Optical monitoring of the Didymos-Dimorphos asteroid system with the Danish telescope around the DART mission impact

The NASA's Double-Asteroid Redirection Test (DART) was a unique planetary defence and technology test mission, the first of its kind. The main spacecraft of the DART mission impacted the target asteroid Dimorphos, a small moon orbiting the asteroid Didymos (65803), on 2022 September 26. The impact brought up a mass of ejecta which, together with the direct momentum transfer from the collision, caused an orbital period change of 33 ± 1 minutes, as measured by ground-based observations. We report here the outcome of the optical monitoring campaign of the Didymos system from the Danish 1.54 m telescope at La Silla around the time of impact. The observations contributed to the determination of the changes in the orbital parameters of the Didymos–Dimorphos system, as reported by Thomas et al., but in this paper we focus on the ejecta produced by the DART impact. We present photometric measurements from which we remove the contribution from the Didymos–Dimorphos system using an H–G photometric model. Using two photometric apertures we determine the fading rate of the ejecta to be 0.115 ± 0.003 mag day−1 (in a 2'' aperture) and 0.086 ± 0.003 mag day−1 (5'') over the first week postimpact. After about 8 days postimpact we note the fading slows down to 0.057 ± 0.003 mag day−1 (2'' aperture) and 0.068 ± 0.002 mag day−1 (5''). We include deep-stacked images of the system to illustrate the ejecta evolution during the first 18 days, noting the emergence of dust tails formed from ejecta pushed in the antisolar direction, and measuring the extent of the particles ejected Sunward to be at least 4000 km.