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Rubble pile asteroids are widely understood to be composed of reaccumulated debris following a catastrophic collision between asteroids in the main asteroid belt, where each disruption can make a family of new asteroids. Near-Earth asteroids Ryugu and Bennu have been linked to collisional families in the main asteroid belt, but surface age analyses of each asteroid suggest these bodies are substantially younger than their putative families. Here we show, through a coupled collisional and dynamical evolution of members of these families, that neither asteroid was likely to have been created at the same time as the original family breakups, but rather are likely remnants of later disruptions of original family members, making them second, or later, generation remnants. Our model finds about 80% and 60% of asteroids currently being delivered to near-Earth orbits from the respective families of New Polana and Eulalia are second or later generation. These asteroids delivered today in the 0.5-1 km size range have median ages since their last disruption that are substantially younger than the family age, reconciling their measured crater retention ages with membership in these families. Asteroids Ryugu and Bennu have been linked to collisional families in the main belt, but their surface ages suggest younger parents than their putative families. Here, the authors show numerical simulations that suggest these asteroids are likely remnants of later disruptions of original family members. [ABSTRACT FROM AUTHOR]

The mutual gravitational interaction of binary asteroids, which make up approximately 15 per cent of the near-Earth asteroid (NEA) population, provides a continuous tidal force, creating ground motion. We explore the potential of kilometre-sized binary asteroids as targets for seismological studies of their interior structure. We use a numerical model wherein each body is constructed of discrete particles interacting via gravity and contact forces. The system's orbital properties are modelled based on those of typical binary NEAs: a secondary body orbits a primary body at a distance of a few to 10 primary radii, resulting in orbital periods of a few tens of hours. We varied the elastic moduli (stiffness) of the constituent particles and measured a strain of a few micrometres caused by the orbiting satellite. Over eight orbital periods, the acceleration of the strain vector along the primary body's equatorial axis indicates that tidally induced ground motion generated by a binary asteroid system is detectable by modern seismometers, like the instruments deployed on the InSight mission to Mars. Owing to the relatively short orbital period of the satellite – a mean of 25.8 h for known binary NEAs – only a modest mission lifetime would be required for a seismometer to adequately characterize an asteroid's interior through tidally induced deformation. Future deployment of seismometers on binary asteroids will allow for a detailed characterization of the structure of these objects. [ABSTRACT FROM AUTHOR]

Two of the instruments onboard the OSIRIS-REx spacecraft, the MapCam color imager and the OVIRS visible and infrared spectrometer, observed the surface of asteroid (101955) Bennu in partially overlapping wavelengths. Significant scientific advances have been enabled by using data from these two instruments in tandem, but a robust statistical understanding of their relationship is needed for future analyses to cross-compare their data as accurately and sensitively as possible. Here we present a cross-instrument comparison of data acquired by MapCam and OVIRS, including methods and results for all global and site-specific observation campaigns in which both instruments were active. In our analysis, we consider both the absolute radiometric offset and the relative (normalized) variation between the two instruments; we find that both depend strongly on the photometric and instrumental conditions during the observation. The two instruments have a large absolute offset (>15%) due to their independent radiometric calibrations. However, they are very consistent (relative offset as low as 1%) when each instrument's response is normalized at a single wavelength, particularly at low phase angles where shadows on Bennu's rough surface are minimized. We recommend using the global datasets acquired at 12:30 pm local solar time for cross-comparisons; data acquired at higher phase angles have larger uncertainties. [ABSTRACT FROM AUTHOR]

