Modified granular impact force laws for the OSIRIS-REx touchdown on the surface of asteroid (101955) Bennu

R-L Ballouz,O Çelik,O S Barnouin,S R Schwartz,E B Bierhaus,Y Zhang,P Sánchez,M C Nolan,D J Scheeres,D S Lauretta,K A Holsapple,D C Richardson,M Baba,H C Connolly,P Michel,K J Walsh

doi:10.1093/mnras/stab2365

Abstract

ABSTRACT The OSIRIS-REx mission collected a sample from the surface of the asteroid (101955) Bennu in 2020 October. 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 (ϕ). 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 ϕ ≳ 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.

Highlights

1.1 The surfaces of asteroidsGranular material, in the form of regolith, is ubiquitous in theT uppermost layer of the surface of airless Solar System bodies, such as the Moon and asteroids
We have demonstrated that the discrepancy between our simulations and experimental results can be explained by the difference in the size of the grains that make up the target
M In Sec. 4.3, we introduced a modified force law for impacts into granular material, Eq (14), that arises from consideration of final depth scaling (Eq (1), U2003) and energy balance (Eq (13))

Summary

Introduction

1.1 The surfaces of asteroidsGranular material, in the form of regolith (that is, broken-up rock particles), is ubiquitous in theT uppermost layer of the surface of airless Solar System bodies, such as the Moon and asteroids. The terms pebble, cobble, and boulder describe individual particles of sizes ranging R from 4 mm to 6.4 cm, 6.4 cm to 2.6 m, and > 2.6 m, respectively (Williams et al 2006). For C simplicity, when we refer to regolith, we mean the population of particles on a planetary surface S that are the size of a pebble or smaller. The main process for regolith formation on a large planetary surface is thought to be the. U comminution of large boulders by impact (e.g., Hörz et al 1975). On the Moon, the impact N excavation of boulders by the formation of large craters is balanced by their subsequent A comminution by the constant bombardment of small meteoroids (e.g., Costello et al 2018; Basilevsky et al 2014).

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