Numerical modeling of lander interaction with a low-gravity asteroid regolith surface

Context.The JAXA asteroid sample return mission Hayabusa2 reached its target (162173) Ryugu in June 2018 and released the European (CNES-DLR) lander MASCOT in October 2018. MASCOT successfully landed on the surface, and the Hayabusa2 Optical Navigation Camera system has been able to image parts of the MASCOT trajectory.

Aims.This work builds on our previous study of interactions between a landing package and a granular material in the context of MASCOT on Ryugu. The purpose is to expand our knowledge on this topic and to help constrain physical properties of surfaces by considering the actual trajectory of MASCOT and observations of Ryugu from Hayabusa2.

Methods.We ran a new campaign of numerical simulations using theN-body codepkdgravwith the soft-sphere discrete element method by expanding the parameter space to characterize the actual landing scenario of MASCOT on Ryugu. The surface was modeled as a granular medium, but we also considered a large boulder in the bed at various depths and a rigid wall representing a cliff. MASCOT was faithfully modeled as the actual lander, and we considered different impact angles, speeds, and surface slopes. We were particularly interested in the outgoing-to-incoming speed ratio of MASCOT during the landing process.

Results.We found that a boulder in the bed generally increases both the stochasticity of the outcomes and the speed ratio, with larger increases when the boulder sits closer to the surface. We also found that the surface slope does not affect our previous results and that the impact speed does not affect the speed ratio for moderate-friction granular material. Finally, we found that a speed ratio as low as 0.3, as estimated in the actual scenario, can occur with a solid-rock surface, not only with a soft surface, because the geometry of the lander is nonspherical. This means that we must infer the physical properties of the surface from outcomes such as the speed ratio with caution: it depends on the lander geometry.