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Impact of a solid object onto a small-body surface can be modeled by thesolid impact onto a hierarchically structured granular target. Impact dragforce model for the hierarchically structured granular target is developedbased on the experiment. We perform a set of granular impact experiments inwhich mechanical strength and porosity of target grains are systematicallyvaried. Tiny glass beads ($5$~$\mu$m in diameter) are agglomerated to formporous grains of $2$--$4$~mm in diameter. Then, the grains are sintered tocontrol their strength. A polyethylene sphere ($12.7$~mm in diameter) isdropped onto a hierarchical granular target consisting of these porous grains.Motion of the penetrating sphere is captured by a high-speed camera andanalyzed. We find that impact drag force produced by the hierarchicallystructured granular target can be modeled by the sum of inertial drag anddepth-proportional drag. The depth-proportional drag in hierarchical granularimpact is much greater than that of the usual granular target consisting ofrigid grains. The ratio between grain strength and impact dynamic pressure is akey dimensionless parameter to characterize this extraordinary largedepth-proportional drag. Grain fracturing plays an important role in the impactdynamics when the impact dynamic pressure is sufficiently larger than the grainstrength. This implies that the effect of grain fracturing should be consideredalso for the impact on a small body. Perhaps, effective strength of the surfacegrains can be estimated based on the kinematic observation of the intrusion ortouchdown of the planetary explorator.

Impact drag force exerting on a projectile penetrating into a hierarchical granular bed