Low-speed impact simulations into regolith in support of asteroid sampling mechanism design I: Comparison with 1-g experiments

This study is carried out in the framework of sample-return missions to asteroids that use a low-speed projectile as the primary component of its sampling mechanism (e.g., JAXA’s Hayabusa and Hayabusa2 missions). We perform numerical simulations of such impacts into granular materials using different projectile shapes under Earth׳s gravity. We then compare the amounts of ejected mass obtained in our simulations against what was found in experiments that used similar setups, which allows us to validate our numerical approach. We then investigate the sensitivity of various parameters involved in the contacts between grains on the amount of mass that is ejected. For the targets, we consider 2 different monodisperse grain-diameter sizes: 5 mm and 3 mm. The impact speed of the projectile is 11 m s−1, and is directed downward, perpendicular to the surface of the targets. Using an implementation of the soft-sphere discrete element method (SSDEM) in the N-body gravity tree code pkdgrav, previously validated in the context of low-speed impacts into sintered glass bead agglomerates, we find a noticeable dependence of the amount of ejected mass on the projectile shape. As found in experiments, in the case of the larger target grain size (5 mm), a conically shaped projectile ejects a greater amount of mass than do projectiles of other shapes, including disks and spheres. We then find that numerically the results are sensitive to the normal coefficient of restitution of the grains, especially for impacts into targets comprising smaller grains (3 mm). We also find that static friction plays a more important role for impacts into targets comprising the larger grains. As a preliminary demonstration, one of these considered setups is simulated in a microgravity environment. As expected, a reduction in gravity increases both the amount of ejected mass and the timescale of the impact process. A dedicated quantitative study in microgravity is the subject of future work. We also plan to study other aspects of the ejection process such as velocity distributions and crater properties, and to adapt our methodology to the conditions of sampling mechanisms included in specific mission designs.