Low-velocity impacts into granular material: application to small-body landing

Abstract

With the flourishing number of small body missions that involve surface interactions, understanding the mechanics of spacecraft - surface interactions is crucial for improving our knowledge about the landing phases of space missions, for preparing spacecraft operations, and for interpreting the results of measurements made during the surface interactions. Given their regolith-covered surfaces, the process of landing on a small body can be considered as an impact at low-velocity onto a granular material in reduced-gravity. In order to study the influence of the surface material, projectile shape, and gravity on the collision dynamics we used two experimental configurations (one for terrestrial gravity experiments and one for reduced-gravity experiments) to perform low-velocity collisions into different types of granular materials: quartz sand, and two different sizes of glass beads (1.5 and 5 mm diameter). Both a spherical and a cubic projectile (with varying impact orientation) were used. The experimental data support a drag model for the impact dynamics composed of both a hydrodynamic drag force and quasi-static resistance force. The hydrodynamic and quasi-static contributions are related to the material frictional properties, the projectile geometry, and the gravity. The transition from a quasi-static to a hydrodynamical regime is shown to occur at lower impact velocities in reduced-gravity trials than in terrestrial gravity trials, indicating that regolith has a more fluid-like behaviour in low-gravity. The reduced quasi-static regime of a granular material under low-gravity conditions leads to a reduction in the strength, resulting in a decreased resistance to penetration and larger penetration depths.

Wheel-regolith interactions on small-body surfaces

“…The numerical simulations allow us to study the flow of the regolith directly under the rover wheel in more detail. Recent impact experiments have shown that granular material becomes fluidized more quickly under reduced gravity conditions [8]. Here, we look for the transition from the quasi-static to the inertial flow regime when the rover starts moving to see if fluidization of the material in low-gravity restricts the maneuverability of the rover.…”

Modified granular impact force laws for the OSIRIS-REx touchdown on the surface of asteroid (101955) Bennu

“…Lauretta & DellaGiustina et al 2019 and references therein). Given that these characteristics are fairly well constrained, the sampling attempt can also act as a useful and rare impact experiment onto particles in a microgravity setting, which is difficult to achieve experimentally on Earth (Brisset et al 2018;Bierhaus et al 2021;Murdoch et al 2021). Based on the spacecraft's reaction, and with the added context of the simulations described here, we may be able to better understand the dynamics of granular material in a microgravity environment.…”

“…Low-speed interactions with regolith surfaces have been studied mainly through laboratory experiments of impacts into granular matter in Earth gravity (U2003, KD2013) and in environments that can simulate low-gravity interactions for a limited time (Brisset et al 2018;Murdoch et al 2021). As we have demonstrated, numerical simulations of granular material are an invaluable tool for exploring these low-energy impact regimes, as we are able to simulate the lowgravity environment of small asteroids easily and recreate the detailed MNRAS 507, 5087-5105 (2021) design characteristics of spacecraft devices or landers (Maurel et al 2018;Thuillet et al 2018;C ¸elik et al 2019.…”

Chrono::GPU: An Open-Source Simulation Package for Granular Dynamics Using the Discrete Element Method

“…To further validate the collision behavior of impact tests, quantities including the peak acceleration of the projectile, penetration depth, and collision time were evaluated and compared against experimental and numerical results [71,72]. Note that the simulation herein used monodisperse particles, whereas polydisperse ones were employed in [72].…”