Microgravity granular mechanics – from recent sample collection, to asteroid evolution, to past planetary formation

Abstract: On 20 October 2020, the OSIRIS-REx spacecraft successfully collected material from the near-Earth asteroid Bennu.  The sample-collection event included two major episodes of energy deposition into the surface and subsurface: first, the sampling device used pressurized gas to mobilize and collect material (“regolith”), and second, the gas from the spacecraft thrusters interacted with the broader sampling region.  I will discuss the results of analysis that considers the timing, magnitude, and consequences of both effects.  The relative response of the surface to these events, in turn, provides novel constraints on the properties of microgravity aggregates, often called “rubble piles”.  These results have significant potential implications because of the widespread presence of microgravity aggregates in both the present and early solar system.  Finally, the regolith response to spacecraft thrusters suggests a methodology for probing surface properties that doesn’t require physical contact by a spacecraft – a technique relevant to future spacecraft encounters with small bodies, such as OSIRIS-APEX with asteroid Apophis.