The expansion of debris flow shed from the primary of 65803 Didymos

This paper investigates the plausible mass shedding from the surface of Didymos A, the primary of Near-Earth binary asteroid 65803 Didymos, and the subsequent orbital evolution of the shedding debris. Shedding conditions are derived semi-analytically. The unstable surface area on Didymos A is determined by combining the analyses and numeric results of SSDEM simulations.We show the observation-basedmodel gives a highly unstable surface for Didymos A. In particular, most of the equatorial ridge is unattachable for cohesionless regolith, and an active region emerges along the ridge, in which extensive avalanches that cause surface mass shedding seem likely to happen. We track the ballistic motion of shedding debris from the unstable area. Month-long simulations show the diverse evolutions of sample particles with different sizes, due to the varying magnitude of solar radiation pressure. Statistics on the dynamical fates of shedding debris give clues on the current development of this binary system. Depending on the observation-based reference model, we find a vast majority of the shedding mass is finally transferred to Didymos B and thus leads to a cumulative growth. The cumulated mass may cause a spiralling-in motion of Didymos B, and the spiralling-in speed depends on the frequency and magnitude of mass shedding. A topography dependence of the mass shedding/transfer is also noticed that the cumulated mass mostly originates from highly elevated regions of the ridge areas. Long term it suggests that the shedding processes prefer to Erode the bulges first and force the equatorial ridge to evolve into good roundness.