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The Lucy Mission is a NASA Discovery-class mission to send a highly capable and robust spacecraft to investigate seven primitive bodies near both the L4 and L5 Lagrange points with Jupiter: the Jupiter Trojan asteroids. These planetesimals from the outer planetary system have been preserved since early in solar system history. The Lucy mission will fly by and extensively study a diverse selection of Trojan asteroids, including all the recognized taxonomic classes, a collisional family member, and a near equal-mass binary. It will visit objects with diameters ranging from roughly 1 km to 100 km. The payload suite consists of a color camera and infrared imaging spectrometer, a high-resolution panchromatic imager, and a thermal infrared spectrometer. Additionally, two spacecraft subsystems will also contribute to the science investigations: the terminal tracking cameras will supplement imaging during closest approach and the telecommunication subsystem will be used to measure the mass of the Trojans. The science goals are derived from the 2013 Planetary Decadal Survey and include determining the surface composition, assessing the geology, determining the bulk properties, and searching for satellites and rings.

A planetary surface’s resistance to change is generally described as its “strength” (units of stress). The surface strength of small, rubble-pile asteroids, which consist of fragments of larger bodies that were collisionally disrupted, is poorly constrained due to their wide departure from terrestrial analogs. Here, we report the observation of an ejecta deposit surrounding an impact crater that limits the maximum surface strength of the near-Earth rubble-pile asteroid (101955) Bennu. The presence of this deposit implies that ejecta were mobilized with velocities less than the escape velocity of Bennu, 20 cm/s. Because ejecta velocities increase with surface strength, the ejecta deposit can only be explained if the effective strength of the surface material near the crater is exceedingly low, ≤100 Pa. This is three orders of magnitude below values commonly used for asteroid surfaces, but is supported by previous observations of an artificial impact crater on a similar asteroid, Ryugu. Our findings indicate a mobile surface that has likely been renewed multiple times since Bennu’s initial assembly and have far-reaching implications for interpreting observations of Bennu and other rubble piles.

International audience; We manually mapped particles ranging in longest axis from 0.3 cm to 95 m on (101955) Bennu for the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission. This enabled the mission to identify candidate sample collection sites and shed light on the processes that have shaped the surface of this rubble-pile asteroid. Building on a global survey of particles, we used higher-resolution data from regional observations to calculate particle size-frequency distributions (PSFDs) and assess the viability of four candidate sites for sample collection (presence of unobstructed particles ≤ 2 cm). The four candidate sites have common characteristics: each is situated within a crater with a relative abundance of sampleable material. Their PSFDs, however, indicate that each site has experienced different geologic processing. The PSFD power-law slopes range from −3.0 ± 0.2 to −2.3 ± 0.1 across the four sites, based on images with a 0.01-m pixel scale. These values are consistent with, or shallower than, the global survey measurements. At one site, Osprey, the particle packing density appears to reach geometric saturation. We evaluate the uncertainty in these measurements and discuss their implications for other remotely sensed and mapped particles, and their importance to OSIRIS-REx sampling operations.

