impact events. The presence of CM chondrite material embedded in other meteorites, their prominence among the micrometeorite flux and the link between CMs and rubble-pile C-type near-Earth asteroids (e.g. Bennu and Ryugu) implies that the CM parent body was disrupted, leaving second-generation CM asteroids to supply material to Earth. [ABSTRACT FROM AUTHOR]

The CM carbonaceous chondrite meteorites provide a record of low temperature (<150 °C) aqueous reactions in the early solar system. A number of CM chondrites also experienced short-lived, post-hydration thermal metamorphism at temperatures of ∼200 °C to >750 °C. The exact conditions of thermal metamorphism and the relationship between the unheated and heated CM chondrites are not well constrained but are crucial to understanding the formation and evolution of hydrous asteroids. Here we have used position-sensitive-detector X-ray diffraction (PSD-XRD), thermogravimetric analysis (TGA) and transmission infrared (IR) spectroscopy to characterise the mineralogy and water contents of 14 heated CM and ungrouped carbonaceous chondrites. We show that heated CM chondrites underwent the same degree of aqueous alteration as the unheated CMs, however upon thermal metamorphism their mineralogy initially (300–500 °C) changed from hydrated phyllosilicates to a dehydrated amorphous phyllosilicate phase. At higher temperatures (>500 °C) we observe recrystallisation of olivine and Fe-sulphides and the formation of metal. Thermal metamorphism also caused the water contents of heated CM chondrites to decrease from ∼13 wt% to ∼3 wt% and a subsequent reduction in the intensity of the 3 μm feature in IR spectra. We estimate that the heated CM chondrites have lost ∼15 - >65% of the water they contained at the end of aqueous alteration. If impacts were the main cause of metamorphism, this is consistent with shock pressures of ∼20–50 GPa. However, not all heated CM chondrites retain shock features suggesting that some were instead heated by solar radiation. Evidence from the Hayabusa2 and ORSIRS-REx missions suggest that dehydrated materials may be common on the surfaces of primitive asteroids and our results will support upcoming analysis of samples returned from asteroids Ryugu and Bennu. [ABSTRACT FROM AUTHOR]

We searched for an optimized protocol for mapping observations from a point spectrometer onto a shape model composed of triangular facets, in the context of NASA's asteroid sample return mission, OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer). Our study was conducted before the spacecraft arrived at the mission target asteroid (101955) Bennu, and we used observational sequence plans of the OSIRIS‐REx Visible and InfraRed Spectrometer (OVIRS). We explored six methods of mapping data to shape model facets, using three spatial resolutions. We attempted to boost map fidelity by increasing the observational coverage of the surface. We find that increasing shape model resolution improves mapping quality. However, once the shape model's mean facet edge length is smaller than two‐fifths of the diameter of the instrument's field of view (FOV), the increase in quality tapers off. The six mapping methods can be broken into two categories: facets that (1) select or (2) combine (average) data from observations. The quality differences between similar averaging methods (clipped average, weighted average, etc.) are insignificant. Selecting the nearest observation to a facet best preserves an enclosed outcrop's shape and signal, but averaging spots are more conservative against errors in photometric modeling. A completely enclosed outcrop border expands into the surrounding region by 0.8–1.5 radii of the instrument's FOV. Regions smaller than the instrument's FOV are present in resulting maps; however, their signal strength is reduced as a function of their size relative to the instrument FOV. Key Points: Spectral map quality is controlled by shape model facet size, boresight spot size, and method of assigning spot data to facetsShape models with facet edges smaller than 2/5 the boresight spot diameter cease to improve map fidelityData selection algorithms perform better than data combination methods for mapping distinct outcrops [ABSTRACT FROM AUTHOR]

