Observational characterization of the ejecta from the DART-Dimorphos impact

Abstract The Double Asteroid Redirection Test1,2 (DART) was intended to test planetary defense strategies via the reaction of the near-Earth asteroid Dimorphos, a satellite of Didymous, to a spacecraft impact. Pre-impact modeling3–7 predicted a momentum gain of the impacted asteroid in the range of 1−5 times that of the impactor. These models further suggest that the velocity distribution of the ejecta depends, for example, on the material cohesion and porosity. Therefore, detailed observations of the velocity and particle-size distribution in the ejecta can be used to calibrate the models and assess the target properties. Here we report on ultraviolet (UV), visible, and near-infrared (NIR) observations of the Dimorphos-DART impact. We show that the ejecta were composed of two main components. A slow component with high scattering efficiency expanding at a few ms−1 with a mass ≳ 2×104 kg, and a fast component moving at 1,600ms−1. Measuring the time scale on which these ejecta become optically thin, the surface brightness evolution, and multi-band Mie scattering, we detect not only the visible scattering ejecta, but also the “unseen" absorbing ejecta. The fast component is composed of ∼ 0.01 to ≈ 0.2μm sized particles, with a total mass of ∼ 103 kg. Most of the momentum gain of Dimorphos, after the impact, was due to the fast ejecta, and so the momentum gain of impacts can be estimated using such observations.