Orbit Period Variations of Dimorphos After the DART Impact

The DART impact will force Dimorphos, the secondary of the Didymos binary asteroid system, to librate in its orbit. Due to spin-orbit coupling, any libration of the secondary about the primary leads to fluctuations in the mutual orbit period, as a result of a periodic exchange of angular momentum between the mutual orbit and Dimorphos's spin (Meyer et al, 2021). Depending on Dimorphos's geophysical properties, this libration can either be stable, with a fluctuating rotation about its principal axis, or unstable, with non-principal axis rotation (Agrusa et al, 2021). The combination of images from DART and LICIACube will provide an estimate for the shape and volume of Dimorphos and Didymos (Daly et al, 2022) and thus a better constraint on the bulk density of the system. With the spacecraft data, we are able to focus on a narrower parameter space for a dynamical analysis and calculate the expected magnitude and frequencies of variations of the orbit period. Based on the available observations following the impact, we will present our findings regarding the libration-induced orbit period variations over time and their observability in lightcurve measurements. The predicted time-varying orbit period will manifest in the lightcurve measurements in the form of deviations of the mutual event timings. Equivalently, the ingress and egress timings of the mutual events will differ from their expected values by up to several minutes. Using dynamical simulations of the binary orbit and corresponding synthetic lightcurves, we will predict the mutual event timing variations for the binary orbit. If these variations can be measured in the actual system, this can constrain the libration state of Dimorphos, as these quantities are closely related. If instead the variations are too small to observe from the ground, we will report our theoretical predictions on how they may appear to the ESA Hera spacecraft during its upcoming survey of Didymos.

References

Agrusa, H. F. et al. The Excited Spin State of Dimorphos Resulting from the DART Impact. Icarus 370, 114624 (2021).

Daly, R.T. et al. Shape Modeling of Dimorphos for the Double Asteroid Redirection Test (DART). In Press (2022).

Meyer, A. J. et al. Libration-induced Orbit Period Variations Following the DART Impact. Planet. Sci. J. 2, 242 (2021).