We describe the capabilities, radiometric stability, and calibration of a custom vacuum environment chamber capable of simulating the near‐surface conditions of airless bodies. Here we demonstrate the collection of spectral measurements of a suite of fine particulate asteroid analogs made using the Planetary Analogue Surface Chamber for Asteroid and Lunar Environments (PASCALE) under conditions like those found on Earth and on airless bodies. The sample suite includes anhydrous and hydrated physical mixtures, and chondritic meteorites (CM, CI, CV, CR, and L5) previously characterized under Earth‐ and asteroid‐like conditions. And for the first time, we measure the terrestrial and extra‐terrestrial mineral end members used in the olivine‐ and phyllosilicate‐dominated physical mixtures under the same conditions as the mixtures and meteorites allowing us better understand how minerals combine spectrally when mixed intimately. Our measurements highlight the sensitivity of thermal infrared emissivity spectra to small amounts of low albedo materials and the composition of the sample materials. As the albedo of the sample decreases, we observe smaller differences between Earth‐ and asteroid‐like spectra, which results from a reduced thermal gradient in the upper hundreds of microns in the sample. These spectral measurements can be compared to thermal infrared emissivity spectra of asteroid (101955) Bennu's surface in regions where similarly fine particulate materials may be observed to infer surface compositions. Plain Language Summary: In this work, we measure fine particulate terrestrial and extra‐terrestrial minerals, physical mixtures made from those minerals, and meteorites using a bespoke environment chamber at the University of Oxford. We selected minerals that were similar in composition to those found in carbonaceous chondrites and physical mixtures were made to simulate the composition of anhydrous and hydrated carbonaceous chondrites. Thermal infrared emissivity spectra were collected under Earth‐ and Bennu‐like conditions using a vacuum chamber capable of simulating the near‐surface conditions of planetary bodies with no appreciable atmosphere. These spectra are invaluable for to interpreting current and future observations of primitive Solar System bodies, particularly those with fine particulate regoliths. Key Points: Thermal infrared spectra of fine particulate minerals, physical mixtures of those minerals, and meteorites were measured under simulated Bennu conditionsComparisons of mineral, physical mixture, and meteorite spectra highlight the spectral behavior when materials are mixed in increasing complexityAs albedo decreases the spectral effects due to thermal gradients due to the vacuum environment of airless bodies are reduced [ABSTRACT FROM AUTHOR]

Bennu is an ~500‐m‐diameter rubble‐pile asteroid that is the target of detailed study by the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS‐REx) mission. Here we use data from the OSIRIS‐REx Laser Altimeter to assess depth‐to‐diameter ratios (d/D) of 108 impact craters larger than 10 m in diameter. The d/D of craters on Bennu ranges from 0.02 to 0.19. The mean is 0.10 ± 0.03. The smallest craters show the broadest range in d/D, consistent with d/D measurements on other asteroids. A few craters have central mounds, which is interpreted as evidence that a more competent substrate lies a few meters beneath them. The range of d/D narrows as crater size increases, with craters larger than 80 m tending toward smaller d/D. At large scales, increases in target strength with depth, combined with target curvature, may affect crater morphometry. Plain Language Summary: Between 2018 and 2020, National Aeronautics and Space Administration (NASA)'s Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS‐REx) spacecraft orbited a small asteroid called Bennu in preparation to collect a sample for return to Earth. Bennu is a "rubble‐pile" asteroid, meaning an aggregate of rock fragments that have coalesced together in space. OSIRIS‐REx observations showed that Bennu has many craters on its surface, which formed when other, smaller objects collided with it in the past. Crater depths and widths (diameters), in addition to relating to the size and speed of the impacting object, also reflect the physical characteristics of the impacted surface. Accordingly, we measured the depths and diameters of many of Bennu's craters to better understand the surface and interior properties of this rubble‐pile asteroid and how it compares to other asteroids. The smaller craters on Bennu have a variety of depths, even among similarly sized craters. The largest are so wide that they appear to be affected by the curvature of Bennu's surface and by the presence of stronger material at depth. We observe mounds inside some of the smaller craters, supporting the idea that a more competent substrate underlies the surface material. Key Points: The depth‐to‐diameter ratio (d/D) of asteroid Bennu's craters >10 m in diameter ranges from 0.02 to 0.19 with a mean of 0.10 ± 0.03Small craters show the greatest diversity in d/D, whereas larger craters (>80 m) span a narrower range in d/D and tend to be shallowerFor craters >80 m, increases in target strength with depth, combined with target curvature, likely contribute to smaller d/D [ABSTRACT FROM AUTHOR]

