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In situ observations of small asteroids show that surfaces covered byboulders and coarse terrain are frequent on such bodies. Regolith grain sizeshave distributions on approximately mm and cm scales, and the behavior of suchlarge grains in the very low-gravity environments of small body surfacesdictates their morphology and evolution. In order to support the understandingof natural processes (e.g., the recapturing of impact ejecta) orspacecraft-induced interactions (e.g., the fate of a small lander), we aim toexperimentally investigate the response of coarse-grained target surfaces tovery-low-speed impacts (below 2 m/s). We present the outcome of 86 low-speedimpacts of a cm-sized spherical projectile into a bed of simulated regolith,composed of irregular mm- and cm-sized grains. These impacts were performedunder vacuum and microgravity conditions. Our results include measurements forthe projectile coefficient of restitution and penetration depth, as well asejecta production, speed, and mass estimation. We find that impact outcomesinclude the frequent occurrence of projectile bouncing and tangential rollingon the target surface upon impact. Ejecta is produced for impact speeds higherthan about 12 cm/s, and ejecta speeds scale with the projectile to target thegrain size ratio and the impact speed. Ejected mass estimations indicate thatejecta is increasingly difficult to produce for increasing grain sizes.Coefficients of restitution of rebounding projectiles do not display adependency on the target grain size, unlike their maximum penetration depth,which can be scaled with the projectile to target grain size ratio. Finally, wecompare our experimental measurements to spacecraft data and numerical work onHayabusa 2's MASCOT landing on the surface of the asteroid Ryugu.

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