Secondary-impact debris in the Didymos system: What could be observed by Hera?

Context. The DART spacecraft impacted the asteroid Dimorphos in 2022. This small moonlet of the asteroid Didymos ejects dust, rocks, and boulder into the space around both asteroids. Part of those ejecta can re-impact the surface of the two asteroids. At the end of 2026, the ESA mission Hera will arrive at the system to analyse the binary system and the consequences of the impacts in detail. Aims. We investigated the effects of low-velocity impacts of rocks and boulders that were originally released after the DART impact on the surface of Didymos and the dynamics of the dust particles that were released by these impacts. We determined whether any of these effects can be observed by the Hera mission. Methods. The iSALE-2D shock physics code was used to simulate the re-impacts of boulders on the surface of the asteroid. To model the dynamics of the boulders and of the subsequent ejecta, we used a numerical model that included the gravity of non-spherical Didymos and Dimorphos, solar gravity, and the radiation pressure. Results. The sesquinary impacts can result in small, shallow craters on the surface of Didymos. For the given low impact speeds, the ejected mass mostly depends on the boulder mass. Ejection speeds range from ~10 to ~80% of the impact speed. The majority of the ejected dust falls back and covers a large area of the surface, mostly at low to medium latitudes. Fewer than 20% of the ejected dust escapes from the system after a few days. The space surrounding the asteroids becomes free from dust after 15–30 days following each sesquinary impact.