Optical observations of Didymos between DART and Hera with the NTT

The binary asteroid system Didymos-Dimorphos is an important planetary defence mission target. In September 2022 the NASA DART mission successfully changed the orbit of Dimorphos around Didymos through kinetic impact. In late 2026 the ESA Hera mission will arrive to study the aftermath of the impact in situ. One of the base requirements of the DART mission was that the orbital period change be measured from ground-based observations. This goal was achieved in the first weeks post-impact (Thomas et al. 2023). However, extended studies have put into question the stability of the new orbit (Scheirich et al. 2024, Naidu et al. 2024, Pravec et al. 2024).We will conduct observations of the Didymos-Dimorphos system in August 2024 with the European Southern Observatorys 3.6m New Technology Telescope (NTT) in Chile. The goal of these observations is to monitor the orbit of the asteroid pair in the 'short-term', ahead of Heras arrival. The observations will be used to reveal whether the orbit is now stable and constrain the rate of change if it is still evolving. This will give us insights into ongoing processes in the system: the ejecta clearing timeline, possible reshaping of Dimorphos through the impact and subsequent landslides, and inform models of energy dissipation in the Didymos-Dimorphos system. We will also investigate whether there is any evidence of on-going mass loss from the system in the form of a tail, as was visible in the months immediately after the DART collision. The outputs of the observing campaign will provide critical input to Hera mission planning.The main challenge of this campaign is the location of the asteroid system during the planned observations. The Didymos-Dimorphos pair will be crossing the Galactic plane. This means we expect extremely crowded star fields for observations of a quick-moving target. The unusual observing circumstances demand deployment of new difference image analysis (DIA) techniques, based on methods developed by e.g. Bramich et al. (2013), Hitchcock et al. (2021). The DIA methods need to be tailored to observations of fast moving targets with medium-sized telescopes, to allow construction of high-quality image templates within the limits of the awarded telescope time.References:Thomas et al. 2023, Nature, 616, 448 Bramich et al. 2013, MNRAS, 428, 2275 Hitchcock et al. 2021, MNRAS, 504, 3561 Scheirich et al. 2024, PSJ, 5, 17 Naidu et al. 2024, PSJ, 5, 74 Pravec et al. 2024 (Icarus, in review)