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

The DART spacecraft impacted Dimorphos, the small moonlet of the 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. We investigated the dynamical evolution of DART-generated impact ejecta and quantified their surface accretion patterns within the Didymos binary system. We constructed high-fidelity ejecta dynamics, including polyhedron asteroid gravity and solar radiation pressure with combined occultations. The initial conditions of the ejecta were generated from the observation-constrained velocity--size distribution and ejecta-cone geometry. In total, 20 million trajectories were 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 it may reach 3--11.5 mm in a localised high-density region. 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.