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 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 (σ c ), 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 σ c 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.

Near-zero cohesion and loose packing of Bennu’s near subsurface revealed by spacecraft contact

“…To explore the sensitivity of the TAGSAM-regolith interaction to the properties listed above, we developed a model of the impact of a TAGSAM-shaped projectile on a regolith bed based on the N -body collisional codes PKDGRAV and GDC-I ( 37 , 38 ).The regolith is modeled as spherical particles that interact with the spacecraft and each other through a soft-sphere discrete element method (SSDEM). SSDEM allows the interacting objects to slightly overlap, enabling the modeling of multicontact frictional and cohesive forces.…”

“…These friction terms allow the spherical particles to mimic the behavior of rough angular particles so that the bulk φ of the system can be systematically varied. These simulations have previously been used to explore the variability in the outcome of a TAGSAM-regolith interaction for different surface geotechnical properties and impact speeds, U ( 37 ). This modeling found two distinct regimes of TAGSAM-regolith behavior for weakly cohesive regolith (σ c ≤ ~10 Pa): (i) For packing fraction P ≤ ~0.5, the forces do not exceed ~60 N, and the force is dominated by a drag force term that depends on φ and ρ; and (ii) for P ≥ ~0.6, the TAGSAM forces exceed ~60 N for φ ≥ ~20°, and TAGSAM only penetrates the first few centimeters for φ ≥ ~30°.…”

“…3 ). In this regime, the force F felt by the spacecraft can be modeled by F = 0.62 tan(φ)ρ U 4/3 ( 37 ). The DTMs of the sample collection site show slopes with maximum of 40° ( 1 , 4 ); thus, we assume this value for the angle of friction (φ ~40°).…”

Ricochets on Asteroids II: Sensitivity of laboratory experiments of low velocity grazing impacts on substrate grain size

“…Crater scaling laws (Holsapple, 1993;Ambroso et al, 2005) and empirical force models for normal impacts into granular media (Tsimring and Volfson, 2005;Katsuragi and Durian, 2007;Goldman and Umbanhowar, 2008;Katsuragi and Durian, 2013) also depend upon the ratio of projectile to substrate bulk density 𝜌 𝑝 /𝜌 𝑠 . Laboratory studies have carried out normal impact experiments into a variety of granular media, however, differences in the force laws or penetration depth are usually attributed to variations in velocity, gravitational acceleration and projectile and substrate densities, rather than to the substrate grain properties such as grain size or shape or friction coefficients (e.g., Ambroso et al 2005; Goldman and Umbanhowar 2008; Katsuragi and Durian 2013), with an exception being Ballouz et al (2021). Impact experiments have previously shown sensitivity to the grain characteristics.…”

Full‐Field Modeling of Heat Transfer in Asteroid Regolith: 2. Effects of Porosity

“…Finally, the method by Ringl et al. (2012) is used, where spheres are sequentially attached to pre‐existing spheres in spaces in the geometry with low local packing fraction values (see also Ballouz et al., 2021). The method can be terminated once a desired packing density is reached, or allowed to run until no further sphere sites can be found after a large number of attempts, as with the Random Sequential Packing Method.…”