  Impact of a solid object onto a small-body surface can be modeled by the
solid impact onto a hierarchically structured granular target. Impact drag
force model for the hierarchically structured granular target is developed
based on the experiment. We perform a set of granular impact experiments in
which mechanical strength and porosity of target grains are systematically
varied. Tiny glass beads ($5$~$\mu$m in diameter) are agglomerated to form
porous grains of $2$--$4$~mm in diameter. Then, the grains are sintered to
control their strength. A polyethylene sphere ($12.7$~mm in diameter) is
dropped onto a hierarchical granular target consisting of these porous grains.
Motion of the penetrating sphere is captured by a high-speed camera and
analyzed. We find that impact drag force produced by the hierarchically
structured granular target can be modeled by the sum of inertial drag and
depth-proportional drag. The depth-proportional drag in hierarchical granular
impact is much greater than that of the usual granular target consisting of
rigid grains. The ratio between grain strength and impact dynamic pressure is a
key dimensionless parameter to characterize this extraordinary large
depth-proportional drag. Grain fracturing plays an important role in the impact
dynamics when the impact dynamic pressure is sufficiently larger than the grain
strength. This implies that the effect of grain fracturing should be considered
also for the impact on a small body. Perhaps, effective strength of the surface
grains can be estimated based on the kinematic observation of the intrusion or
touchdown of the planetary explorator.
