Numerical modeling of lander interaction with a low-gravity asteroid regolith surface Application to MASCOT on board Hayabusa2

Context. Landing on the surface of small bodies is particularly challenging, as the physical properties of the surface material are not well known and the mechanical response of this material in a low-gravity environment is not well understood. Aims. In order to improve our understanding of low-speed impact processes on granular media and their outcome in low-gravity environments, we consider the landing of the package MASCOT, to be released by the JAXA asteroid sample return mission Hayabusa2 on (162173) Ryugu in October 2018. Beyond addressing the theoretical aspects of the mechanical response of granular media in low gravity, this study also supports both engineering and scientific teams of Hayabusa2 in the search for the lander and in the determination of Ryugu's surface properties. Methods. A campaign of hundreds of numerical simulations using the soft-sphere discrete element method implemented in the N-body code pkdgrav were performed to study the interaction between the lander and the low-gravity surface of the asteroid made of a granular medium representing the regolith. Assuming a broad range of regolith properties, and the lander's trajectory and motion, we analyzed the outcomes of the landing (distance traveled by the lander, penetration depth, and shape of the traces left in the regolith surface) to determine the influence of the many parameters defining the properties of MASCOT and of the grains, and the ingoing motion of the lander. Results. We identify well-marked trends for the fate of the lander and the traces left in the granular material. Distances traveled by the lander are greater and penetrations are shallower for gravel-like media than for less frictional material. A similar trend is found for grazing impacts as opposed to vertical ones. Different regolith properties also generate different traces on the ground after the impact.