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  • Solar Spin Machine

Photonics Spectra
May 2007
Michael A. Greenwood

Sure, most governments twist events to their advantage when necessary. But the sun, it turns out, is the ultimate spin machine.

Astronomers from Cornell University in Ithaca, N.Y., and from Queen’s University Belfast in the UK have evidence that sunlight alone packs enough photonic punch to influence the movement of smaller celestial bodies, including some asteroids.


For four years, the international team of researchers observed the rotation of 2000 PH5, a misshapen chunk of cosmic debris about 115 m in diameter, via a worldwide network of optical telescopes as well as radar installations located in Puerto Rico and California. They discovered that the rate of PH5’s axial spin, clocked at slightly more than 12 minutes per rotation, was getting faster at the rate of ~1 ms annually, as reported in the March 8 online issue of Science Express.

They attributed this subtle acceleration to torque created by the YORP (Yarkovsky-O’Keefe-Radzievskii-Paddack) effect. After ruling out several other possible forces, the researchers concluded that bombarding photons from the sun carry enough momentum to cause the near-Earth asteroid to spin faster.

The YORP effect holds that photons are absorbed by an object and reradiated as heat. It has two possible results: The impact of the photons can provide a tiny nudge forward to the rotation, or the photons can act as microbrakes and slow the spin. The effect is particularly noticeable on smaller objects and, over long periods of time, can result in significant changes in rotation. The researchers compared the phenomenon to air molecules striking the blades of a windmill.

Astronomers have been baffled for years by the seemingly anomalous rotation characteristics of asteroids. Some spin rapidly, whereas others turn at a lethargic pace. The YORP effect seems to explain the differences.

The researchers believe that PH5’s incremental acceleration will continue indefinitely. In about 1000 years it should be turning a full second faster; about 500,000 years from now, its rotational period could be cut in half — to six minutes.

Beyond that, things do not look very rosy for the rock that was first discovered in 2000. Eventually, it could be turning so rapidly — a day could be over in 20 seconds — that it could spin itself into annihilation.

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