March 7, 2007

Asteroids spin at YORP speed, thanks to the effects of sunlight, Cornell and Belfast astronomers discover

Sunlight alone can change the way an asteroid and other small bodies spin in space, suggests a new study led by astronomers at Cornell and Queen's University Belfast. Their observations provide the most conclusive evidence to date that an effect of sunlight called YORP plays a direct role in the evolution of asteroids.

Cornell graduate student Patrick Taylor and assistant professor of astronomy Jean-Luc Margot mapped the shape and located the spin pole of a 100-meter-diameter (about 300 feet) near-Earth asteroid called (54509) 2000 PH5 (abbreviated to PH5) between 2001 and 2005, using radar at the National Science Foundation's (NSF) Arecibo Observatory in Puerto Rico and NASA's Goldstone telescope in California.

Meanwhile, a team led by astronomers Stephen Lowry and Alan Fitzsimmons in Belfast used telescopes around the world to measure PH5's light curve, the varying brightness of the asteroid as it rotates. They found that PH5's spin, already unusually fast at about 12 minutes per rotation, is accelerating by about one millisecond per year.

The researchers, reporting on Science magazine's online service, Science Express, on March 8, say that by ruling out other potential forces on PH5, such as tidal torques, they were able to demonstrate that the most likely culprit for the acceleration is the YORP effect from sunlight.

The acronym, from the tongue-twisting Yarkovsky-O'Keefe-Radzievskii-Paddack effect, is a phenomenon that occurs when photons from the sun are absorbed by a body and reradiated as heat. In the process, two forces influence the object: one from the impact of the photons, providing a tiny push, and the other as a recoil effect when the object emits the absorbed energy. For small, irregularly shaped objects like PH5, YORP can cause measurable changes in motion.

On average, asteroids rotate every four to 12 hours. But the smallest asteroids (with a diameter of less than 10 kilometers, or about 6 miles) tend to spin either unusually slowly or unusually quickly -- and astronomers have long wondered why.

"It is one of the significant and longstanding questions in asteroid science," said Margot. "YORP is more effective on small objects, so it can nicely explain this."

YORP could also explain why some asteroids come in pairs. Most asteroids are actually loosely bound clumps of rubble with very little internal cohesion, so an object with an increasing spin rate could eventually spin faster than its own strength and gravity can endure -- ultimately flying apart to form two objects. Several dozen asteroids are known to be binaries, with potentially many more undiscovered.

PH5 was discovered in 2000 by the Massachusetts Institute of Technology's near-Earth asteroid search program. When it was observed, it was about five times more distant than the moon.

Before the researchers could attribute the asteroid's accelerating spin to YORP, they had to discount the other possible torques that could be influencing its rotation. Using a shape model produced from high-resolution images gathered by the Arecibo telescope, the team led by Lowry and Fitzsimmons found that tidal torques as the asteroid passed near Earth were not strong enough to account for the acceleration. In fact, tidal forces are just as likely to decelerate the spin.

Beyond the finding's significance to asteroid science, it is also a testament to the unique capabilities of the Arecibo telescope, which is managed for the NSF by the National Astronomy and Ionosphere Center at Cornell.

"Arecibo is absolutely critical for this experiment," said Margot. And while one millisecond may sound trivial, he added, even a change that small adds up. "The length of the day on PH5 can be halved in half a million years," he said. "Anything, even a minute change in our lifetime, can have a dramatic effect in geological timescales."