Didymos’s Reshaping Scale Revealed by Detection of Its Spin Slow-down

NASA’s Double Asteroid Redirection Test (DART) conducted a successful kinetic deflection experiment on Dimorphos, the smaller companion in the near-Earth binary asteroid (65803) Didymos. Earlier light-curve work detected that Didymos’s spin slowed down by 0.18 ± 0.03 s. The most plausible explanation is that Didymos’s spin-driven reshaping increased its moment of inertia, slowing it under the asteroid’s angular momentum conservation. Here we extend this scenario and develop a simple analytical model to infer that the DART-driven ejecta contributes to Didymos’s spin-up, and thus it requires a higher amount of surface mass wasting than originally reported to experience the observed spin-down. Setting accumulated ejecta mass and initial speed as free parameters, we find that the ∼10 6 kg ejecta mass may give an equator thickness change less than ∼1.8 cm, while the ∼10 8 kg ejecta mass may cause a thickness change of ∼18 cm. The resulting thickness changes are higher by a factor of a few to an order of magnitude. We also investigate whether accumulated ejecta and spin-driven reshaping can make the mutual orbital period shorter by an additional ∼30 s, which was observed during a few weeks since the DART impact after the initial ∼33-minute orbital period drop. Our model suggests that the later-stage mutual orbit period drop only by reshaping may be ∼1.65 s. Thus, ejecta accumulation and spin-driven reshaping only do not explain this later-stage mutual orbit period change. ESA’s Hera, which is en route to Didymos, will offer insights into the asteroid’s spin-down mechanisms.