Modified granular impact force laws for the OSIRIS-REx touchdown on the
surface of asteroid (101955) Bennu
Abstract
The OSIRIS-REx mission collected a sample from the surface of the
asteroid (101955) Bennu in October 2020. Here we study the impact of the
OSIRIS-REx Touch-and-Go Sampling Acquisition Mechanism (TAGSAM)
interacting with the surface of an asteroid in the framework of granular
physics. Traditional approaches to estimating the penetration depth of a
projectile into a granular medium include force laws and scaling
relationships formulated from laboratory experiments in
terrestrial-gravity conditions. However, it is unclear that these
formulations extend to the OSIRIS-REx scenario of a 1300-kg spacecraft
interacting with regolith in a microgravity environment. We studied the
TAGSAM interaction with Bennu through numerical simulations using two
collisional codes, pkdgrav and GDC-i. We validated their accuracy by
reproducing the results of laboratory impact experiments in terrestrial
gravity. We then performed TAGSAM penetration simulations varying the
following geotechnical properties of the regolith: packing fraction (P),
bulk density, inter-particle cohesion (σc), and angle of friction (ɸ).
We find that the outcome of a spacecraft-regolith impact has a
non-linear dependence on packing fraction. Closely packed regolith
(P≳0.6) can effectively resist the penetration of TAGSAM if ɸ≳28° and/or
σc≳50 Pa. For loosely packed regolith (P≲0.5), the penetration depth is
governed by a drag force that scales with impact velocity to the 4/3
power, consistent with energy conservation. We discuss the importance of
low-speed impact studies for predicting and interpreting
spacecraft-surface interactions. We show that these low-energy events
also provide a framework for interpreting the burial depths of large
boulders in asteroidal regolith.