Future missions carrying seismometer payloads to icy ocean worlds will measure global and local seismicity to determine where the ice shell is seismically active. We use two locations, a seismically active site on Gulkana Glacier, Alaska, and a more seismically quiet site on the northwestern Greenland Ice Sheet as geophysical analogs. We compare the performance of a single‐station seismometer against a small‐aperture seismic array to detect both high (>1 Hz) and low (<0.1 Hz) frequency events at each site. We created catalogs of high frequency (HF) and low frequency (LF) seismicity at each location using an automated short‐term average/long‐term average technique. We find that with a 1‐m small‐aperture seismic array, our detection rate increased (9% for Alaska and 46% for Greenland) over the single‐station approach. At Gulkana, we recorded an order of magnitude greater HF events than the Greenland site. We ascribe the HF events sources to a combination of icequakes, rockfalls, and ice‐water interactions, while very HF events are determined to result from bamboo poles that were used to secure gear. We further find that local environmental noise reduces the ability to detect LF global tectonic events. Based upon this study, we recommend that (a) future missions consider the value of the expanded capability of a small array compared to a single station, (b) design detection algorithms that can accommodate variable environmental noise, and (c) assess the potential landings sites for sources of local environmental noise that may limit detection of global events. Plain Language Summary: To better prepare for future planetary missions, we deployed seismometers on glaciers and ice sheets, environments on Earth that mimic those of icy ocean worlds. We compare the ability of a single seismometer versus several seismometers in detecting different types of earth and ice quakes and to compare widely different sites with respect to local environmental noise such as ice cracking, melt water from the glacier, and rock falls off nearby mountains. We find that multiple instruments separated by only 1 m can better detect large tectonic events than only one instrument. Further, if the site has low level of environment noise, we detect more large tectonic events. Small local events, however, can help characterize the local environment. We also detected events from equipment left at our field site. Future missions would benefit from sending multiple seismometers instead of just one. If a mission wants to study the whole planet or moon, then the landing site should be situated away from any active surface features and a single seismometer should be sufficient. If the goal is to study a specific active feature or region, then the landing site needs to be close to that feature. Key Points: Small‐aperture arrays (2 m) have higher detection rates (9%–46%) of regional and teleseismic events compared to a single‐stationLow noise sites allow more detection of global tectonic events, but offer less characterization of local environmentA single‐station seismometer in an icy ocean world analog setting can effectively detect and locate local seismicity [ABSTRACT FROM AUTHOR]

Active asteroids are those that show evidence of ongoing mass loss. We report repeated instances of particle ejection from the surface of (101955) Bennu, demonstrating that it is an active asteroid. The ejection events were imaged by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft. For the three largest observed events, we estimated the ejected particle velocities and sizes, event times, source regions, and energies. We also determined the trajectories and photometric properties of several gravitationally bound particles that orbited temporarily in the Bennu environment. We consider multiple hypotheses for the mechanisms that lead to particle ejection for the largest events, including rotational disruption, electrostatic lofting, ice sublimation, phyllosilicate dehydration, meteoroid impacts, thermal stress fracturing, and secondary impacts. [ABSTRACT FROM AUTHOR]

An asteroid's history is determined in large part by its strength against collisions with other objects1,2 (impact strength). Laboratory experiments on centimetre-scale meteorites3 have been extrapolated and buttressed with numerical simulations to derive the impact strength at the asteroid scale4,5. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples6 and images of the asteroid (25143) Itokawa7. It has not yet been possible, however, to assess directly the impact strength, and thus the absolute surface age, of the boulders that constitute the building blocks of a rubble-pile asteroid. Here we report an analysis of the size and depth of craters observed on boulders on the asteroid (101955) Bennu. We show that the impact strength of metre-sized boulders is 0.44 to 1.7 megapascals, which is low compared to that of solid terrestrial materials. We infer that Bennu's metre-sized boulders record its history of impact by millimetre- to centimetre-scale objects in near-Earth space. We conclude that this population of near-Earth impactors has a size frequency distribution similar to that of metre-scale bolides and originates from the asteroidal population. Our results indicate that Bennu has been dynamically decoupled from the main asteroid belt for 1.75 ± 0.75 million years. Analysis of the size and depth of craters on boulders on the asteroid (101955) Bennu indicates that Bennu has been in near-Earth space for 1.75 ± 0.75 million years. [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]

Rock breakdown due to diurnal thermal cycling has been hypothesized to drive boulder degradation and regolith production on airless bodies. Numerous studies have invoked its importance in driving landscape evolution, yet morphological features produced by thermal fracture processes have never been definitively observed on an airless body, or any surface where other weathering mechanisms may be ruled out. The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission provides an opportunity to search for evidence of thermal breakdown and assess its significance on asteroid surfaces. Here we show boulder morphologies observed on Bennu that are consistent with terrestrial observations and models of fatigue-driven exfoliation and demonstrate how crack propagation via thermal stress can lead to their development. The rate and expression of this process will vary with asteroid composition and location, influencing how different bodies evolve and their apparent relative surface ages from space weathering and cratering records. In their study, the authors discuss the potential of thermal weathering on airless bodies. As a case study, they use boulder and fracture morphologies on asteroid Bennu. [ABSTRACT FROM AUTHOR]

