Cameras aboard NASA's OSIRIS-REx spacecraft captured close-up shots of material being ejected from the surface of the near-Earth asteroid Bennu. The images offer a detailed look at small-scale mass loss events on an active asteroid, whereas before, observations have been limited to only the largest phenomena. More than 20,000 known near-Earth asteroids are traversing our Solar System. From Earth, the vast majority appear inert with no evidence of smaller bits breaking free from their surface. Although rare, a small number of asteroids have been observed actively ejecting dust and particles in quantities large enough to create temporary clouds or comet-like tails viewable from Earth-based telescopes. While multiple mechanisms have been suggested to help explain mass loss in active asteroids, the process isn't well understood. In late 2018, the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) spacecraft arrived at Bennu - a roughly spherical near-Earth asteroid about 500 meters in diameter. Using images taken by the OSIRIS-REx navigation camera, Dante Lauretta and colleagues describe close-up observations of mass loss from Bennu's surface. Appearing as bright points of light in the images, Lauretta et al. identified small, centimeter-scale objects traveling just above the surface of Bennu. Further analysis of the trajectory of the material revealed three discrete events during which material was ejected from the surface. According to the results, some of the shed objects remained in Bennu's orbit for several days before re-impacting with the surface, while others escaped into interplanetary space. The authors suggest that micro-meteorite impacts as well as dehydration and thermal stress fracturing are the possible causes of the particle ejection. In a related Perspective, Jessica Agarwal writes, "Bennu shows that - from Earth's perspective - an apparently inactive asteroid can harbor complex dynamics of debris re-impacting the surface or feeding the interplanetary dust cloud, with yet unknown implications for its evolution."
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Journal
Science