Rotational lightcurves of Dimorphos and constraints on its post-DART impact spin state

Abstract

Dimorphos, the secondary member of the binary asteroid (65803) Didymos, was impacted by NASA’s Double Asteroid Redirection Test (DART) spacecraft on September 26, 2022. Images taken with the DART’s DRACO camera before the impact showed that Dimorphos’s original shape was nearly rotationally symmetric, close to an oblate spheroid, and it was probably in a synchronous spin state. As such, it did not show a detectable secondary brightness variation before the DART impact in the integral light from the binary system. However, numerical impact models predicted that the DART impact could change both its shape and spin state, signatures of which could be detected with high-quality lightcurve observations. We have analyzed the best photometric observations of the Didymos–Dimorphos system that were taken during its favorable observing and geometric conditions in December 2022 and January 2023 and detected a significant secondary rotational lightcurve with amplitudes (in the total light from the primary and secondary) ranging from 0.008 to 0.031 mag at 7 distinct epochs. We estimate that the apparent cross section of Dimorphos varied with a relative amplitude (normalized to the mean cross section) between ±0.07 and ±0.24 over its rotation on the individual epochs. The observed changes of Dimorphos’s apparent cross-section amplitude over its rotation suggest an attitude instability, showing variations of Dimorphos’s obliquity. The lightcurve minima are approximately aligned, to within 30∘in mean anomaly, with the mutual events between the components of the binary system. The observations suggest that Dimorphos is in an excited, non-principal axis (NPA) spin state where, on average, it is tidally locked. By comparing the observations to high-fidelity simulations, we find that a moderate amount of NPA rotation in Dimorphos after the DART impact is required to reproduce the observed lightcurves. This NPA rotation is limited to around on-average synchronous or anti-synchronous configurations of Dimorphos. We also find Dimorphos elongations with the equatorial axis ratios a/b between 1.1 and 1.4 are generally consistent with the observed mean apsidal precession rate. We predict that when the ESA’s Hera spacecraft arrives to the Didymos–Dimorphos system in late 2026, it will still find Dimorphos in the NPA spin state, which places additional constraints on the operations of the spacecraft to achieve its objectives.