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

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

Research output: Contribution to journalArticlepeer-review

22 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)5087-5105
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Nov 1 2021


  • methods: numerical
  • minor planets, asteroids: general
  • minor planets, asteroids: individual: 101955

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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