Mobilization of Unconsolidated Granular Material on Asteroid (101955) Bennu by Spacecraft Interaction

Abstract NASA’s OSIRIS-REx mission demonstrated the potential for robotic spacecraft to probe physical properties of planetary bodies. In 2020, the spacecraft autonomously collected granular material from Bennu, a small unconsolidated asteroid, leaving behind a region excavated by the combined effects of the gas-driven sampler and backaway thrusters. Disambiguating the physical responses to these two energy-injection events offers an opportunity to characterize a microgravity asteroid’s near-surface properties and understand how thruster-surface interactions could be utilized by future missions. We do so here using data from the spacecraft’s instruments and telemetry in conjunction with detailed modeling of the thrusters. The sampler initially formed a crater 0.5–0.7 meters in radius before thruster activation. The thrusters deposited four regions of high-pressure gas ∼1–6 meters from the contact location, with lower pressures inside and beyond this region. As a result, the crater expanded into a region undergoing active erosion from the thrusters, and its final dimensions were increased by thruster effects. The total erosion and redeposition depend on the pre-existing mass distribution and topography of the area. Varying erosion responses to the thrusters indicate variability in material properties laterally, and, combined with accelerometer data, as a function of depth. The efficacy of the thrusters to mobilize material over a broad area (>100 square meters), and at very small pressures (perhaps as low as 0.005 Pa), motivates their use to interrogate small-body surface properties, particularly in the spacecraft’s planned 2029 encounter with asteroid Apophis.