  In situ observations of small asteroids 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. 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). 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. 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, 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.
