Impact and intrusion experiments on the deceleration of low-velocity impactors by small-body regolith

Abstract

Previous laboratory impact experiments into sand and glass beads have enriched our understanding of the cratering process on granular media common on asteroids and planetary regolith. However, less attention has been paid to the fate of the projectile, such as its penetration depth in the granular medium, although this may be important for the regolith mixing process. We conducted laboratory experiments on the deceleration of projectiles with low impact velocities to understand the re-accumulation process of ejecta on small asteroids. Glass beads were used as a model of a granular target. Impact experiments using 6-mm plastic projectiles with velocities of ∼70ms−1 were performed on the Earth’s surface and under microgravity. Measurements of the resistance force of the glass beads against slow intrusion and penetration were also performed. In the impact experiments, the projectiles were decelerated mainly as a result of drag proportional to the square of the velocity. The drag coefficient was 0.9–1.5. Additionally, we found a possible term proportional to the projectile velocity corresponding to the viscous drag with a viscosity up to 2Pas. These forces are consistent with numerical simulations that we carried out. The slow intrusion and penetration measurements showed that the velocity-independent resistance force per unit area on a projectile is roughly 20 times larger than the lithostatic pressure. The penetration depth of re-accumulated ejecta was examined based on the drag parameters obtained in this study. A simple configuration was used to visualize the dependence of penetration depth on the drag parameters. The penetration depth was more sensitive to the drag parameters in the case of small particles impacting a relatively small model asteroid.