The Dimorphos Boulder Swarm

Abstract: We present deep Hubble Space Telescope images taken to examine the ejecta from the DART spacecraft impact into asteroid Dimorphos. The images reveal an extensive population of co-moving boulders, the largest of which is about 7 m in diameter (geometric albedo 0.15 assumed). Measurements of 37 boulders show a mean sky-plane velocity dispersion of 0.30+/-0.03 m/s, only slightly larger than the 0.24 m/s gravitational escape velocity from the Didymos/Dimorphos binary system. The total boulder mass, 5e6 kg (density 2200 kg/m3 assumed), corresponds to about 0.1 percent of the mass of Dimorphos and the boulders collectively carry about 3e-5 of the kinetic energy delivered by the DART spacecraft impact. The sky-plane distribution of the boulders is asymmetric, consistent with impact into an inhomogeneous, likely rubble-pile, body. Surface boulder counts on Didymos show that the observed boulder swarm could be ejected from as little as 2 percent of the surface of Dimorphos (for example a circular crater at the impact point about 50 m in diameter). The large, slow-moving boulders are potential targets to be investigated in-situ by the upcoming ESA HERA mission.