Ricochets on Asteroids II: Sensitivity of laboratory experiments of low velocity grazing impacts on substrate grain size

Abstract

We compare low velocity impacts that ricochet with the same impact velocity and impact angle into granular media with similar bulk density, porosity and friction coefficient but different mean grain size. The ratio of projectile diameter to mean grain length ranges from 4 in our coarsest medium to 50 in our finest sand. Using high speed video and fluorescent markers, we measure the ratio of pre-to post-impact horizontal and vertical velocity components, which we refer to as coefficients of restitution, and the angle of deflection caused by the impact in the horizontal plane. Coefficients of restitution are sensitive to mean grain size with the ratio associated with the horizontal velocity component about twice as large for our coarsest gravel as that for our finest sand. This implies that coefficients for hydro-static-like, drag-like and lift-like forces, used in empirical force laws, are sensitive to mean grain size. The coefficient that is most strongly sensitive to grain size is the lift coefficient which decreases by a factor of 3 between our coarsest and finest media. The deflection angles are largest in the coarser media and their size approximately depends on grain size to the 3/2 power. This scaling is matched with a model where momentum transfer takes place via collisions with individual grains. The dependence of impact mechanics on substrate size distribution should be considered in future models for populations of objects that impact granular asteroid surfaces.

Sub-surface granular dynamics in the context of oblique, low-velocity impacts into angular granular media

“…Perhaps surface topography alone could be the culprit, since the grain size used in this work is quite large relative to the impactor. This explanation would agree with theories from Wright et al (2021), where it was suggested that as grain size relative to the projectile decreases, the response of the projectile stabilizes and becomes less dependent upon surface topography. Of course, the invisible force chain network that absorbs the impactor energy may also play a role and that impacting at a different location may change how it is able to respond.…”

“…To form the granular bed, the grains are allowed to fall freely and settle under Earth's gravity into the rigid bin. The interaction between all of the grains as well as the impactor are defined using a friction coefficient of = 0.85, appropriate for coarse gravel (Wright et al, 2021). A ramped acceleration-based shake is applied to the settled granular bed to further settle the system.…”

“…projectile ricocheted off the media or not (Wright et al, 2020(Wright et al, , 2021. This body of work provides valuable information about the surface and overall impactor behavior in low velocity impact conditions, but the dynamics of sub-surface grains and how these correspond to the impact dynamics are rarely studied.…”