The exploration of near‐Earth asteroids has revealed dynamic surfaces characterized by mobile, unconsolidated material that responds to local geophysical gradients, resulting in distinct morphologies and boulder distributions. The OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer) mission confirmed that asteroid (101955) Bennu is a rubble pile with an unconsolidated surface dominated by boulders. In this work, we documented morphologies indicative of mass movement on Bennu and assessed the relationship to slope and other geologic features on the surface. We found globally distributed morphologic evidence of mass movement on Bennu up to ~70° latitude and on spatial scales ranging from individual boulders (meter scale) to a single debris flow ~100 m long and several meters thick. The apparent direction of mass movement is consistent with the local downslope direction and dominantly moves from the midlatitudes toward the equator. Mass movement appears to have altered the surface expression of large (≥30m diameter) boulders, excavating them in the midlatitudes and burying them in the equatorial region. Up to a 10 ± 1 m depth of material may have been transported away from the midlatitudes, which would have deposited a layer ~5 ± 1 m thick in the equatorial region assuming a stagnated flow model. This mass movement could explain the observed paucity of small (<50‐m diameter) craters and may have contributed material to Bennu's equatorial ridge. Models of changes in slope suggest that the midlatitude mass movement occurred in the past several hundred thousand years in regions that became steeper by several degrees. Plain Language Summary: Mass movement is the flow of loose material such as rock fragments across the surface of a planetary body (for instance, a landslide). We searched images of the surface of asteroid (101955) Bennu for evidence of mass movement. We found that rocks of various sizes have moved downslope, and evidence of this movement is apparent at most locations on the asteroid. By measuring the distribution of, and surface elevation around, the largest boulders on the surface of Bennu, we also found that the downslope movement of material appears to have excavated large boulders from the subsurface in the midlatitudes and buried large boulders near the equator. Our observation that material on Bennu has moved in what is currently the local downslope direction is not necessarily expected, because the downslope direction can change based on how quickly the asteroid is rotating, which varies over time. Thus, we can infer that this movement happened in the geologically recent past—probably within the past several hundred thousand years. These results can help us understand how geologic features like craters are erased, how the equatorial ridge formed, and how Bennu (and potentially other asteroids) change shape over time. Key Points: Signatures of mass movement on Bennu are globally distributed at multiple spatial scalesMass movement may have removed a ~10‐m‐thick layer of material from the midlatitudes and deposited a ~5‐m‐thick layer near the equatorMass movement that left visible evidence on Bennu occurred within the past several hundred thousand years [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The asteroid Bennu, the target of the OSIRIS‐REx sample return mission, was observed to be ejecting tiny rocks shortly after the spacecraft entered orbit. The three largest ejection events took place in the late afternoon local time, with an average interval of 2 weeks. Each event launched multicentimeter‐sized and smaller rocks into temporary orbits, where some escaped and others reimpacted Bennu. Given that all inner solar system objects are bombarded by cometary dust particles, we used a NASA model constructed to evaluate spacecraft impact risk to explore whether impacts could be the source of these events. We found that millimeter‐sized cometary dust particles not only strike Bennu in the late afternoon, matching observations, but also produce enough ejected debris to explain the orbiting particles, provided that the material being pummeled is weak. Key Points: Meteoroids derived from comets strike Bennu near perihelion once every 2 weeks on average with an impact kinetic energy >7,000 JThey can explain the particle sizes (<10 cm), speeds (<3.3 m s−1), and timing (late afternoon) of Bennu's largest observed particle ejection eventsFor meteoroid impacts to match observations, Bennu's surface must be as porous and structurally weak as common soils [ABSTRACT FROM AUTHOR]

The finding by the OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer) mission that its target (101955) Bennu is an active asteroid has raised questions as to whether the observed particle ejection events are driven by temperature. To investigate sublimation of water ice and rock thermal fracture as possible temperature‐driven causes, we modeled the global temperatures of Bennu and searched for correlations with the identified ejection points on the asteroid surface. We computed temperatures with the Advanced Thermophysical Model and the 75‐cm‐resolution global shape model of Bennu derived by the OSIRIS‐REx mission. We find that ~1,856 m2 of Bennu's polar regions have orbit‐averaged temperatures that are sufficiently cold to enable water ice, if buried within the top few meters of the surface, to remain stable over geological timescales. Millimeter thick layers of surface water ice are also stable over ~103‐year timescales within polar centimeter‐scale cold traps. However, we do not find evidence of conditions enabling ice stability in the warmer equatorial regions, where ejection events have been observed, implying that sublimation of water ice is not the cause of particle ejection. Conversely, rock thermal fracture remains a possible mechanism of particle ejection. We find high amplitudes of diurnal temperature variation, a proxy for the efficacy of thermal fracturing, at all latitudes on Bennu due to its extreme ruggedness. Therefore, if rock thermal fracture is the mechanism, particles could be ejected from any latitude, which is consistent with the continued observations of particle ejection by OSIRIS‐REx. Plain Language Summary: The OSIRIS‐REx mission discovered that particles are being ejected periodically from the surface of near‐Earth asteroid Bennu. Some hypotheses for the process (es) driving these ejection events relate to temperature. These include sublimation of volatile substances such as water (like in a comet) and thermal fracturing (cracking of rocks driven by day‐night temperature changes). To evaluate these hypotheses, we performed numerical simulations of temperatures across the surface of Bennu over its orbit. Temperatures on the majority of the surface, including at the ejection sites, are too warm for water ice to be present, even if covered by dust. We therefore conclude that sublimation of water ice is not responsible for the particle ejections. Nevertheless, portions of the polar regions are cold enough that subsurface water ice could exist. Small (centimeter‐scale) cold traps near the poles could store surface water ice for up to ~1,000 years. We find that thermal fracturing is a viable mechanism to explain the particle ejections because Bennu exhibits large day‐night differences in temperature. These large temperature differences occur even at high latitudes on the sunward facing sides of boulders. This widespread viability of thermal fracturing is consistent with the observation of particles ejecting from various latitudes on Bennu. Key Points: Modeled temperatures indicate that water ice sublimation is not the process ejecting particles from the surface of BennuSubsurface water ice however could be stable in small regions near the polesThe diurnal temperature curve has a large amplitude at all latitudes, which supports thermal fracturing as a cause of the ejection events [ABSTRACT FROM AUTHOR]

