Numerical modelling of medium-speed impacts on a granular surface in a low-gravity environment application to Hayabusa2 sampling mechanism

• Published in: Monthly notices of the Royal Astronomical Society 2020-01, Vol.491 (1), p.153-177
• Main Authors: Thuillet, Florian, Michel, Patrick, Tachibana, Shogo, Ballouz, Ronald-L, Schwartz, Stephen R
• Format: Article
• Language: English
• Subjects: Astrophysics; Earth and Planetary Astrophysics; Sciences of the Universe
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stz3010

Even if craters are very common on Solar System body surfaces, crater formation in granular media such as the ones covering most of visited asteroids still needs to be better understood, above all in low-gravity environments. JAXA’s sample return mission Hayabusa2, currently visiting asteroid (162173) Ryugu, is a perfect opportunity for studying medium-speed impacts into granular matter, since its sampling mechanism partly consists of a 300 m s−1 impact. In this paper, we look at medium-speed impacts, from 50 to 300 m s−1, into a granular material bed, to better understand crater formation and ejecta characteristics. We then consider the sampler horn of Hayabusa2 sampling mechanism and monitor the distribution of particles inside the horn. We find that the cratering process is much longer under low gravity, and that the crater formation mechanism does not seem to depend on the impact speed, in the considered range. The Z-model seems to rightly represent our velocity field for a steady excavation state. From the impact, less than $10{{\ \rm per\ cent}}$ is transmitted into the target, and grains are ejected mostly with angles between 48° and 54°. Concerning the sampling mechanism, we find that for most of the simulations, the science goal of 100 mg is fulfilled, and that a second impact increases the number of ejecta but not necessarily the number of collected particles.