A Change of the Rotation Period of Asteroid (65803) Didymos Caused by the DART Impact

Abstract On 2022 September 26, the NASA Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the secondary component of the binary asteroid (65803) Didymos. This experiment tested the kinetic impactor technology for diverting dangerous asteroids. Due to the impact, the binary system’s angular momentum has changed, resulting in a significant change in the orbital period of Dimorphos. Precise values of the pre- and postimpact orbital periods were derived from a large set of photometric light curves measured for the Didymos–Dimorphos system during six apparitions from 2003 to 2023. We used these data to detect a possible change in the rotation period of the primary as a consequence of the impact. We analyzed the binary system’s light curves using the binary asteroid light-curve decomposition method. We selected parts of the light curves covering orbital phases outside mutual events, which represent the primary rotational light curves. We applied the light-curve inversion method to construct a convex shape model of Didymos and determine its rotation period before and after the impact. These two periods were treated as independent free parameters of the modeling. We found a value of 2.260 389 1 ± 0.000 000 2 hr for the preimpact period and 2.260 440 ± 0.000 008 hr for the postimpact period. Their difference 0.18 ± 0.03 s is small yet significant, indicating that the rotation of Didymos became slower after the DART impact. The most plausible physical explanation is Didymos’s postimpact reshaping, making its shape more oblate.