The principal objective of the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission is to retrieve a sample of the asteroid (101955) Bennu and return it to Earth. OSIRIS-REx arrived at Bennu in December 2018. Images of the asteroid by the OSIRIS-REx Camera Suite (OCAMS) were photogrammetrically controlled to produce a global basemap and site-specific image mosaics essential to the selection of a primary and backup sample site, which were announced in December 2019. In the control process, OCAMS images were registered to shape models created from OSIRIS-REx Laser Altimeter (OLA) data and from the process of stereophotoclinometry. This paper summarizes the photogrammetric control to date of images collected at Bennu. We briefly review the mission and the OCAMS imaging sensors. We then describe the photogrammetric control process for the global mapping campaign and targeted reconnaissance surveys of candidate sample sites. Finally, we discuss ongoing and future work. [ABSTRACT FROM AUTHOR]

The OSIRIS-REx Camera Suite (OCAMS) onboard the OSIRIS-REx spacecraft is used to study the shape and surface of the mission's target, asteroid (101955) Bennu, in support of the selection of a sampling site. We present calibration methods and results for the three OCAMS cameras—MapCam, PolyCam, and SamCam—using data from pre-flight and in-flight calibration campaigns. Pre-flight calibrations established a baseline for a variety of camera properties, including bias and dark behavior, flat fields, stray light, and radiometric calibration. In-flight activities updated these calibrations where possible, allowing us to confidently measure Bennu's surface. Accurate calibration is critical not only for establishing a global understanding of Bennu, but also for enabling analyses of potential sampling locations and for providing scientific context for the returned sample. [ABSTRACT FROM AUTHOR]

The Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer (OSIRIS‐REx) mission observed the Moon during the spacecraft's Earth gravity assist in 2017. From the spacecraft view, the lunar phase was 42°, and the in‐view hemisphere was dominated by anorthositic highlands terrain. Lunar spectra obtained by the OSIRIS‐REx Visible and InfraRed Spectrometer show evidence of several candidate absorption features. We observe the 2.8‐μm hydration band, confirming the spectral results from other missions, but detected in full‐disk spectra. We also tentatively identify weak spectral features near 0.9 and 1.3 μm, consistent with lunar regolith containing a mixture of plagioclase and orthopyroxene minerals, as expected for highlands terrain. Plain Language Summary: The Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer mission obtained observations of Earth and the Moon during a 2017 gravity assist maneuver. The Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer Visible and InfraRed Spectrometer acquired several spectra that covered the fully illuminated portion of the Moon. These spectra show absorption features consistent with the mineral mixtures expected in the lunar Highlands terrains. Near‐simultaneous images show that the side of the Moon that was visible is dominated by this type of terrain. In addition, an infrared absorption, consistent with water‐bearing minerals, is also observed, confirming the spectral identifications made by previous missions that have observed the Moon. Key Points: The OSIRIS‐REx mission obtained observations of Earth and the Moon during a 2017 gravity assist maneuverVisible and near‐infrared spectra of the Moon show features consistent with plagioclase‐dominated mixtures expected in highlands terrainThe 2.8‐μm hydration band is observed in full‐disk spectra, confirming previous spacecraft observations [ABSTRACT FROM AUTHOR]

NASA'S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at the near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth1. Bennu is a low-albedo B-type asteroid2 that has been linked to organic-rich hydrated carbonaceous chondrites3. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Therefore, the primary mission objective is to return a sample of Bennu to Earth that is pristine—that is, not affected by these processes4. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu's global properties, support the selection of a sampling site and document that site at a sub-centimetre scale5–11. Here we consider early OSIRIS-REx observations of Bennu to understand how the asteroid's properties compare to pre-encounter expectations and to assess the prospects for sample return. The bulk composition of Bennu appears to be hydrated and volatile-rich, as expected. However, in contrast to pre-encounter modelling of Bennu's thermal inertia12 and radar polarization ratios13—which indicated a generally smooth surface covered by centimetre-scale particles—resolved imaging reveals an unexpected surficial diversity. The albedo, texture, particle size and roughness are beyond the spacecraft design specifications. On the basis of our pre-encounter knowledge, we developed a sampling strategy to target 50-metre-diameter patches of loose regolith with grain sizes smaller than two centimetres4. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 metres in extent, the sampling of which poses a substantial challenge to mission success. Observations of asteroid (101955) Bennu with NASA's OSIRIS-REx spacecraft reveal an unexpected surficial diversity that poses a challenge to the success of the sample-return mission. [ABSTRACT FROM AUTHOR]

