As a consequence of the DART impact, a substantial amount of ejected material was thrown into space. This process removed kinetic energy from the system, and needs to be accounted for when Dimorphos’s remaining kinetic energy is determined. HST has been periodically observing Dimorphos since the collision with DART, and in a HST/WFC3 optical image from approximately 3 months post-impact, shown here, as many as 37 boulders can be seen (circled and numbered). David Jewitt and colleagues have examined this image in order to estimate the properties of the ejected material, finding that these boulders have carried away a miniscule 3 × 10–5 of the kinetic energy of the system (D. Jewitt et al. Astrophys. J. Lett. 952, L12; 2023).
The boulders shown in the HST image are up to 7 m in diameter, weigh on average about 135 tonnes each, and are spread over 10,000 km (in sky-projected distance). They are distributed asymmetrically around Didymos–Dimorphos (saturated in the image), consistent with impact into an inhomogeneous body. The 160-m diameter Dimorphos is thought to be a rubble pile held together by gravity, like asteroids Bennu and Ryugu, rather than a solid body. From the image, the velocity dispersion (in the sky plane) of the rocks can be calculated, and the mean is only slightly larger (25%) than Dimorphos’s escape velocity, signifying that the boulders are some of the slowest-moving bodies ejected from the system. Given the distribution of surface boulders in a pre-impact image from DART, the rocks seen escaping could have been ejected from a crater as small as 50 m in diameter.
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