The Dimorphos Boulder Swarm

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 comoving boulders, the largest of which is ∼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 −1 , only slightly larger than the 0.24 m s −1 gravitational escape velocity from the Didymos–Dimorphos binary system. The total boulder mass, M b ∼ 5 × 10 6 kg (density 2200 kg m −3 assumed), corresponds to about 0.1% of the mass of Dimorphos, and the boulders collectively carry about 3 × 10 −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% 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.