During its approach to asteroid (101955) Bennu, NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft surveyed Bennu’s immediate environment, photometric properties, and rotation state. Discovery of a dusty environment, a natural satellite, or unexpected asteroid characteristics would have had consequences for the mission’s safety and observation strategy. Here we show that spacecraft observations during this period were highly sensitive to satellites (sub-meter scale) but reveal none, although later navigational images indicate that further investigation is needed. We constrain average dust production in September 2018 from Bennu’s surface to an upper limit of 150 g s–1 averaged over 34 min. Bennu’s disk-integrated photometric phase function validates measurements from the pre-encounter astronomical campaign. We demonstrate that Bennu’s rotation rate is accelerating continuously at 3.63 ± 0.52 × 10–6 degrees day–2, likely due to the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, with evolutionary implications. [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]

Instroduction: The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft arrived at its target, near-Earth asteroid (101955) Bennu, in December 2018. After one year of operating in proximity, the team selected a primary site for sample collection. In October 2020, the spacecraft descended to the surface of Bennu and collected a sample. The spacecraft departed Bennu in May 2021 and will return the sample to Earth in September 2023 [3]. The analysis of the returned sample will produce key data to determine this B-type asteroid's history and that of its components and precursor objects. Bennu's mineralogy, as inferred from spectral characterization [2], is dominated by (1) hydrated silicates, (2) carbonates, (3) magnetite, (4) organic components. Other phases, such as sulfides, that are not spectrally active may also be abundant in the returned sample. The presence of organics on its surface make it an object of astrobiological interest, as it may hold important information early solar system conditions, the origin of organics on Earth, and the delivery of water to Earth. Preliminary Examination: The Preliminary Examination phase of the mission will begin once the sample canister is opened at NASA's Johnson Space Center (JSC). One of the primary goals of Preliminary Examination is the selection of returned samples for the OSIRIS-REx Sample Analysis Team to address mission science goals (up to 25 wt. percent), and samples archived by NASA (75 wt. percent) to include samples for international agencies (CSA 4 wt. percent and JAXA 0.5 wt. percent), samples for remote storage at White Sands Complex (WSC), and samples for hermetically sealed or cold storage. Imaging Objectives: Quantitative reflectance imaging of the returned sample will occur soon after the Sample Return Capsule is opened at JSC. This characterization provides quantitative information on the reflectance of the sample for discrete wavelengths comparable to the filter set on MapCam [5] with additional wavelength bands added for more comprehensive broadband spectral characterization. The objective of this investigation is to rapidly assess the lithological diversity using normal reflectance and spectral band ratios, similar to the color imaging of Bennu [1]. After evaluating the number of distinct lithologies, we will organize our scientific investigations with reference to our coordinated analysis investigation plan to ensure characterization of all major lithologies and a subset of minor lithologies, depending the heterogeneity of the material. Science Operations Proficiency Integrated Exercise 1 (SOPIE-1): A prototype Quantitative Reflectance Imaging System (QRIS) was used at OSIRIS-REx's Sample Analysis SOPIE-1 in February 2022 to image a sample of unknown composition under five wavelengths of light. Through analysis of the SOPIE-1 images, the OSIRIS-REx team identified 6 of the 8 unique lithologies present in the sample. Following SOPIE-1, we conducted further analysis and identified a candidate stone to represent one of the missing lithologies. Conclusion: The successful use of the prototype QRIS in this exercise suggests that a similar system, with a more sophisticated camera, mount, and software, will be sufficient for subsequent SOPIEs and the real Bennu sample selection. The detector, lighting, and optics procured during this project will be key components in producing sciencequality images at the SOPIE-2 in August 2022. [ABSTRACT FROM AUTHOR]