The Touch And Go Camera System (TAGCAMS) is a three-camera-head instrument onboard NASA’s OSIRIS-REx asteroid sample return mission spacecraft. The purpose of TAGCAMS is to facilitate navigation to the target asteroid, (101955) Bennu; confirm acquisition of the asteroid sample; document asteroid sample stowage; and provide supplementary imaging for OSIRIS-REx science investigations. During the almost two-year OSIRIS-REx outbound cruise phase we pursued nine TAGCAMS imaging campaigns to check, calibrate and characterize the camera system’s performance before asteroid arrival and proximity operations began in late 2018. The TAGCAMS in-flight calibration dataset provides the relevant information to enable the three cameras to complete their primary observation goals during asteroid operations. The key performance parameters that we investigated in flight included: linearity, responsivity (both point source and extended body), dark current, hot pixels, pointing, image geometry transformation, image quality and stray light. Analyses of the in-flight performance either confirmed the continued applicability of the TAGCAMS ground test results or substantially improved upon the ground test knowledge. In addition, the TAGCAMS calibration observations identified the source of a spacecraft outgassing feature that guided successful remediation efforts prior to asteroid arrival. [ABSTRACT FROM AUTHOR]

This paper explores the implications of the observed Bennu particle ejection events for that asteroid's spin rate and orbit evolution, which could complicate interpretation of the Yarkovsky‐O'Keefe‐Radzievskii‐Paddack (YORP) and Yarkovsky effects on this body's spin rate and orbital evolution. Based on current estimates of particle ejection rates, we find that the overall contribution to Bennu's spin and orbital drift is small or negligible as compared to the Yarkovsky and YORP effects. However, if there is a large unseen component of smaller mass ejections or a strong directionality in the ejection events, it could constitute a significant contribution that could mask the overall YORP effect. This means that the YORP effect may be stronger than currently assumed. The analysis is generalized so that the particle ejection effect can be assessed for other bodies that may be subject to similar mass loss events. Further, our model can be modified to address different potential mechanisms of particle ejection, which are a topic of ongoing study. Plain Language Summary: The near‐Earth asteroid Bennu has been observed to be ejecting particles of rock from its surface. The possible effect of these particle ejections on the asteroid's spin rate and orbit is studied using numerical modeling. We show that the effect is likely minimal, although for certain ejection geometries, their effect on the spin rate could be more important. The loss of mass caused by the particle ejections would likely limit the asteroid's rotational acceleration, suggesting that the acceleration effect on this body may be larger than assumed. We consider how different mechanisms of ejection could be modeled using our approach. Key Points: The net contribution of particle ejection from Bennu's surface on its spin rate and orbit evolution is minimal under some assumptionsIf there are systematics in ejection geometry, there may be meaningful contributions to rotational accelerationOur model can be tuned to address different potential mechanisms of particle ejection [ABSTRACT FROM AUTHOR]

The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft launched on September 8, 2016, beginning a seven-year journey to return at least 60 g of asteroid material from (101955) Bennu to Earth. During the outbound cruise, Doppler tracking of the spacecraft observed a small but measurable acceleration when the sample return capsule (SRC) was first placed in sunlight. Subsequent analysis determined that outgassing from the SRC is the most likely cause for the acceleration. This outgassing received combined engineering and scientific attention because it has potential implications both for spacecraft navigation performance and for contamination of the collected samples. Thermal modeling, laboratory studies of SRC materials, and monitoring of the acceleration are all consistent with H 2 O as the main component of the outgassing. Dedicated, in-flight campaigns continued to expose the SRC to sunlight until the acceleration dropped to the acceleration noise floor. Any residual amounts of H 2 O outgassing are not considered to be a hazard with regards to mission operations or pristine sample acquisition. The sample stow procedure has been updated to ensure that no direct line of site exists between any residual outgassing and the samples during future operations. Similar outgassing of the Stardust SRC probably also occurred. No adverse contamination of Stardust samples was observed that could be associated with this process. Future missions that use similar reentry vehicles should consider procedures to test for and, if necessary, mediate such outgassing after launch. • OSIRIS-REx accelerated slightly when sunlight hit its sample return capsule (SRC). • Outgassing from the SRC is the most likely cause of the acceleration. • Thermal modeling, lab studies, and spacecraft monitoring are all consistent with H 2 O outgassing. • Outgassing of remaining H 2 O is not a hazard to mission operations or pristine sample acquisition. • Similar outgassing of the Stardust SRC probably also occurred. [ABSTRACT FROM AUTHOR]

