Spacecraft sample collection and subsurface excavation of asteroid (101955) Bennu

Abstract

Carbonaceous asteroids, such as (101955) Bennu, preserve material from the early Solar System, including volatile compounds and organic molecules. We report spacecraft imaging and spectral data collected during and after retrieval of a sample from Bennu’s surface. The sampling event mobilized rocks and dust into a debris plume, excavating a 9-m-long elliptical crater. This exposed material that is darker, spectrally redder, and more abundant in fine particulates than the original surface. The bulk density of the displaced subsurface material was 500–700 kg per cubic meter, about half that of the whole asteroid. Particulates that landed on instrument optics spectrally resemble aqueously altered carbonaceous meteorites. The spacecraft stored 250 ± 101 g of material, which will be delivered to Earth in 2023.

Near-zero cohesion and loose packing of Bennu’s near subsurface revealed by spacecraft contact

“…The measured pregas maximum force of 10 to 15 N would therefore require a bulk density of ρ = 440 to 600 kg m −3 for the uppermost 6 to 7 cm and likely the upper 10 to 20 cm that would have been immediately affected during the interaction ( 37 ). These findings of bulk density are consistent with independent results derived from the size of the crater formed during contact and gas release ( 1 ).…”

“…In this regime, the force F felt by the spacecraft can be modeled by F = 0.62 tan(φ)ρ U 4/3 ( 37 ). The DTMs of the sample collection site show slopes with maximum of 40° ( 1 , 4 ); thus, we assume this value for the angle of friction (φ ~40°). The measured pregas maximum force of 10 to 15 N would therefore require a bulk density of ρ = 440 to 600 kg m −3 for the uppermost 6 to 7 cm and likely the upper 10 to 20 cm that would have been immediately affected during the interaction ( 37 ).…”

“…The goal of NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security–Regolith Explorer) mission was to collect and return at least 60 g of surface material from Bennu ( 1 ). Bennu is a small (~500 m in diameter), near-Earth asteroid with a carbonaceous composition ( 2 , 3 ).…”

“…TAGSAM was designed to contact the surface for several seconds, during which it would release nitrogen gas and capture the subsequently mobilized particles in its sample collection chamber. This operation took place successfully on 20 October 2020 near the center of the ~20-m-diameter Hokioi crater in the Northern Hemisphere of Bennu ( 1 ). TAGSAM contacted the surface of Bennu at 10 cm s −1 with negligible lateral velocity.…”

The ESA Hera Mission: Detailed Characterization of the DART Impact Outcome and of the Binary Asteroid (65803) Didymos

“…Finally, the interaction of the OSIRIS-REx sampling tool (TAGSAM), which made a slow impact into the surface of Bennu, also revealed the very low cohesion and density of the surface of the asteroid (Lauretta et al 2022;Walsh et al 2022). Furthermore, although there is some indication of subsurface stiffness (Barnouin et al 2019), impact craters on Bennu may also be controlled by either gravity scaling or strength scaling assuming a very low strength (<100 Pa), except for small craters that are affected by armoring (Bierhaus et al 2022).…”