Low-velocity impacts into granular material: application to small-body landing

With the flourishing number of small body missions that involve surface interactions,understanding the mechanics of spacecraft - surface interactions is crucial for improving ourknowledge about the landing phases of space missions, for preparing spacecraft operations,and for interpreting the results of measurements made during the surface interactions. Giventheir regolith-covered surfaces, the process of landing on a small body can be considered asan impact at low-velocity onto a granular material in reduced-gravity.In order to study the influence of the surface material, projectile shape, and gravity onthe collision dynamics we used two experimental configurations (one for terrestrial gravityexperiments and one for reduced-gravity experiments) to perform low-velocity collisions intodifferent types of granular materials: quartz sand, and two different sizes of glass beads (1.5and 5 mm diameter). Both a spherical and a cubic projectile (with varying impact orientation)were used.The experimental data support a drag model for the impact dynamics composed of both ahydrodynamic drag force and quasi-static resistance force. The hydrodynamic and quasi-staticcontributions are related to the material frictional properties, the projectile geometry, and thegravity.The transition from a quasi-static to a hydrodynamical regime is shown to occur at lowerimpact velocities in reduced-gravity trials than in terrestrial gravity trials, indicating thatregolith has a more fluid-like behaviour in low-gravity. The reduced quasi-static regime of agranular material under low-gravity conditions leads to a reduction in the strength, resultingin a decreased resistance to penetration and larger penetration depths.