Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect is causing the spin of asteroid 2000 PH5 to speed up at the rate of about one millisecond per year

"Astronomers have found that the weak but constant force of sunlight is causing the spin of a small asteroid to accelerate. In a paper published online by Science last week, a group of astronomers said than the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect is causing the spin of asteroid 2000 PH5 to speed up at the rate of about one millisecond per year. The YORP effect is created when an asteroid or other body absorbs sunlight and reradiates it as heat. While normally a feeble force, the YORP effect can cause measurable changes in small, irregularly-shaped bodies like this asteroid. Researchers ruled out all other known potential causes for the change in the asteroid's spin, including tidal forces created when the asteroid passes the vicinity of the Earth. The YORP effect may explain why very small asteroids spin either very quickly or very slowly."

From Science Express (08 March 2007):

Published Online March 8, 2007
Science DOI: 10.1126/science.1139040
Science Express Index
Submitted on December 19, 2006
Accepted on February 23, 2007

Direct Detection of the Asteroidal YORP Effect
Stephen C. Lowry 1*, Alan Fitzsimmons 1, Petr Pravec 2, David Vokrouhlicky 3, Hermann Boehnhardt 4, Patrick A. Taylor 5, Adrian Galád 6, Mike Irwin 7, Jonathan Irwin 7, Peter Kusnirák 2
1 School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN, UK.
2 Astronomical Institute, Academy of Sciences of the Czech Republic, Fricova 1, CZ-25165 Ondrejov, Czech Republic.
3 nstitute of Astronomy, Charles University, V Holesovickách 2, 18000 Prague 8, Czech Republic.
4 Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany.
5 Department of Astronomy, Cornell University, Ithaca, NY 14853, USA.
6 Department of Astronomy, Physics of the Earth, and Meteorology, Faculty of Mathematics, Physics and Informatics, Comenius University, 842 48 Bratislava, Slovakia.
7 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK.
* To whom correspondence should be addressed. Stephen C. Lowry , E-mail: s.c.lowry@qub.ac.uk

The Yarkovsky-O'Keefe-Radzievskii-Paddack effect is believed to alter the spin states of small bodies in the solar system. However, evidence for the effect has so far been indirect. Here we report precise optical photometric observations acquired over four years of a small near-Earth asteroid (54509) 2000 PH5. We find the asteroid has been continuously increasing its rotation rate {omega} over this period by d{omega}/dt = 2.0 (±0.2) x 10-4 degrees/day2. We simulated the close Earth approaches from 2001-2005, showing that gravitational torques cannot explain the observed spin-rate increase. Dynamical simulations also suggest that 2000 PH5 may reach a rotation period of ~20 seconds towards the end of its expected lifetime.

Link: Science Express Abstract (08 March 2007)
Link: PDF of Supporting Material for Paper

Link: New York Times article
Link: SPACE.com article
Link: Reuters article
Link: New Scientist article
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Link: Cornell Univ. press release