YORP-Driven Spin State Evolution of Meter-Sized Asteroids

The spin states of meter-sized asteroids have implications for solar system evolution and proposed NEO exploration missions. Unfortunately, the small size of these objects makes them very challenging to detect and study. Nevertheless, it may be possible to gain insight about the spin state evolution of these small asteroids by studying more convenient analogues, namely retired artificial earth satellites. These satellites are easily observed with ground-based telescopes and some have well-documented geometry, mass distributions, and optical properties. Of particular interest are defunct satellites in geosynchronous earth orbit (GEO). At these altitudes, atmospheric drag is zero and spin states are dominated by solar radiation and gravitational torques, in line with their natural counterparts. Recent study of the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect for defunct GEO satellites has uncovered rich dynamical structure. Using both full and semi-analytical spin-averaged models, we have found that solar radiation torques can drive these bodies repeatedly between uniform and non-principal axis tumbling via so-called "tumbling cycles". Furthermore, there is a strong tendency for the tumbling satellite's rotational angular momentum vector to continuously track and precess around the sun line, with the angle between the sun line and pole dictating evolution. As a result, earlier orbit-averaged models are inappropriate for these bodies. Finally, resonant YORP tumbling states with commensurate tumbling periods were also found in simulations. Intriguingly, these behaviors are consistent with observed spin state evolution of retired GEO satellites. Optical and radar-based spin state analysis of the GOES 8-12 satellites has shown transitions from uniform rotation to tumbling, subsequent tumbling spin up, systematic pole motion, and resonant tumbling states. Applying these YORP models to meter-sized pseudo asteroids, we find that the solar torque structure permits sun-tracking precession in many cases. For some asteroid shapes, tumbling cycles are also observed in addition to sun-tracking precession. For completeness, spin-averaged gravitational torques are also considered.