Asteroids Spin Faster Due to Solar Power, Studies Show

Brian Handwerk
for National Geographic News
March 7, 2007
Sunlight can speed up or slow down the spin of small asteroids, according to a trio of related papers appearing this week. The discovery offers the first direct evidence of a predicted asteroid behavior.

"The solar system is a very dynamic place, and our star affects all worlds—even small ones," said Patrick Taylor, a graduate student at Cornell University in New York State.

Taylor co-authored two studies that will appear tomorrow in the early online journal Science Express.

The third study, led by Mikko Kaasalainen of the University of Helsinki in Finland, appears today in the early online edition of the journal Nature.

All three papers looked at a phenomenon called the YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack) effect.

When the sun warms an asteroid's surface, the rocky object reradiates heat into the void (related: asteroid facts).

Heat emissions produce a slight recoil that alters the asteroid's spin—the same principle by which light shined on a pinwheel can sometimes cause it to rotate even without a breeze.

The YORP effect alters the asteroid's rate of rotation and the direction it spins on its axis based on the object's size and shape. More asymmetrical asteroids are more susceptible to YORP.

Solving Asteroid Puzzles?

YORP's tiny torque is almost imperceptible in real time. The asteroid in which Taylor observed the effect was seen to be rotating faster by only about a millisecond a year.

Another asteroid studied by Kaasalainen and colleagues also increased its spin, gaining an extra rotation around its axis over the past 40 years.

Yet the effect's potential impact can be significant.

"YORP can have a large effect over million-year time scales," said Stephen Lowry, co-author of both Science Express studies.

YORP may be at least partially responsible for puzzling phenomena like binary asteroids—systems in which two asteroids orbit one another like a planet and its moon.

"YORP could, in theory, cause an asteroid to [increase its spin] so fast that it could break apart by centrifugal forces and perhaps lead to the birth of new binary-asteroid systems," said Lowry, of Queen's University Belfast in the United Kingdom.

YORP also likely played a significant role in putting small asteroids into their current orbits. YORP is a variation on an already detected phenomenon called the Yarkovsky effect, in which sunlight alters an asteroid's spin along its axis, changing its orbital path.

The notion of the Yarkovsky effect being able to nudge small asteroids into new orbits has led some scientists to ponder its usefulness for deflecting space rocks that might be on a collision course with Earth.

"Painting" a threatening asteroid with white dust could, in theory, boost the Yarkovsky effect and alter the asteroid's path enough to avoid a hit.

(Related news: "'Gravity Tractor,' Super Telescopes Enlisted to Battle Killer Asteroids" [February 17, 2007].)

Cornell University's Taylor notes that the YORP effect can strengthen or weaken the Yarkovsky effect. But he remains skeptical that modifying an asteroid's YORP would be the most practical method for speeding up an asteroid's shift into a new orbit.

Instead Taylor and colleagues argue that just finding proof of YORP is an important step.

The discovery will further our understanding of the physical properties of asteroids and their movements, Lowry said, providing clues to the solar system's earliest days.

"Asteroids are the only remaining relics from the formation era of our solar system," Lowry said.

"You could think of them as solar system fossils."

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