Though rare, asteroid impacts are inevitable, and with the current state of technology, kinetic impactors are the preferred but not the complete solution. If the time to impact is short, or the threatening body too large, nuclear deflection serves as a final option. This work is part of an integrated study by National Aeronautics and Space Administration (NASA) and the National Nuclear Security Administration (NNSA) to better determine the relative efficacy of these complimentary approaches. In particular, we examine the important material properties that affect each approach, to improve critical characterization efforts, and reduce uncertainty in the limits of the impactor technology. Impact speeds for kinetic impactors on Near-Earth Object (NEO) intercept trajectories commonly range from 5 to 20 km/s, resulting in significant crater ejecta and a momentum enhancement above that carried by the impactor. This enhancement depends substantially on the strength and porosity of the asteroid, as well as the impact speed. Here simulations from different codes are presented, along with constraints from experimental measurements. The uncertainties due to ignorance of the strength and porosity of the impact point are significant in determining the limits of impactor sufficiency. The nuclear approach is considered within the context of current capabilities, posing no need to test, as extant and well-understood devices are sufficient for the largest known Potentially Hazardous Objects (PHOs). Results of x-ray sources with realistic spectra as well as blackbody spectra are given, along with some assessment on composition dependence. • Asteroid Bennu can be successfully deflected by extant nuclear devices. • Kinetic impact, even with large momentum enhancement, is unlikely to deflect Bennu. • Nuclear deflection efficacy depends upon the x-ray source spectrum. • Nuclear deflection efficacy is enhanced by near-surface, unbound volatiles. • Code comparisons for kinetic and nuclear deflection simulations show good agreement. [ABSTRACT FROM AUTHOR]

This paper presents an improved algorithm for the 2D and 3D Fourier forward modeling of gravity fields caused by polyhedral bodies with constant and exponential density distributions. Three modifications have been made to the Fourier forward algorithm introduced in a previous paper. First, vertex-based Fourier-domain expressions are used instead of the original face-based Fourier-domain expressions, which simplify the computation of the anomaly spectrum considerably, especially in 3D modeling problems. Second, instead of using a pure Gauss-FFT sampling of the anomaly spectrum, we apply an improved sampling strategy by combining a nonuniform spherical sampling with a low-order Gauss-FFT sampling. In this way, the number of samplings required in the Fourier domain reduces to about 1 3 and 1 7 of those required in a pure Gauss-FFT algorithm for 2D and 3D modeling problems, respectively. A significant acceleration over the original algorithm is achieved. Third, we incorporate all three types of nonuniform fast Fourier transform algorithms to transform directly a uniform or nonuniform anomaly spectrum to gravity fields either on a regular grid, or at a set of arbitrary positions. Extra interpolation operations are no longer needed. Synthetic numerical tests show that for gravity vector components, the new algorithm runs about 3 times faster in 2D modeling and 7 times faster in 3D modeling than the original ones, while maintaining the same level of accuracy. For the gravity potential, the new algorithm is significantly superior to the pure Gauss-FFT solution both in numerical accuracy and in efficiency. We apply this novel approach to compute the gravitational fields of asteroid 101955 Bennu and comet 67 P/Churyumov –Gerasimenko. The 2D algorithm works very efficiently for the computation of gravity fields on horizontal planes. The 3D algorithm is valid both outside, on, and inside the source's bounding surface, with relative errors less than 0.1% for the gravity potential and less than 2% for the gravity vector. By comparing to modeling results of analytical and spherical harmonic-based solutions, we generally conclude that the Fourier-based algorithm introduced here is an attractive alternative to these conventional solutions, especially for nonspherical, irregularly shaped bodies with complex geometries. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: We observed near‐Earth asteroid (101955) Bennu—the target of the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS‐REx) spacecraft mission—using ground‐based telescopes and the Hubble Space Telescope. Our measurements show that its rotation has been speeding up since 1999. This change could be due to a change in the shape of Bennu or to the Yarkovsky‐O'Keefe‐Radzievskii‐Paddack effect: As the sunlight received by an asteroid is reflected or radiated back to space, the change in direction of the light coming in and going out pushes on the asteroid and can cause it to spin faster or slower, depending on its shape and orientation. The specific properties of the surface, such as the position of boulders, can have a large influence on the outcome. In the next few years, the OSIRIS‐REx team will study Bennu and independently measure its rotation, helping us to better understand the structure and dynamical evolution of small asteroids. Key Points: Asteroid (101955) Bennu has been rotating faster over time since 1999An acceleration in its rotation rate is needed to fit all the photometric measurements of asteroid BennuThe measured acceleration of Bennu is consistent with theoretical calculations of the total effects of the YORP effect [ABSTRACT FROM AUTHOR]

