Modeling of impact and surface processes on small bodies : interpretation of observations, implications for their physical properties and support to space mission operations

This thesis aims for a better understanding of the dynamics of regolith in a low-gravity environment through numerical simulations. It is incorporated within the framework and operations of two sample-return missions towards near-Earth asteroids, JAXA’s Hayabusa2 and NASA’s OSIRIS-REx. The simulations were performed with the numerical N-body code pkdgrav adapted to model the interactions with granular medium. Comparing the results of low-speed impact experiments in glass beads with numerical simulations, great agreements were found, demonstrating the validity of these simulations to this application. The form of the Coulomb force (friction force opposing the penetration) was also derived and seems to go from constant to proportional to penetration depth for an increasing size or impactor-grain size ratio. Moreover, inclined plane simulations were also conducted to investigate the relations between parameters of pkdgrav and the continuum approach using a constitutive law μ(I) relating the friction parameter and inertial number. For a moderate friction, such a relation can be established; however, the discrepancies between methods in the velocity profiles are too large for high-friction materials such as gravel. Concerning the Hayabusa2 mission, after a brief analysis of Ryugu’s geophysics are presented the studies on CNES-DLR MASCOT lander and on the sampling mechanism. MASCOT simulations were performed in order to better understand the impact response of the lander on assumed surface granular materials, and to support the engineering team in the landing site selection and the interpretation of landing outcome. Among the results is the increase in the distance traveled after impact for shallower beds, more-grazing impacts, higher-friction materials, and with MASCOT landing on its back corner. It is also shown that the post-impact traces left by MASCOT depend on the lander’s attitude and the surface friction properties. Furthermore, additional simulations were performed with a boulder and a side wall, to model the actual landing context. These lead to the realization that outgoing-to-incoming speed ratios as low as 0.3 could be due to microbounces (quick succession of contacts), and not necessarily to a soft surface/boulder. Then is presented a numerical study of Hayabusa2 sampling, firstly without the modeling of the structure of the sampler, to derive general results on impact cratering in low gravity on a granular material and compare them to the literature. For instance, it was found that streamlines in the bed are well represented by the analytical Z-model, and that the ejecta quantity seems to scale with the impact velocity. With the horn, the majority of simulations fulfill the scientific objective of collecting at least 100 mg. Finally are introduced the OSIRIS-REx mission and its target (101955) Bennu. Two phenomena observed on the asteroid, i.e., particle ejections and terraces, are treated as applications of previous chapters. Some particle ejections can potentially be explained by re-impacts of particles after a first ejection, modeled with an adaptation of MASCOT simulations. In addition, using inclined planes, a preliminary study aims at understanding the formation of terraces on Bennu. To conclude, a large range of simulations of granular material dynamics in different conditions were performed and applied to Hayabusa2 and OSIRIS-REx results and devices. However, the results and associated developed numerical tools are general enough to be applied to future missions devoted to small body exploration and interaction, such as the JAXA MMX mission to Phobos and ESA Hera/NASA DART asteroid deflection missions.