Context. On September 26, 2022, the NASA DART mission impacted the asteroid Dimorphos, the smaller component of the Didymos binary asteroid system. This provided a unique opportunity to observe, in real time, the evolution of the ejecta cloud produced by the impact and the formation of a tail.
Aims: We present observations performed with the MUSE instrument at the Very Large Telescope to characterise the morphology, spectral properties, and evolution of the ejecta. The Didymos system was observed with MUSE on 11 nights from just before impact to almost one month post-impact, using both wide-field observations without adaptive optics and narrow-field observations with adaptive optics.
Methods: We produced white light images that were used to study the morphology of the ejecta at different spatial scales. The spectral information was used to search for gas emission from either exposed ice or propellant, and to study the spatial and temporal variation of the ejecta dust reflectance through reflectance maps.
Results: We searched for, but did not detect, emission from [OI], Xe, NH2, and H2O+ in a 1'×1' field of view in our observations starting almost 4h after impact. We detected a number of morphological features, including a short-lived ejecta cloud visible on September 27 towards the east, spirals, clumps, and a tail that started forming only a few hours after impact. The analysis of the reflectance maps showed that the initial ejecta was bluer than the system before impact, while the tail and spirals were redder than the initial ejecta, consistent with them being made of larger particles. Over the few weeks following impact, the tail became redder. No significant colour differences could be seen between the clumps and the initial ejecta.