Double Trouble: Polarimetric Insights into the Didymos-Dimorphos post-DART Evolution

On September 26, 2022, the NASA DART (Double Asteroid Redirection Test) spacecraft struck Dimorphos, the moonlet of Didymos, to test near-Earth object deflection through impact in the context of planetary defence (Daly et al. 2023). As well as causing a change in Dimorphos’ orbital period (Thomas et al. 2023), the impact caused a massive dust cloud to be ejected from the surface (e.g. Li et al. 2023; Opitom et al. 2023). By studying the characteristics and behaviour of the ejecta cloud, the DART mission provided a rare opportunity to peer under the surface of an asteroid and further our understanding of their global properties.

Didymos-Dimorphos was monitored in polarimetric mode before and after the impact (Bagnulo et al. 2023; Gray et al. 2024). The post-impact measurements revealed a significant drop in polarisation, suggesting differences between the material in the ejecta and that on the original regolith surface. Remarkably, even months after the impact, the polarisation remained persistently lower than pre-impact, despite photometric measurements showing that the system faded back to its “normal” brightness around 23 days post-impact (Graykowski et al. 2023). This suggests the presence of residual ejecta material still within the system months after the impact, either in orbit or deposited on the asteroid surface(s). This contrast in findings demonstrates the sensitivity of polarimetric measurements, revealing details often unattainable from traditional observations techniques.

In this presentation, we will be discussing these previous results, as well as presenting new polarimetric measurements obtained with VLT this year. In 2024, Didymos-Dimorphos is making another close approach to Earth, offering an opportunity to study the system around two years after the DART impact. Our goal with these observations is to establish whether the polarisation of the system has returned to the pre-impact level or remains at a lower level as observed post-impact and, thus, clarify if or how much ejecta material has remained in the system. This may ultimately benefit the Hera team (Michel et al. 2018) for planning the spacecraft trajectory to avoid damage by impacts of dust particles. Eventually, instruments on board Hera will reveal the spatial distribution of fresh material on the surfaces of the system. This unique opportunity will allow us to test and demonstrate the reliability and importance of polarimetric measurements of small solar system bodies.

Daly, R., et al. (2023), Natur, 616, 443; Thomas, C. A., et al. (2023), Natur, 616, 448; Li, J.-Y., et al. (2023), Natur, 616, 452; Opitom, C., et al. (2023), A&A, 671, L11; Bagnulo, S., et al. (2023), ApJL, 945, L38; Gray, Z., et al. (2024), PSJ, 5, 18; Graykowski, A., et al. (2024), Natur, 616, 452; Michel, P., et al. (2018), AdSpR, 62, 2261.