Small body surfaces are covered by granular material of varying particle sizes and depths. This material is observed to be easily mobilised from the surface through natural processes or spacecraft interaction. The material can escape or re-impact to the surface as a result of this process. The latter may be considered as a low-speed cratering activity under microgravity. In this paper, a set of simulations of normal impacts at 5–10 cm/s under 9.81 × 10⁻⁵m/s² is presented with gravel-type realistic material properties. The simulated craters are investigated for their qualitative impact outcomes, as well as quantitatively for the coefficient of restitution and crater and ejecta scaling properties. The results are compared with the results of impacts in glass beads type materials from authors’ previous work under the same conditions and wider literature experimental and observational literature. It was shown that most impacts result in a rebound with non-negligible coefficient of restitution values. The crater sizes are smaller compared to those in glass beads material and follows the crater scaling relationships across different impact energies with a μ coefficient of 0.55. Ejection is shown to continue beyond the final size with a significant quantity of material mobilised at speed below escape speed under the selected gravity level. The depth-to-diameter ratio of the craters is within the range of small craters of asteroid Bennu, with qualitative impact outcomes displaying similarities with those found in this paper. These results suggest a possible low-speed impact and secondary cratering activity in small bodies as a result of natural and artificial particle ejection events, such as in the case of Bennu and Didymos.

The OSIRIS-REx mission collected a sample from the surface of the asteroid (101955) Bennu in 2020 October. Here, we study the impact of the OSIRIS-REx Touch-and-Go Sampling Acquisition Mechanism (TAGSAM) interacting with the surface of an asteroid in the framework of granular physics. Traditional approaches to estimating the penetration depth of a projectile into a granular medium include force laws and scaling relationships formulated from laboratory experiments in terrestrial-gravity conditions. However, it is unclear that these formulations extend to the OSIRIS-REx scenario of a 1300-kg spacecraft interacting with regolith in a microgravity environment. We studied the TAGSAM interaction with Bennu through numerical simulations using two collisional codes, pkdgrav and gdc-i. We validated their accuracy by reproducing the results of laboratory impact experiments in terrestrial gravity. We then performed TAGSAM penetration simulations varying the following geotechnical properties of the regolith: packing fraction (P), bulk density, inter-particle cohesion (σc), and angle of friction (φ). We find that the outcome of a spacecraft-regolith impact has a non-linear dependence on packing fraction. Closely packed regolith (P ? 0.6) can effectively resist the penetration of TAGSAM if φ ? 28° and/or σc ? 50 Pa. For loosely packed regolith (P ? 0.5), the penetration depth is governed by a drag force that scales with impact velocity to the 4/3 power, consistent with energy conservation. We discuss the importance of low-speed impact studies for predicting and interpreting spacecraft-surface interactions. We show that these low-energy events also provide a framework for interpreting the burial depths of large boulders in asteroidal regolith.

This paper outlines the strategic approach to realize a human mission to an asteroid, focusing specifically on the proximity operations. The risks and challenges posed by asteroid surfaces to in-situ investigations force the proximity operations to be done by the intermediary of robotic explorers. In this architecture, a precursor is sent years in advance to a potential target asteroids. Its main goals are the characterization of the gravity field and of the surface behavior. If the target is found suitable, the manned mission then proceeds. With their main spacecraft stationed on a stable orbit around the asteroid, the astronauts are transported to the surface via a small, unpressurized spacecraft. Hovering a few meters above the surface, they deploy and command small robotic landers that perform scientific operations at the surface.