The OSIRIS‐REx Asteroid Sample Return Mission is the third mission in National Aeronautics and Space Administration (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 several foundational data products. Acquiring the necessary inputs to these data products requires observational strategies that are defined specifically to overcome the challenges associated with mapping a small irregular body. We present these strategies in the context of assessing candidate sample sites at Bennu according to a framework of decisions regarding the relative safety, sampleability, and scientific value across the asteroid's surface. To create data products that aid these assessments, we describe the best practices developed by the OSIRIS‐REx team for image‐based mapping of irregular small bodies. We emphasize the importance of using 3‐D shape models and the ability to work in body‐fixed rectangular coordinates when dealing with planetary surfaces that cannot be uniquely addressed by body‐fixed latitude and longitude. Plain Language Summary: The OSIRIS‐REx Asteroid Sample Return Mission must map asteroid (101955) Bennu using the OSIRIS‐REx Camera Suite. Here we present the techniques that are established to accomplish this goal. Mapping helps us find the best place on the surface of Bennu from which to gather a sample. Because asteroids are small bodies with weak gravitational fields, maneuvering a spacecraft around them can be challenging. Considering these complexities, we have found ways to gather images of Bennu needed for creating maps. Additionally, due to the irregular shape of many asteroids, producing 2‐D maps in terms of latitude and longitude may be insufficient for describing their surface geography. To that end, we have developed software that is capable of creating and displaying image maps in 3‐D. Key Points: The OSIRIS‐REx Asteroid Sample Return Mission performs image‐based mapping of (101955) Bennu to aid in the selection of a sample‐siteWe develop observational strategies to perform mapping to address the challenges associated with surveying a small bodyWe identify pitfalls and best practices for mapping images of small bodies with large concavities, elongated axes, or overhanging terrain [ABSTRACT FROM AUTHOR]

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 (8.8 W average), and robust instrument with the sensitivity needed to detect a 5% spectral absorption feature on a very dark surface (3% reflectance) in the inner solar system (0.89–1.35 AU). It, in combination with the other instruments on the OSIRIS-REx Mission, will provide an unprecedented view of an asteroid's surface. [ABSTRACT FROM AUTHOR]

The OSIRIS-REx mission will conduct a Radio Science investigation of the asteroid Bennu with a primary goal of estimating the mass and gravity field of the asteroid. The spacecraft will conduct proximity operations around Bennu for over 1 year, during which time radiometric tracking data, optical landmark tracking images, and altimetry data will be obtained that can be used to make these estimates. Most significantly, the main Radio Science experiment will be a 9-day arc of quiescent operations in a 1-km nominally circular terminator orbit. The pristine data from this arc will allow the Radio Science team to determine the significant components of the gravity field up to the fourth spherical harmonic degree. The Radio Science team will also be responsible for estimating the surface accelerations, surface slopes, constraints on the internal density distribution of Bennu, the rotational state of Bennu to confirm YORP estimates, and the ephemeris of Bennu that incorporates a detailed model of the Yarkovsky effect. [ABSTRACT FROM AUTHOR]

NASA's OSIRIS-REx asteroid sample return mission spacecraft includes the Touch And Go Camera System (TAGCAMS) three camera-head instrument. The purpose of TAGCAMS is to provide imagery during the mission to facilitate navigation to the target asteroid, confirm acquisition of the asteroid sample, and document asteroid sample stowage. The cameras were designed and constructed by Malin Space Science Systems (MSSS) based on requirements developed by Lockheed Martin and NASA. All three of the cameras are mounted to the spacecraft nadir deck and provide images in the visible part of the spectrum, 400–700 nm. Two of the TAGCAMS cameras, NavCam 1 and NavCam 2, serve as fully redundant navigation cameras to support optical navigation and natural feature tracking. Their boresights are aligned in the nadir direction with small angular offsets for operational convenience. The third TAGCAMS camera, StowCam, provides imagery to assist with and confirm proper stowage of the asteroid sample. Its boresight is pointed at the OSIRIS-REx sample return capsule located on the spacecraft deck. All three cameras have at their heart a 2592 × 1944 pixel complementary metal oxide semiconductor (CMOS) detector array that provides up to 12-bit pixel depth. All cameras also share the same lens design and a camera field of view of roughly 44 ∘ × 32 ∘ with a pixel scale of 0.28 mrad/pixel. The StowCam lens is focused to image features on the spacecraft deck, while both NavCam lens focus positions are optimized for imaging at infinity. A brief description of the TAGCAMS instrument and how it is used to support critical OSIRIS-REx operations is provided. [ABSTRACT FROM AUTHOR]

