Modeling the Coupled Dynamics of an Asteroid with Surface Boulder Motion

Abstract A new model is presented to investigate the interaction between the asteroid's 6-degrees-of-freedom dynamics and the state of boulders on its surface. The model considers boulder motion on the surface of the asteroid as well as boulder launch, orbit, crash and escape in two ways. There are two mechanisms that change the angular velocity of the asteroid. First, there can be a momentum transfer during launch and crash of boulders, and second when a boulder moves the inertia tensor of the combined system changes. If a boulder launches and escapes the system, an effective Δ V is applied to the asteroid inertial velocity. By using defined polyhedral asteroid geometries and a given boulder distribution this hybrid model provides insight into the behavior of a specific asteroid while only accounting for tens or hundreds of bodies in the simulated system. This approach gives the model an advantage in detail in comparison to simpler analytical investigations, while also providing a computational advantage in comparison to a full N-body simulation. Surface moving boulders tend to reach the equator and launch from it or rest near it. The resulting orbits are of low inclination and follow chaotic trajectories leading to crash events. This model indicates that motion of several meter-sized boulders on the surface of a Bennu-like top-shaped asteroid can induce wobble in the spin axis on the order of 0.01° and a change of asteroid period on the order of several seconds. This phenomena, if it happens repeatedly, can alter the YORP and Yarkovsky evolution of small asteroids.