Presentation #202.05 in the session Asteroid Dynamics.
Near-Earth binary asteroids are typically made up of a fast-rotating primary with a much smaller secondary locked in a synchronous spin-orbit resonance. However, exceptions to this paradigm exist, notably 1991 VH, which appears to have an asynchronous secondary in a chaotically rotating state. Multiple observation campaigns for 1991 VH reveal a non-constant orbit period and secondary spin rate. Using dynamical modeling of the binary asteroid, we attempt to explain these observations and gain insight into the true dynamics of the system. We study a variety of possible spin configurations of the secondary in order to fit the observations. One possibility is the secondary may inhabit a rotation state in which it remains on-average synchronous to the primary for a time, before entering a fully chaotic tumbling state and settling back into an on-average synchronous state, but oriented in the opposite direction, repeating this cycle over time. With even a small eccentricity (<0.1, as is the case for 1991 VH), many secondary shapes enter this rotation state naturally with an infinitesimal perturbation to the spin obliquity, leaving the stable, synchronous resonance. A discrete FFT analysis on pseudo-lightcurves of this rotation state reveals a 1/2 resonance with its nutation rate dominates in frequency with a period of 11-12 hours. These results are consistent with observations, and over time this frequency shifts with the same magnitude seen in observations due to spin-orbit coupling in binary asteroids. Beyond this, we also consider additional secondary spin configurations consistent with observations.