Long-term dynamics around the Didymos–Dimorphos binary asteroid of boulders ejected after the DART impact

Context. In 2022, the DART mission spacecraft impacted the asteroid Dimorphos, the secondary body of the binary Didymos system, ejecting a large number of dust particles, rocks and boulders. The ESA Hera mission will reach the system in 2026 for post-impact studies and a possible detection of orbiting fragments. Aims. We aim to investigate the long-term dynamics of the large boulders ejected by DART to test if any of these objects survive in orbit until the arrival of the Hera mission. Methods. To model the dynamics of the boulders, we used a numerical model that includes the gravity of non-spherical Didymos and Dimorphos, the solar gravity, and the radiation pressure. The SPICE kernels are used to define the correct reference frame for the integrations. Results. The dynamics of the boulders is highly chaotic, and 1% of the initial boulders survive at least for four years on quasi-stable orbits. These orbits are characterised by wide oscillations in eccentricity in antiphase with those in inclination (including spin flips), a mechanism similar to the Kozai one. This behaviour may protect these bodies from close encounters with both asteroids. We also computed the distribution on the surfaces of the asteroids of sesquinary impacts, which may influence the dust emission (after the initial DART impact) and the surface composition of the asteroids. Conclusions. The probability of observing boulders by the mission Hera is small but non-negligible, and an almost constant flux of escaping boulders is expected in the coming years since their lifetime after the DART impact covers a large time interval. Most re-impacts on Dimorphos occur in the hemisphere opposite the impact site, preferentially close to the equatorial plane.