Proximal boulders and momentum transfer from DART-scale 3D impact simulations on asteroid Dimorphos

ABSTRACT The NASA Double Asteroid Redirection Test (DART) mission successfully changed the trajectory of asteroid Dimorphos by hypervelocity kinetic impact. The spacecraft imaged a boulder-strewn surface, yet the asteroid’s internal structure remains unknown. To understand how boulders surrounding the impact point influence ejecta and momentum transfer, 3D impact simulations were performed using the isale shock physics code, varying the number, size, burial depth, and radial distance of proximal boulders. Results show that boulders positioned off-centre, adjacent to the impact site, can locally suppress or redirect ejecta, producing peripheral effects on the momentum enhancement factor, $\beta$, with net variations within 8 per cent compared to the no-boulder case. These effects are confined to a radial zone extending up to around 10 meters (around 16 impactor radii) from the impact point, within which proximal boulders can efficiently govern ejecta and momentum transfer. Outside this zone, the influence of boulders on deflection efficiency becomes negligible. This underscores the limited influence of boulders surrounding the impact site and points to the dominant role of those within the projectile-target coupling zone, offering insights for planetary defense applications. To probe impact ejecta over extended time-scales, axisymmetric 2D simulations were conducted with a mass-strength rescaling that conserves boulder mass density and strength per boulder ‘ring’. This reproduces $\beta$ trends in 3D while enabling longer 2D simulations of boulder-rich rubble-pile targets in spheroidal volumes. Simulated $\beta$ values remain close to the observed range, suggesting plausible interior solutions for Dimorphos and supporting efforts to probe its interior ahead of ESA’s Hera mission.