The recoil due to the reflection and emission of photons from a Sun-irradiated surface is a major driver of dynamical evolution for small asteroids—especially the sorts that pose an impact hazard for Earth. The net recoil force (the Yarkovsky effect) drives evolution of the orbital elements; the net recoil torque (the YORP effect) drives evolution of the spin rate and axis orientation. Both effects are sensitively dependent on the spin state; hence understanding how spins evolve under the influence of YORP is crucial for understanding how orbits evolve under the influence of Yarkovsky. Previous work showed that monolithic, rigid asteroids should follow a largely deterministic “YORP cycle,” with long phases of rotational acceleration and deceleration. I will demonstrate, however, that YORP is so hypersensitive to the detailed topography of the surface that slight motions of loose material can qualitatively alter the torque and interrupt the cycle. The fact that most asteroids are probably not monolithic, but instead loosely-bound aggregates, has led to suggestions that continuous YORP acceleration may drive centrifugal mass shedding and the formation of binaries. However, we have performed the first self-consistent simulations of the YORP effect on dynamically evolving aggregates, and the results indicate that acceleration is rarely continuous. Instead, repeated reconfigurations of the body under the changing centrifugal force result in a random walk in spin rate and obliquity. This stochastic YORP evolution is qualitatively different from the YORP cycle, and, moreover, correctly predicts the distribution of orbits for asteroid families evolving under the Yarkovsky effect. These results have significant implications for binary formation and the feeding of asteroids onto Earth-crossing orbits, as well as for our understanding of the material properties of potential impactors.
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