Orbital and physical characterization of asteroid Dimorphos following the DART impact

The Double Asteroid Redirection Test (DART) mission impacted Dimorphos, the satellite of binary near-Earth asteroid (65803) Didymos, on 2022 September 26 UTC. We estimate the changes in the orbital and physical properties of the system due to the impact using ground-based photometric and radar observations, as well as DART camera observations. We estimate that the DART impact instantaneously changed the along-track velocity of Dimorphos by −2.63 ± 0.06 mm s−1. Initially, after the impact, Dimorphos’s orbital period had changed by −32.7 minutes ± 16 s to 11.377 ± 0.004 h. We find that over the subsequent several weeks the orbital period changed by an additional 34±15 s, eventually stabilizing at 11.3674 ± 0.0004 h. The total change in the orbital period was −33.25 minutes ± 1.5 s. The post-impact orbit exhibits an apsidal precession rate of 6.7 ± 0.2 deg day−1. Under our model, this rate is driven by the oblateness parameter of Didymos, J2, as well as the spherical harmonics coefficients, C20 and C22, of Dimorphos’s gravity. Under the assumption that Dimorphos is a triaxial ellipsoid with a uniform density, its C20 and C22 estimates imply axial ratios, a/b and a/c, of about 1.3 and 1.6 respectively. Pre-impact images from DART indicate Dimorphos’s shape was close to that of an oblate spheroid, and thus our results indicate that the DART impact significantly altered the shape of Dimorphos.