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D. P. Rubincam (NASA/GSFC)
The absorption of sunlight and its reemission as thermal radiation may change the spin state of small (< 6 km) asteroids on geologically significant timescales. The mechanism is the Yarkovsky-Radzievskii effect, as elaborated by John A. O’Keefe and Stephen Paddack (called YORP for short). In the YORP effect a dark asteroid absorbs most of the sunlight falling on it; this energy is reradiated in the infrared. If the asteroid has a certain amount of “windmill” shape to it, the momentum carried off by the infrared photons can torque the asteroid, causing it to speed up or slow down its rotation. Both 951 Gaspra and the Martian moon Deimos have enough of a windmill shape for YORP to significantly alter their rotation speeds on the 200 million year timescale, assuming that their spin axes are normal to their orbital planes (and if Deimos were a main belt asteroid). Smaller asteroids with the same shapes can spin up or down even faster, due to a radius-squared dependence of the spin timescale. Asteroids closer to the Sun can also change faster, due to stronger sunlight. Rapidly spinning 1566 Icarus and slowly tumbling 4179 Toutatis may be examples of YORP-induced spin changes.
The author(s) of this abstract have provided an email address for comments about the abstract: rubincam@denali.gsfc.nasa.gov