The vapour plume resulting from the DART impact on Dimorphos

Abstract

The DART spacecraft performed the first test of kinetic impactor technology to deflect asteroids, by colliding with the 160 m-diameter asteroid Dimorphos. Pre-impact predictions of observable phenomena from Earth concentrated on the production of slow-moving ejecta with velocities of <= 100 m/s, resulting from the crater formation by DART. A major surprise was the appearance after the impact of a spectacular semi-circular shell of material moving away from the asteroid system at high velocity. We observed this feature using a multitude of ground-based telescopes. Moving relative to the asteroid system at a sky-plane velocity of 1.7 - 1.8 km/s, the evolution of the plume surface brightness was consistent with optically thin material that conserved particle density as it expanded away from the asteroid system. Combining observations in multiple filters showed the appearance and evolution of the plume was consistent with its brightness being dominated by atomic line emission from Na I and K I, spectroscopically detected at the time of impact by other researchers. Assuming excitation from resonance fluorescence, we derive upper limits to the vapor generated by the DART impact of ~2.4 kg of Na I vapor and ~100 g of K I vapor. Similar velocities measured in filters dominated by Na I or K I emission imply an initial collision-dominated phase for the vapor or interaction with solid ejecta. Modelling of the impact using iSALE-2D implies that most, if not all, of this material likely originated from heating of the impact site. Extrapolating from the small detected mass in the plume, the plume provided at most a minor contribution to the momentum change caused by the DART impact.