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A. W. Harris (Space Science Institute)
The YORP radiation torque on asteroid spins does not depend strongly on spin rate, thus one might expect that some objects should have reached an end state of coming to a complete rotational halt, at which point they would become synchronously locked with their orbit period, or perhaps a harmonic of it, as is the case for end states of tidal evolution. Asteroids such as 288 Glauke, with a rotation period of 2 months, or 253 Mathilde, with a period of half a month, have presumably been slowed by YORP by one or two orders of magnitude from their initial spin rates, yet have entered "tumbling" (non-principal axis) rotation states and not gone the last little way to spin-orbit synchronization. Collisions offer a plausible explanation. For a YORP torque scaled from calculations by Vokrouhlicky and Capek (Icarus 159, 449-467, 2002), the time scale for Glauke to slow from its present spin to a total halt is ~10 million years. From the main belt population and collision frequency estimates given by Davis et al. (Asteroids III, pp. 545-558, 2002), an asteroid the size of Glauke should experience a collision large enough to excite its present spin rate about once in 5 million years. A similar comparison for Mathilde yields time scales of a few tens of millions of years, but again comparable to each other. Thus within uncertainties of both effects, it appears plausible that the spin states of large slow rotators are a balance between YORP slowing and collisional excitation. This would also naturally explain the tumbling rotation states. This explanation does not work so well for small, slowly rotating NEAs. Among sub-km sized asteroids, the time scale of YORP alteration is so fast that collisions appear inadequate to establish an "equilibrium" spin rate or induce tumbling. For 25143 Itokawa, 350 m in diameter with a period of 12 hours, the time scale for YORP to bring it to a complete halt from its present spin is only about 40,000 years. The expected time between collisions large enough to excite this level of rotation is ten times longer.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.