Data gathered during the sampling of asteroid Bennu by the OSIRIS-REx mission shows that the asteroid’s surface consists of low-cohesion rubble, according to two new studies – one each in Science and Science Advances. According to the Science study by Dante Lauretta and colleagues, about 250 grams of sample was collected, which will be brought to Earth in 2023 for laboratory analysis. The Science Advances study by Kevin Walsh and colleagues analyzed the forces experienced by the spacecraft, finding that Bennu’s low gravity has resulted in a granular surface bed with weak cohesion between particles. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft spent about two years surveying Bennu, a carbonaceous rubble-pile asteroid about 500 meters in diameter. After considering the best locations to collect a sample, the mission team chose a site within a 20-meter crater, nicknamed Nightingale. The spacecraft descended to the surface and collected the sample in October 2020, Lauretta et al. note. The spacecraft’s Touch-and-Go Sample Acquisition Mechanism (TAGSAM) made contact and began sinking into the asteroid’s surface before it released a jet of nitrogen gas that mobilized sub-surface material and guided it into a collection chamber. By analyzing imaging and spectral data taken during and after the sample’s retrieval, the team found that sub-surface material is darker and contains more fine particles than the overlying surface. The process produced a debris plume and a new 9-meter-long elliptical crater.
Walsh et al. investigated the physical properties of the material up to 10 centimeters below Bennu’s surface, using images and accelerometer data. They reconstructed the forces exerted on the spacecraft in the short span of time between when it first contacted Bennu’s surface and when it released the nitrogen gas. They found that the near-subsurface material is loosely packed and less dense than the average of the whole asteroid, with very low cohesion. The high porosity and low material strength allow dust and other small particles to move within the sub-surface of the asteroid. Spectral and thermal data gathered during the mission suggests these results apply to the whole asteroid, not just the sampling site.
**Non-standard embargo lift time for Science Advances study: 14:00 (2:00 pm) US ET on Thursday, 7 June, synched with Science paper embargo lift time**
Spacecraft sample collection and subsurface excavation of asteroid (101955) Bennu
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