Title:Primary Surface Particle Motion and YORP-Driven Expansion of Asteroid Binaries
Author:Eugene G. Fahnestock, University of Michigan

This presentation will focus on recent investigation of the motion of loose material from the surface of the primary in any close binary belonging to the class of asynchronous binary asteroids that is the main subject of this workshop. I examine such dynamics within the 1999 KW4 system in particular, it being the best characterized example from this class. I first employ high-fidelity dynamic simulation of regolith material particles in the full gravity field of the components, themselves propagating according to their full-detail mutual gravitation. For this one models the regolith particles as test particles having no direct influence on the dynamics of the components. However, in addition to tracking the final dispositions (return impact, possibly transfer impact, possibly escape) of the dynamically simulated particles, from their response one calculates the particle-mass-specific angular momentum changes to the system components and mutual orbit that should correspond to such motion, averaged over all particles of a common disposition. I obtain results for lofting of regolith at different initial primary spin rates and initial relative poses that determine the lofting location. Using the dynamic simulation's particle trajectory output to construct the core probability matrices, I also perform probability-based mapping of many particles initially on the primary surface forward in time for very long durations, O (10^4 yr), with feasibly low computational cost. Tracking changes to mass, inertia dyad, rotation states, and centroid position and velocity for each binary component during this probability-based simulation allows changes to the angular momenta of primary, secondary, and mutual orbit in response to the particle motion to be tracked. This independently reproduces and visibly demonstrates the angular momentum transfer mechanism behind binary evolution towards separation, from which we quantify the relevant timescale for comparison with the tidal evolution timescale, dynamical lifetime of such pairs, etc. It is found that lofting of primary surface material due to supercritical primary spin may occur episodically to near-continuously, depending upon the magnitude of primary angular acceleration driving the lofting. While I also characterize average mass lofting rate and average amount of material in flight vs. the applied angular acceleration magnitude and particle size and quantity, the lofting's typically episodic nature demands caution in interpretation of these results.