Dynamical evolution and surface accretion of DART impact ejecta in the (65803) Didymos system

The DART spacecraft impacted Dimorphos, the small moonlet of Didymos binary system, on 26 September 2022. The impact ejected dust, fragments, and boulders into the near-binary environment. In November 2026, ESA's Hera mission is expected to arrive at the binary system to characterise both asteroids and investigate the post-impact consequences in detail. In this research, we aim to investigate the dynamical evolution of DART-generated impact ejecta and to quantify their surface accretion patterns within the Didymos binary system. High-fidelity ejecta dynamics, including polyhedron asteroid gravity and solar radiation pressure with combined occultations, are constructed. The ejecta initial conditions are generated from the observation-constrained velocity-size distribution and ejecta-cone geometry. In total, 20 million trajectories are integrated to characterise the ejecta evolution and surface accretion. More than 93.5% of DART-generated ejecta particles escape from the system within two years, while only approximately 0.002% remain in the near-binary environment. The deposited layer on Dimorphos reaches the order of 1.5 mm at mid-to-low latitudes. On Didymos, the accreted layer is mostly thinner than 0.3 mm, but may reach 3-11.5 mm in a localised high-density region. The results indicate that, most DART-generated ejecta are removed from the binary system, while a small but dynamically meaningful subset remains near the system or accretes onto the asteroid surfaces. The surface accretion distribution is strongly controlled by the initial ejecta-cone geometry, especially the cone-axis direction.