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An asteroid's history is determined in large part by its strength against collisions with other objects.sup.1,2 (impact strength). Laboratory experiments on centimetre-scale meteorites.sup.3 have been extrapolated and buttressed with numerical simulations to derive the impact strength at the asteroid scale.sup.4,5. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples.sup.6 and images of the asteroid (25143) Itokawa.sup.7. 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 [plus or minus] 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 [plus or minus] 0.75 million years., Author(s): R.-L. Ballouz [sup.1] , K. J. Walsh [sup.2] , O. S. Barnouin [sup.3] , D. N. DellaGiustina [sup.1] , M. Al Asad [sup.4] , E. R. Jawin [sup.5] , [...]

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 Earth.sup.1. Bennu is a low-albedo B-type asteroid.sup.2 that has been linked to organic-rich hydrated carbonaceous chondrites.sup.3. 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 processes.sup.4. 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 scale.sup.5-11. 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 inertia.sup.12 and radar polarization ratios.sup.13--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 centimetres.sup.4. 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., Author(s): D. S. Lauretta [sup.1] , D. N. DellaGiustina [sup.1] , C. A. Bennett [sup.1] , D. R. Golish [sup.1] , K. J. Becker [sup.1] , S. S. Balram-Knutson [sup.1] [...]

Spacecraft missions have observed regolith blankets of unconsolidated subcentimetre particles on stony asteroids 1-3 . Telescopic data have suggested the presence of regolith blankets also on carbonaceous asteroids, including (101955) Bennu 4 and (162173) Ryugu 5 . However, despite observations of processes that are capable of comminuting boulders into unconsolidated materials, such as meteoroid bombardment 6,7 and thermal cracking 8 , Bennu and Ryugu lack extensive areas covered in subcentimetre particles 7,9 . Here we report an inverse correlation between the local abundance of subcentimetre particles and the porosity of rocks on Bennu. We interpret this finding to mean that accumulation of unconsolidated subcentimetre particles is frustrated where the rocks are highly porous, which appears to be most of the surface 10 . The highly porous rocks are compressed rather than fragmented by meteoroid impacts, consistent with laboratory experiments 11,12 , and thermal cracking proceeds more slowly than in denser rocks. We infer that regolith blankets are uncommon on carbonaceous asteroids, which are the most numerous type of asteroid 13 . By contrast, these terrains should be common on stony asteroids, which have less porous rocks and are the second-most populous group by composition 13 . The higher porosity of carbonaceous asteroid materials may have aided in their compaction and cementation to form breccias, which dominate the carbonaceous chondrite meteorites 14 ., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)