The Dynamical State of the Didymos System Before and After the DART Impact

NASA’s Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the natural satellite of (65803) Didymos, on 2022 September 26, as a first successful test of kinetic impactor technology for deflecting a potentially hazardous object in space. The experiment resulted in a small change to the dynamical state of the Didymos system consistent with expectations and Level 1 mission requirements. In our pre-encounter paper \citep{Richardson2022}, we put forward predictions for the pre- and post-impact dynamical state of the Didymos system. Here we assess these predictions, update preliminary findings published after the impact, report on new findings related to dynamics, and provide implications for ESA's Hera mission to Didymos, scheduled for launch in 2024 with arrival in late December 2026. Pre-encounter predictions tested to date are mostly in line with observations, with the biggest surprise being the flattened appearance of Didymos compared to the radar model and the apparent pre-impact oblate shape of Dimorphos (where the expectation had been to find a prolate secondary). New findings include the fact that Dimorphos likely became prolate as a result of the impact or entered a tumbling rotation state, or both.