In May of 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third mission in the New Frontiers program. The other two New Frontiers missions are New Horizons, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on Jan. 1, 2019, and Juno, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch 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 August 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 Bennus 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.Comment: 89 pages, 39 figures, submitted to Space Science Reviews - OSIRIS-REx special issu

The shapes of asteroids reflect interplay between their interior properties and the processes responsible for their formation and evolution as they journey through the Solar System. Prior to the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission, Earth-based radar imaging gave an overview of (101955) Bennu’s shape. Here, we construct a high-resolution shape model from OSIRIS-REx images. We find that Bennu’s top-like shape, considerable macroporosity, and prominent surface boulders suggest that it is a rubble pile. High-standing, north-south ridges that extend from pole to pole, many long grooves, and surface mass wasting indicate some low levels of internal friction and/or cohesion. Our shape model indicates that, similar to other top-shaped asteroids, Bennu formed by reaccumulation and underwent past periods of fast spin leading to its current shape. Today, Bennu might follow a different evolutionary pathway, with interior stiffness permitting surface cracking and mass wasting.

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. CitationHamilton, V.E., et al., "Evidence for widespread hydrated minerals on asteroid (101955) Bennu." Nature astronomy 3, 4 (2019): p.

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. Size-frequency distribution (SFD): the abundance of an entity (e.g., craters or rocks) as a function of the diameter of that entity Primary crater: crater produced by impact of interplanetary debris Secondary crater: crater produced by impact of ejecta thrown out by a primary impact on that same body Planetocentric debris: solid material produced within a planetary system that escapes from the body of origin but remains in orbit around the planet and may impact a satellite Crossover diameter: crater diameter below which secondaries are most abundant and above which primaries are most abundant viii Contents

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