Low-speed Impact Simulations into Regolith in Support of Asteroid Sampling Mechanism Design I.: Comparison with 1-g Experiments

摘要

This study is carried out in the framework of sample-return missions toasteroids that use a low-speed projectile as the primary component of itssampling mechanism (e.g., JAXA's Hayabusa and Hayabusa2 missions). We performnumerical simulations of such impacts into granular materials using differentprojectile shapes under Earth's gravity. We then compare the amounts of ejectedmass obtained in our simulations against what was found in experiments thatused similar setups, which allows us to validate our numerical approach. Forthe targets, we consider 2 different monodisperse grain-diameter sizes: 5 mmand 3 mm. The impact speed of the projectile is 11 m s$^{-1}$ directeddownward, perpendicular to the surface of the targets. Using an implementationof the soft-sphere discrete element method (SSDEM) in the $N$-Body gravity treecode PKDGRAV, previously validated in the context of low-speed impacts intosintered glass bead agglomerates, we find a noticeable dependence of the amountof ejected mass on the projectile shape. As found in experiments, in the caseof the larger target grain size (5 mm), a conically shaped projectile ejects agreater amount of mass than do projectiles of other shapes, including disks andspheres. We then find that numerically the results are sensitive to the normalcoefficient of restitution of the grains, especially for impacts into targetscomprised of smaller grains (3 mm). We also find that static friction plays amore important role for impacts into targets comprised of the larger grains. Asa preliminary demonstration, one of these considered setups is simulated in amicrogravity environment. As expected, a reduction in gravity increases boththe amount of ejected mass and the timescale of the impact process. Ourmethodology is also adaptable to the conditions of sampling mechanisms includedin specific mission designs.