OSIRIS-REx will return pristine samples of carbonaceous asteroid Bennu. This article describes how pristine was defined based on expectations of Bennu and on a realistic understanding of what is achievable with a constrained schedule and budget, and how that definition flowed to requirements and implementation. To return a pristine sample, the OSIRIS-REx spacecraft sampling hardware was maintained at level 100 A/2 and <180 ng/cm2 of amino acids and hydrazine on the sampler head through precision cleaning, control of materials, and vigilance. Contamination is further characterized via witness material exposed to the spacecraft assembly and testing environment as well as in space. This characterization provided knowledge of the expected background and will be used in conjunction with archived spacecraft components for comparison with the samples when they are delivered to Earth for analysis. Most of all, the cleanliness of the OSIRIS-REx spacecraft was achieved through communication among scientists, engineers, managers, and technicians. [ABSTRACT FROM AUTHOR]

The Canadian Space Agency (CSA) has contributed to the Origins Spectral Interpretation Resource Identification Security-Regolith Explorer (OSIRIS-REx) spacecraft the OSIRIS-REx Laser Altimeter (OLA). The OSIRIS-REx mission will sample asteroid 101955 Bennu, the first B-type asteroid to be visited by a spacecraft. Bennu is thought to be primitive, carbonaceous, and spectrally most closely related to CI and/or CM meteorites. As a scanning laser altimeter, the OLA instrument will measure the range between the OSIRIS-REx spacecraft and the surface of Bennu to produce digital terrain maps of unprecedented spatial scales for a planetary mission. The digital terrain maps produced will measure $\sim7~\mbox{cm}$ per pixel globally, and $\sim3~\mbox{cm}$ per pixel at specific sample sites. In addition, OLA data will be used to constrain and refine the spacecraft trajectories. Global maps and highly accurate spacecraft trajectory estimates are critical to infer the internal structure of the asteroid. The global and regional maps also are key to gain new insights into the surface processes acting across Bennu, which inform the selection of the OSIRIS-REx sample site. These, in turn, are essential for understanding the provenance of the regolith sample collected by the OSIRIS-REx spacecraft. The OLA data also are important for quantifying any hazards near the selected OSIRIS-REx sample site and for evaluating the range of tilts at the sampling site for comparison against the capabilities of the sample acquisition device. [ABSTRACT FROM AUTHOR]

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 January 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 Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu's resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023. [ABSTRACT FROM AUTHOR]

Top Shaped Asteroids (TSAs) have proven to be common amongst the near-Earth rubble pile population, with multiple examples confirmed via groundbased radar and spaceborne optical sensors through the past 20 years. A substantial body of literature has developed, exploring the formation of these unique shapes either through rotation-induced landslides and creep, or collisional reaccumulation. Models of such mass movements can provide good explanations for mid and low latitude material redistribution, but Bennu also shows a significant increase in radius in the high polar regions, which is harder to explain with these processes. The discovery of repeated and probably ongoing particle ejections around the 500 m diameter asteroid Bennu by the OSIRIS-REx mission suggests that we need to consider an alternate or additional mechanism which, we show, can anticipate the detailed variety of TSA shapes. This paper explores asteroid shape evolution as the result of particle ejections, modeled as being simply correlated with latitude via diurnal heating (or meteorite impacts), and re-accumulation using simulations including gravity and solar radiation pressure. Asteroid outlines are evolved with time as a function of particle ejection velocities and asteroid rotation rates. Bennu's shape can be anticipated by our model with RMS surface errors of less than 1.1% (2.7 m) although some southern latitudes have errors up to 10 m. Straightforward variation in conditions can produce shapes matching other TSAs. However, the observed particle fluxes on Bennu are approximately 3 orders of magnitude too low for this to be the only shaping mechanism on Bennu. The time necessary to form these shapes by our mechanism alone is far longer than the lifetimes of near-Earth asteroids, unless fluxes were once much greater, or there was an underlying oblate shape. • Particle ejections from a rubble pile asteroid can produce 'top shapes' consistent with observations. • Assuming particle launches are latitude dependant significantly improves shape matching. • Observed particle flux is too low to account for current shapes given the lifetime of near Earth asteroids. • Low rotation rates can lead to prolate growth and rotational instability [ABSTRACT FROM AUTHOR]

