Regolith behavior under asteroid-level gravity conditions: low-velocity impacts into mm- and cm-sized grain targets

Abstract

Context. In situ observations of small asteroids, such as Itokawa, Ryugu, and Bennu, show that surfaces covered by boulders and coarse terrain are frequent on such bodies. Regolith grain sizes have distributions on approximately mm and cm scales, and the behavior of such large grains in the very low-gravity environments of small body surfaces dictates their morphology and evolution. Aims. In order to support the understanding of natural processes (e.g., the recapturing of impact ejecta) or spacecraft-induced interactions (e.g., the fate of a small lander), we aim to experimentally investigate the response of coarse-grained target surfaces to very-low-speed impacts (below 2 m s−1). Methods. We present the outcome of 86 low-speed impacts of a cm-sized spherical projectile into a bed of simulated regolith, composed of irregular mm- and cm-sized grains. These impacts were performed under vacuum and microgravity conditions. Our results include measurements for the projectile coefficient of restitution and penetration depth, as well as ejecta production, speed, and mass estimation. As part of our data analysis, we compared our data set with impacts performed in similar conditions with fine grain regolith targets to determine the dependence of our measurements on the target grain size. Results. We find that impact outcomes include the frequent occurrence of projectile bouncing and tangential rolling on the target surface upon impact. Ejecta is produced for impact speeds higher than about 12 cm s−1, and ejecta speeds scale with the projectile to target the grain size ratio and the impact speed. Ejected mass estimations indicate that ejecta is increasingly difficult to produce for increasing grain sizes. Coefficients of restitution of rebounding projectiles do not display a dependency on the target grain size, unlike their maximum penetration depth, which can be scaled with the projectile to target grain size ratio. Finally, we compare our experimental measurements to spacecraft data and numerical work on Hayabusa 2’s MASCOT landing on the surface of the asteroid Ryugu.

Low-velocity impacts into granular material: application to small-body landing

“…There were no in-situ acceleration measurements during the impacts so the data cannot be used for the analyses presented here. Nonetheless, different responses of the regolith, such as the quantity and velocity of ejecta, where observed in low gravity conditions for impact speeds above and below a threshold of 0.2-0.4 m.s −1 (Colwell 2003;Brisset et al 2020).…”

“…Experiments with impact velocities ranging from 0.01 to 2.3 m.s −1 have been performed in low-gravity using parabolic flights, a drop-tower and the ISS (Colwell & Taylor 1999;Colwell 2003;Colwell et al 2008;Brisset et al 2018Brisset et al , 2020. There were no in-situ acceleration measurements during the impacts so the data cannot be used for the analyses presented here.…”

“…By performing experiments at low collision velocities (the maximum penetration depth was approximately a sphere diameter), Ambroso et al (2005a,b) found 1 = 2/3, 2 = 1/2 and 3 = 2/3. Meanwhile Brisset et al (2020) report 1 = 1/2, 2 = 1/4 and & Volfson (2005) found that 1 = 4/5, 2 = 2/5, and 3 = 3/5.…”

Probing regolith-covered surfaces in low gravity

“…There have been a number of recent studies investigating low-velocity impacts on granular surfaces in low gravity [4,5]. These studies, however, are mostly focused on fundamental properties of asteroid and planetary surfaces and the nature of granular collisions, rather than the applications of digging or anchoring that are relevant to low-gravity sampling missions.…”

Experiments on rebounding slow impacts under asteroid conditions

“…While there are a number of experiments on ejecta generation by granular impacts in the context of asteroids and planetesimals [5,6] or on the gravity dependence of the BNE [7], little experimentation has been performed on the ballistic sorting hypothesis under realistic conditions. Here we detail an experiment that is able to mimic the granular surface of a rubble pile asteroid.…”