Introduction: The OSIRIS-REx mission’s observation campaigns [1] using the PolyCam instrument, part of the OSIRIS-REx Camera Suite (OCAMS) [2 3], have returned images of the surface of near-Earth asteroid (NEA) (101955) Bennu. These unprecedented-resolution images resolved cavities on Bennu’s boulders (Fig. 1) that are near-circular in shape and have diameters ranging from 5 cm to 5 m. We made hundreds of measurements of these cavities in image and laser altimeter data and found more than 100 boulders that exhibit at least one on their surface (Fig. 1). The most likely mechanism for the creation of these cavities is impacts. However, it is unclear whether these “mini-craters” were formed during Bennu’s residence in the main asteroid belt, or if they were formed more recently, after Bennu became a near-Earth asteroid (NEA). We use our observations of mini-craters to derive the strength of solid C-type objects against impacts. Our results have implications for Bennu’s history in the main-belt and in near-Earth space. The Strength of Monolithic C-type Objects: The strength of asteroids against collisions is crucial for understanding the surface evolution of airless planetary bodes, the dynamical evolution of asteroids throughout Solar System history, and the incorporation of planetesimals into planets [4,5]. Laboratory data on centimeter-scale meteorites have been extrapolated and buttressed with numerical simulations and analytic formalisms to derive the cratering threshold at the asteroid scale [6–8]. However, thus far it has not been possible to directly assess the strength of the boulders that constitute the building blocks of a rubble-pile asteroid. Apollo lunar rocks and spacecraft missions to near-Earth asteroids indicate only two modes of impact-induced breakdown of boulders: 1) abrasion by micro-meteorites (sand-blasting), and 2) catastrophic rupture by a single large impact [9–11]. Widespread cratering on boulders has not been observed heretofore. Figure 1: Centimeter-scale impact features on the sur-face of a boulder (image 20190703T044506S720_pol, taken July 3, 2019, by the OCAMS PolyCam imager; 1 cm/pixel). We developed a new method to derive the cratering efficiency and the disruption threshold of C-type objects by combining scaling laws and observations of craters on C-type boulders and asteroids [12­–14]. We postulate that the largest crater on a boulder of a given size signifies an impact energy close to that required for disrupting that boulder. This type of analysis has been previously done for the study of the largest craters on planetary bodies larger than tens of kilometers using scaling laws [6] and laboratory experiments [15]. Here, we extend that analysis to objects of smaller size. We find that the crater to impactor size ratio on C-type objects is ~ 15, and that the collisional disruption of 1-m radius boulders on the surface of Bennu is efficient in the main belt (~ 1 Myr) but effectively ceases in near-Earth space as their collisional lifetime (~50 Myr) becomes greater than the dynamical lifetime of NEAs ( Towards a mini-crater clock for NEA surfaces: Detailed analysis of the surface density of mini-craters on Bennu’s boulders may provide a new way to determine relative ages of different regions of the surface. This can be used as a basis of comparison or calibration for alternative approaches to chronology, such as assessments of the small crater population on Bennu’s surface [12] or space-weathering on Bennu’s surface [17]. Our findings may be validated by analyzing the cosmic ray exposure ages of the returned sample. Acknowledgements This work was supported in part by NASA grant NNX15AH90G awarded by the Solar System Workings program. S.R.S. acknowledges support from NASA Grant no. 80NSSC18K0226 as part of the OSIRIS-REx Participating Scientist Program. This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. P.M. acknowledges support from the Centre National d’Études Spatiales and from the Academies of Excellence on Complex Systems and Space, Environment, Risk and Resilience of the Initiative d’EXcellence “Joint, Excellent, and Dynamic Initiative” (IDEX JEDI) of the Université Côte d’Azur. We are grateful to the entire OSIRIS-REx team for making the encounter with Bennu possible. References [1] Lauretta et al. (2017) Space Science Reviews 212, 925.[2] Rizk, B. et al. (2018) Space Science Reviews, 214, 26. [3] Golish, D.R. et al. (2020) Space Sci. Rev. 216, 12. [4] Michel, P. et al. (2001) Science, 294, 1696. [5] Bottke, W.F. et al. (2005) Icarus, 179, 63. [6] Holsapple, K.A. (1993) AREAP, 21, 333. [7] Benz, W., & Asphaug, E. (1994). Icarus, 107, 98. [8] Jutzi, M. et al. (2010) Icarus, 207, 54. [9] Hörz, F., et al. (1975) The Moon, 235. [10] Cheng, A.F. (2002) Asteroids III, University of Arizona Press, 351-366. [11] Nakamura, A.M. et al. (2008) Earth, Planets, and Space, 60, 7. [12] Walsh, K.J. et al. (2019) Nature Geoscience, 12, 242. [13] Sugita, S. et al. (2019) Science, 364, 252. [14] Veverka, J. et al. (1997) Science, 278, 2109. [15] Leliwa-Kopystyński, J. et al. (2008) Icarus, 195, 817. [16] Michel, P., & Delbo, M. (2011) Icarus, 209, 520. [17] Trang, D. et al. (2019) LPSC L, Abstract #2172.[1] Lauretta et al. (2017) Space Science Reviews 212, 925

