Rubble Pile Rock Size Distribution Influence on Momentum Enhancement in Hypervelocity Cratering Experiments in Crushed Basalt with Comparison to DART

Experiments were performed impacting targets made of various size distributions of basalt rock held together with a grout and sand mixture.  These targets (with masses over 200 kg) were hung in a pendulum so that the momentum transferred to the projectiles could be measured.  They were impacted with a 3.0-cm-diameter aluminum sphere.  The impactor size and the largest crushed basalt rock size were on the same order.  The experiments were performed with a large two-stage light gas gun.  The impacts were performed at speeds from 5.25 to 5.50 km/s.  Due to the crater ejecta motion, the impacts produce a large momentum transfer to the target, characterized by β > 1, where β is the ratio of the momentum transferred to the target to the momentum of the incoming impactor.  The momentum enhancement βs were between 3 and 5 and showed a strong rock-size-distribution dependence.  The origin of the size distribution dependence could relate to the relative size of the crushed rock pieces to the impactor size or to the absolute rock size distribution.  Computations are presented and compared with the experiments.  The targets are intended to simulate a rubble-pile asteroid and the results are compared to the Double Asteroid Redirection Test (DART) spacecraft impact into the asteroid Dimorphos.  Though the asteroid-simulant targets and the solid-sphere impactor are not highly representative of Dimorphos and the DART spacecraft, there is good agreement between the experimental momentum enhancement β measurements.  Momentum enhancement is central to the topic of deflecting an asteroid with a hypervelocity impactor.