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We present the first self-consistent simulations of the coupled spin-shapeevolution of small gravitational aggregates under the influence of the YORPeffect. Because of YORP's sensitivity to surface topography, even smallcentrifugally driven reconfigurations of aggregates can alter the YORP torquedramatically, resulting in spin evolution that can differ qualitatively fromthe rigid-body prediction. One third of our simulations follow a simpleevolution described as a modified YORP cycle. Two-thirds exhibit one or more ofthree distinct behaviors---stochastic YORP, self-governed YORP, and stagnatingYORP---which together result in YORP self-limitation. Self-limitation confinesrotation rates of evolving aggregates to far narrower ranges than thoseexpected in the classical YORP cycle, greatly prolonging the times over whichobjects can preserve their sense of rotation. Simulated objects are initiallyrandomly packed, disordered aggregates of identical spheres in rotatingequilibrium, with low internal angles of friction. Their shape evolution ischaracterized by rearrangement of the entire body, including the deep interior.They do not evolve to axisymmetric top shapes with equatorial ridges. Mass lossoccurs in one-third of the simulations, typically in small amounts from theends of a prolate-triaxial body. We conjecture that YORP self-limitation mayinhibit formation of top-shapes, binaries, or both, by restricting the amountof angular momentum that can be imparted to a deformable body. Stochastic YORP,in particular, will affect the evolution of collisional families whose orbitsdrift apart under the influence of Yarkovsky forces, in observable ways.

Coupled Spin and Shape Evolution of Small Rubble-Pile Asteroids: Self-Limitation of the YORP Effect