Janus is a NASA SIMPLEx mission currently in Phase B. The SIMPLEx program is designed around the idea of using secondary launch opportunities to explore interplanetary destinations. The Janus mission concept plans to take advantage of the NASA Psyche launch to send two spacecraft to fly by Near Earth Objects of interest. A specific point design has been developed that sends two spacecraft to two binary asteroid systems, (175706) 1996 FG3 and (35107) 1991 VH, both of which have been observed repeatedly with photometry, spectrometry and radar. The Janus mission sends light-weight, low-cost spacecraft built by Lockheed Martin to encounter these high-science value small body targets. The science instruments are a visible and IR imager, from Malin Space Science Systems. The spacecraft will perform a rigorous remote sensing campaign when the object is a point source, and when resolved. The spacecraft will track the binary asteroid systems through closest approach, allowing for a combination of absolute surface resolution, relative resolution across the target asteroids and phase angle coverage unparalleled in previous asteroid flyby missions. Janus science will combine flyby observations of the target binary asteroids with ground-based observations, enabling the high resolution imaging and thermal data to be placed into a global context and leveraging all available data to construct an accurate topographical and morphological model of these bodies. Based on these measurements, the formation and evolutionary implications for small rubble pile asteroids will be studied. The science team members all have experience on asteroid missions or have made extensive ground based observations of NEAs. The industry team has extensive experience in the design, fabrication and operation of interplanetary spacecraft and instrumentation. Acknowledgements: The Janus mission is supported by NASA under a contract from the SIMPLEx Program Office. Part of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.

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.

Surface roughness is an important metric in understanding how the geologic history of an asteroid affects its small-scale topography and it provides an additional means to quantitatively compare one asteroid with another. In this study, we report the first detailed global surface roughness maps of 25143 Itokawa at horizontal scales from 8–32 m. Comparison of the spatial distribution of the surface roughness of Itokawa with 433 Eros, the other asteroid for which this kind of analysis has been possible, indicates that the two asteroids are dominated by different geologic processes. On Itokawa, the surface roughness reflects the results of down-slope activity that moves fine grained material into geopotential lows and leaves large blocks in geopotential highs. On 433 Eros, the surface roughness is controlled by geologically-recent large impact craters. In addition, large longitudinal spatial variations of surface roughness could impact the role of YORP on Itokawa.

Geotechnical properties of a granular material affect all surface operations from mobility to landing and excavation. As such, significant efforts to study and model these properties are necessary before sending a spacecraft. Lack of knowledge of regolith material properties adversely affected Apollo, Lunokhod, and Mars Exploration Rover missions; hence additional measures need to be undertaken to prevent potential failures or delays of future missions, in particular missions to explore low-gravity asteroidal surfaces. Geotechnical properties of regolith include cohesion and friction angle, which affect material strength. Friction angle is gravity-dependent, whereas cohesion is not. It is therefore much easier to study and model surface regolith on planetary bodies with significant gravity such as the Moon or Mars. If gravity becomes extremely low, for example, on asteroids, cohesive forces start to dominate. This chapter addresses geotechnical properties of asteroid regolith and their implications for safe mission surface operations. The chapter starts with a high-level overview of soil mechanics followed by an overview of asteroid's regolith from past and current missions. Models related to regolith are presented with specific emphasis on sources of cohesion. Several examples of surface operations are given (landing, boulder retrieval, excavation) to illustrate the effect of various properties on the hardware.