The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission revealed that the asteroid Bennu has ~1500 impact craters (Bierhaus et al., 2022). Here we use data from the OSIRIS-REx laser altimeter (OLA) to measure the depths, d, and diameters, D, of a sample of small (D ≤ 10 m) impact craters that span a broad range of latitudes and longitudes. d/D in this sample ranges from 0.04 to 0.27, with a mean of 0.13 ± 0.04. Compared to larger (D ≥ 10 m) craters on Bennu, those studied here have a greater average d/D. The mean d/D of craters on Bennu's younger, Rugged Unit is statistically larger than the mean d/D of those on the older, Smooth Unit. One of the studied craters contains an interior mound that may indicate the presence of stronger material at depth. A large fraction of craters studied here are located near large boulders whose lengths are >50% the diameter of the crater. Some of these large boulders cross the crater rim crest, protruding into and perched above the crater interior. In the most extreme cases, the boulder length is ~2.5 times the crater diameter. The d/D of craters near large boulders is bimodal, which could be a consequence of how boulders affect crater formation. Laboratory experiments and results from Hayabusa2's Small Carry-On Impactor experiment indicate that preexisting boulders and heterogeneities affect crater formation and d/D. The abundance of small craters on Bennu makes the asteroid a rich resource for understanding impact processes. • Craters on Bennu smaller than ~10 m have depth-diameter (d/D) ratios between 0.04 and 0.27, with a mean of 0.13 0.04.Craters on the Rugged Unit have a larger average d/D than those on the Smooth Unit. • The number of craters with a central mound is consistent with predictions based larger craters. • The distribution of d/D suggests that crater formation is affected by how far large boulders protrude above the crater. [ABSTRACT FROM AUTHOR]

Assessments of asteroid deflection strategies depend on material characterization to reduce the uncertainty in predictions of the deflection velocity resulting from impulsive loading. In addition to strength, equation of state, the initial state of the material including its competency (i.e. fractured or monolithic) and the amount of micro- or macroscopic porosity are important considerations to predict the thermomechanical response. There is recent interest in observing near-Earth asteroid (101955) Bennu due to its classification of being potentially hazardous with close approaches occurring every 6 years. Bennu is relatively large with a nominal diameter of 492 m, density estimates ranging from 0.9-1.26 g/cm 3 and is composed mainly of carbonaceous chondrite. There is a lack of data for highly porous carbonaceous chondrite at very large pressures and temperatures. In the absence of the specific material composition and state (e.g. layering, porosity as a function of depth) on Bennu we introduce a continuum constitutive model based on the response of granular materials and provide impact and standoff explosion simulations to investigate the response of highly porous materials to these types of impulsive loading scenarios. Simulations with impact speeds of 5 km/s show that the shock wave emanating from the impact site is highly dispersive and that a 10% porous material has a larger compacted volume compared with a 40% porous material with the same bulk density due to differences in compaction response. A three-dimensional simulation of a 190 kT standoff explosion 160 m off the surface of a shape model of Bennu estimated a deflection velocity of 10 cm/s. [ABSTRACT FROM AUTHOR]

Global geologic maps are useful tools for efficient interpretation of a planetary body, and they provide global context for the diversity and evolution of the surface. We used data acquired by the OSIRIS-REx spacecraft to create the first global geologic map of the near-Earth asteroid (101955) Bennu. As this is the first geologic map of a small, non-spherical, rubble-pile asteroid, we discuss the distinctive mapping challenges and best practices that may be useful for future exploration of similar asteroids, such as those to be visited with the Hera and Janus missions. By mapping on two centimeter-scale global image mosaics (2D projected space) and a centimeter-scale global shape model (3D space), we generated three input maps respectively describing Bennu's shape features, geologic features, and surface texture. Based on these input maps, we defined two geologic units: the Smooth Unit and the Rugged Unit. The units are differentiated primarily on the basis of surface texture, concentrations of boulders, and the distributions of lineaments, mass movement features, and craters. They are bounded by several scarps. The Rugged Unit contains abundant boulders and signs of recent mass movement. It also has fewer small (<20 m), putatively fresh craters than the Smooth Unit, suggesting that such craters have been erased in the former. Based on these geologic indicators, we infer that the Rugged Unit has the younger surface of the two. Differential crater size-frequency distributions and the distribution of the freshest craters suggest that both unit surfaces formed ~10–65 million years ago, when Bennu was located in the Main Asteroid Belt, and the Smooth Unit has not been significantly resurfaced in the past 2 million years. Meanwhile, the Rugged Unit has experienced resurfacing within the past ~500,000 years during Bennu's lifetime as a near-Earth asteroid. The geologic units are consistent with global diversity in slope, surface roughness, normal albedo, and thermal emission spectral characteristics. The site on Bennu where the OSIRIS-REx mission collected a regolith sample is located in the Smooth Unit, in a small crater nested within a larger one. So although the Smooth Unit is an older surface than the Rugged Unit, the impact-crater setting indicates that the material sampled was recently exposed. Several similarities are apparent between Bennu and asteroid (162173) Ryugu from a global geologic perspective, including two geologic units distinguishable by variations in the number density of boulders, as well as in other datasets such as brightness. • We created a global geologic map of rubble-pile asteroid Bennu. • Two geologic units are present: one smooth, one rugged. • The Smooth Unit is a > 2 Myr old surface that was shaped in the Main Asteroid Belt. • The Rugged Unit was resurfaced via mass movement in the past ~0.5 Myr. [ABSTRACT FROM AUTHOR]