Dynamics of asteroid systems post-rotational fission

Abstract

Asteroid binaries found among the near-Earth objects are believed to have formed from rotational fission. In this paper, we study the dynamical evolution of asteroid systems the moment after fission. The model considers two bodies the moment after a contact binary separates due to rotational fission. Both bodies are modeled as ellipsoids, and the secondary is given an initial rotation angle about its body-fixed y-axis. Moreover, we consider six different cases, three where the density of the secondary varies and three where the shape of the secondary varies. The simulations consider 45 different initial tilt angles of the secondary, each with 37 different mass ratios. We start the dynamical simulations at the moment the contact binary reaches a spin fission limit, and our model ensures that the closest distance between the surfaces of the two bodies is always kept at 1 cm. The forces, torques, and gravitational potential between the two bodies are modeled using a newly developed surface integration scheme, giving exact results for two ellipsoids. We find that more than 80% of the simulations end with the two bodies impacting, and collisions between the bodies are more common when the density of the secondary is lower, or when it becomes more elongated. In comparison with observed data on asteroid pairs, we find that variations in density and shape of the secondary can account for some of the spread seen in the rotation period for observed pairs. Furthermore, the secondary may also reach a spin limit for surface disruption, creating a ternary or multiple system. We find that secondary fission typically occurs within the first five hours after the contact binary separates, and is more common when the secondary is less dense or more elongated.