The cameras of New Horizons will provide robust data sets that should be imminently amenable to geological analysis of the Pluto system’s landscapes. In this paper, we begin with a brief discussion of the planned observations by the New Horizons cameras that will bear most directly on geological interpretability. Then we broadly review the major geological processes that could potentially operate on the surfaces of Pluto and its major moon Charon. We first survey exogenic processes (i.e. those for which energy for surface modification is supplied externally to the planetary surface): impact cratering, sedimentary processes (including volatile migration), and the work of wind. We conclude with an assessment of the prospects for endogenic activity in the form of tectonics and cryovolcanism.

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, 75 pages, 28 figures, 2 supplements, accepted for publication in Space Science Reviews

Small craters (less than one kilometer diameter) can be primary craters produced by impact of interplanetary debris, or they can be secondary craters produced by fallback of high-velocity ejecta blocks from much larger but infrequent primary impacts. The prevalent assumption over recent decades has been that primaries are most abundant, so most small craters are independent random events and can be used for dating. However, recent results from Europa and Mars support the early theory that distant secondaries globally dominate the number of small lunar craters; this would invalidate part of production functions that have been widely used for age dating. Crater excavation results in higher mean ejection velocities for smaller fragments, resulting in a steeper size-frequency distribution for secondary craters than is produced by the same size-frequency distribution of interplanetary debris. This review also discusses how small craters can sometimes be used to derive meaningful age constraints.

We examine a population of Pwyll secondary craters, as well as several small crater populations seen in six high-resolution sequences of Europa, taken by the Galileo spacecraft. We conclude that post-Pwyll (the youngest large impact on Europa) endogenic surface activity occurred in the Conamara Chaos region. The Pwyll impact deposited a high-density secondary crater population in this area. The two terrains in Conamara Chaos show very different crater densities with one terrain containing anywhere from a few to 10 times higher density than the other. Surface activity within the one terrain, which degraded and erased part of that terrain's crater population relative to the other terrain, best explains the density difference. Because Pwyll is a young impact crater, 18 million years old or younger, subsequent endogenic surface activity is consistent with Europa remaining active today. The steepness of the average differential power-law slope (−4.2) of the small-crater size distribution suggests that many of the small craters are secondaries. We demonstrate that the amount of mass ejected by Pwyll and the other large craters on Europa is potentially enough to create the majority of the small crater population via the secondary cratering process, allowing the possibility that most small craters on Europa are secondary craters.

A new analysis of images of Jupiter's icy moon Europa taken by the Galileo probe, may prompt a rethink on the nature of cratered surfaces in general. For many years the impact craters on solid planets and smaller bodies have been regarded as mainly caused by direct (primary) impact of asteroids and comets. But Bierhaus et al. conclude that the Galileo data show that more than 95% of Europa's small craters, less than a kilometre in diameter, are ‘secondary’, formed by material ejected during a primary impact of a comet or asteroid. That means that there have been few small comets passing close to Jupiter in recent years, but brings their numbers into line with model predictions. Recalculations could be in order in other matters too, since cratering is used as a measure of relative surface ages for Solar System objects. For several decades, most planetary researchers have regarded the impact crater populations on solid-surfaced planets and smaller bodies as predominantly reflecting the direct (‘primary’) impacts of asteroids and comets1. Estimates of the relative and absolute ages of geological units on these objects have been based on this assumption2. Here we present an analysis of the comparatively sparse crater population on Jupiter's icy moon Europa and suggest that this assumption is incorrect for small craters. We find that ‘secondaries’ (craters formed by material ejected from large primary impact craters) comprise about 95 per cent of the small craters (diameters less than 1 km) on Europa. We therefore conclude that large primary impacts into a solid surface (for example, ice or rock) produce far more secondaries than previously believed, implying that the small crater populations on the Moon, Mars and other large bodies must be dominated by secondaries. Moreover, our results indicate that there have been few small comets (less than 100 m diameter) passing through the jovian system in recent times, consistent with dynamical simulations3,4,5,6.

