The influence of gravity on granular impacts

Abstract

Context. Impacts on small-body surfaces can occur naturally during cratering events or even strategically during carefully planned impact experiments, sampling maneuvers, and landing attempts. A proper interpretation of impact dynamics allows for a better understanding of the physical properties and the dynamical process of their regolith-covered surfaces and their general evolution. Aims. This work aims to first validate low-velocity, low-gravity impact simulations against experimental results, and then to discuss the observed collision behaviors in terms of a popular phenomenological collision model and a commonly referenced scaling relationship. Methods. We performed simulations using the soft-sphere discrete element method and two different codes, Chrono and pkdgrav. The simulations consist of a 10-cm-diameter spherical projectile impacting a bed of approximately 1-cm-diameter glass beads at collision velocities up to 1 m s−1. The impact simulations and experiments were conducted under terrestrial and low-gravity conditions, and the experimental results were used to calibrate the simulation parameters. Results. Both Chrono and pkdgrav succeed in replicating the terrestrial gravity impact experiments with high and comparable computational performance, allowing us to simulate impacts in other gravity conditions with confidence. Low-gravity impact simulations with Chrono show that the penetration depth and collision duration both increase when the gravity level decreases. However, the presented collision model and scaling relationship fail to describe the projectile’s behavior over the full range of impact cases. Conclusions. The impact simulations reveal that the penetration depth is a more reliable metric than the peak acceleration for assessing collision behavior in a coarse-grained material. This observation is important to consider when analyzing lander-regolith interactions using the accelerometer data from small-body missions. The objective of future work will be to determine the correct form and applicability of the cited collision models for different impact velocity and gravity regimes.