Orbital and physical characterization of asteroid Dimorphos following the DART impact

The Double Asteroid Redirection Test (DART) mission struck Dimorphos, the satellite of the binary near-Earth asteroid (65803) Didymos, on September 26, 2022, UTC. By analyzing ground-based photometric and radar data, along with observations from DART's camera, we assessed the changes in the system's orbital and physical characteristics due to the impact. We estimate that the impact instantaneously altered Dimorphos's along-track velocity by -2.63 ± 0.06 mm/s. Initially, this collision induced a change in the orbital period of Dimorphos by -32.7 minutes ± 16 seconds, resulting in a new period of 11.377 ± 0.004 hours. Subsequently, over several weeks, the orbital period further adjusted by an additional 34 ± 15 seconds, ultimately stabilizing at 11.3674 ± 0.0004 hours. In total, the orbital period changed by -33.25 minutes ± 1.5 seconds. Post-impact observations revealed an apsidal precession rate of 6.7 ± 0.2 degrees per day. In our model, this rate is influenced by the oblateness parameter of Didymos (J2) and the spherical harmonics coefficients (C20 and C22) of Dimorphos's gravitational field. Assuming that Dimorphos is a triaxial ellipsoid with a uniform density, our C20 and C22 estimates suggest axial ratios of approximately 1.3 for a/b and 1.6 for a/c. Pre-impact imagery from DART showed Dimorphos's shape resembled an oblate spheroid, and thus our results indicate that the impact significantly modified its shape.