EJECTA- AND CRATER SIZE-SCALING OF EXPERIMENTAL IMPACTS

One of the most promising means of learning how initial impact conditions are related to the processes leading to the formation of planetary-scale craters is through dimensionless scaling relationships. Such derivations have led to great insight into the cratering process, yet have typically treated targets as continuous media. In many cases, however, planetary materials represent irregular and discontinuous targets, the effects of which on the scaling relationships are still poorly understood. We continue to examine the effects of varying impact conditions on the excavation and final dimensions of craters formed in sand. This contribution presents some of the data collected during and after the impact of glass and aluminum spheres into medium- and coarse-grained sand targets.

Experiments are performed with the Vertical Impact Facility at the Johnson Space Center during which the ejecta were documented with the Ejection-Velocity Measurement System (EVMS). Projectiles impacted the target at speeds ranging from 0.32 to 1.9 km/s and at normal incidence angles. The EVMS produces stroboscopic images of ejecta in flight by flashing a “sheet” of laser light at programmed rates; the illumination sequence initiates at impact, which serves as the reference time for subsequent measurement of particle kinematics. The “sheet” is aligned such that fragments of the target with multiple images in the photograph were traveling radially from the impact site.

The velocities of a large number of ejected fragments are measured in each photograph, and those values can be decomposed into their respective speed and launch-angle components. Evolution of ejection speed with crater growth is fit using ejecta-scaling theory and crater size-scaling relationships are applied to the final crater dimensions. Results have implications for the role of target material grain size as well as the mechanism by which the projectile deforms.