spacetoday.net: Sunlight affects asteroid spin

Posted: Mon, Mar 12, 2007, 8:25 AM ET (1225 GMT)
Radar image of asteroid 2000 PH5 (Cornell Univ.)

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.