YORP Alteration of Asteroid Spins: Why are Slow Rotators Tumbling and not Synchronized?

Details YORP Alteration of Asteroid Spins: Why are Slow Rotators Tumbling and not Synchronized? YORP Alteration of Asteroid Spins: Why are Slow Rotators Tumbling and not Synchronized?

Nov 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004dps....36.4804h&link_type=abstract

American Astronomical Society, DPS meeting #36, #48.04; Bulletin of the American Astronomical Society, Vol. 36, p.1185

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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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