An innovative approach that enables greatly increased return from planetary science remote sensing missions is described. Our concept, called Multiple Instrument Distributed Aperture Sensor (MIDAS), provides a large-aperture, wide-field telescope at a fraction of the cost, mass and volume of conventional space telescopes, by integrating advanced optical interferometry technologies. All optical assemblies are integrated into MIDAS as the primary remote sensing science payload, thereby reducing the cost, resources, complexity, integration and risks of a set of back-end science instruments (SI’s) tailored to a specific mission, such as advanced SI’s now in development for future planetary remote sensing missions. MIDAS interfaces to multiple SI’s for redundancy and to enable synchronized concurrent science investigations, such as with multiple highly sensitive spectrometers. Passive imaging modes with MIDAS enable high resolution remote sensing at the diffraction limit of the overall synthetic aperture, sequentially by each science instrument as well as in somewhat lower resolution by multiple science instruments acting concurrently on the image, such as in different wavebands. Our MIDAS concept inherently provides nanometer-resolution hyperspectral passive imaging without the need for any moving parts in the science instruments. In its active remote sensing modes using an integrated laser subsystem, MIDAS enables LIDAR, vibrometry, illumination, various active laser spectroscopies such as ablative, breakdown, fluorescence, Raman and time-resolved spectroscopy. The MIDAS optical design also provides high-resolution imaging for long dwell times at high altitudes, thereby enabling real-time, wide-area remote sensing of dynamic changes in planet surface processes. These remote sensing capabilities significantly enhance astrobiologic, geologic, atmospheric, and similar scientific objectives for planetary exploration missions.

An innovative approach that enables greatly increased return from earth and planetary science remote sensing missions is described. Our concept, called Multiple Instrument Distributed Aperture Sensor (MIDAS), provides a large-aperture, wide-field, diffraction-limited telescope at a fraction of the cost, mass and volume of conventional space telescopes, by integrating advanced optical interferometry technologies. All optical assemblies are integrated into MIDAS as the primary remote sensing science payload, thereby reducing the cost, resources, complexity, integration and risks of a set of back-end science instruments (SI's) tailored to a specific mission, such as advanced SI's now in development for earth and planetary remote sensing missions. MIDAS interfaces to multiple SI's for redundancy and to enable synchronized concurrent science investigations, such as with multiple highly sensitive spectrometers. Passive imaging modes with MIDAS enable remote sensing at diffraction-limited resolution sequentially by each science instrument, as well as in somewhat lower resolution by multiple science instruments acting concurrently on the image, such as in different wavebands. Our MIDAS concept inherently provides nanometer-resolution hyperspectral passive imaging without the need for any moving parts in the science instruments. In its active remote sensing modes using an integrated laser source, MIDAS enables LIDAR, vibrometry, illumination, various active laser spectroscopies such as ablative, breakdown or time-resolved spectroscopy. The MIDAS optical design also provides high-resolution imaging for long dwell times at high altitudes, thereby enabling real-time, wide-area remote sensing of dynamic changes in planet surface processes.

The science capabilities and features of an innovative and revolutionary approach to remote sensing imaging systems aimed at increasing the return on future planetary science missions many fold are described. Our concept, called Multiple Instrument Distributed Aperture Sensor (MIDAS), provides a large-aperture, wide-field, diffraction-limited telescope at a fraction of the cost, mass and volume of conventional space telescopes, by integrating advanced optical imaging interferometer technologies into a multi-functional remote sensing science payload. MIDAS acts as a single front-end actively controlled telescope array for use on common missions, reducing the cost, resources, complexity, and risks of developing a set of back-end science instruments (SIs) tailored to each specific mission. By interfacing to multiple science instruments, MIDAS enables either sequential or concurrent SI operations in all functional modes. Passive imaging modes enable remote sensing at diffraction-limited resolution sequentially by each SI, as well as at somewhat lower resolution by multiple SIs acting concurrently on the image, such as in different wavebands. MIDAS inherently provides nanometer-resolution hyperspectral passive imaging without the need for any moving parts in the SI's. Our optical design features high-resolution imaging for long dwell times at high altitudes