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Sunlight increases space-rock spin

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Two separate asteroids have turned the sun’s rays into an engine of sorts, accelerating their spin rates by reflecting absorbed sunlight into space, according to a trio of new studies.

The asteroids, known as 2000 PH5 and 1862 Apollo, are the first to yield direct measurements of sunlight’s spin-altering effect on relatively small space rocks, researchers said.

“Asteroids are among the least altered remnants from the formation of our solar system,” astronomer Stephen Lowry, who led the 2000 PH5 study from of Queens University in Belfast, Ireland, told “So by studying them we are looking back in time to the conditions present when our solar system formed some 4.5 billion years ago.”

When a large Sun-facing portion of an asymmetrical space rock rotates away from the star and into night, it can radiate more energy than that of smaller areas and tweak its spin rate. The phenomenon, long inferred but now spotted directly, is named the YORP effect (Yarkovsky-O’Keefe-Radzievskii-Paddack) after the researchers who first described it.

“YORP also plays an important role in changing the orbits of asteroids between Mars and Jupiter, including their delivery to planet-crossing orbits,” Lowry said. A "planet crossing orbit" is the sort that makes astronomers on Earth worry, of course.

The effect appears to be small for asteroid 2000 PH5, speeding its spin rate up just fractions of a second per year. But for the larger 1862 Apollo, the cumulative YORP effect over the last 40 years now allows the space rock to complete one extra spin per trip around the Sun, the new studies found.

Space rock shimmy
Asteroid 2000 PH5 is a near-Earth space rock with an average radius of just 187 feet (57 meters) and a spin rate of once every 12.17 minutes, making for a very short “day” on its surface. For comparison, the Earth completes one turn on its axis every 24 hours.

Visual studies of the space rock between 2001 and 2005 found that its 12-minute day shortened by one millisecond per year, indicating that its “daily” spin was growing faster over time, Lowry said. The research is detailed in the March 9 issue of Science alongside a separate study of the same asteroid, led by researcher Patrick Taylor at Cornell University in Ithaca, New York, who used radar observations that support the 2000 PH5’s YORP effect.

“We expect the changes to be this small, as in theory it takes millions of years to see a large effect,” Lowry said, adding that his team used an armada of telescopes to track the asteroids’ brightness and rotation.

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But a swifter example of YORP in action was reported by a research team led by Mikko Kaasalainen, of Finland’s Rolf Nevanlinna Institute, which found that the 4,593-foot-wide (1,400-meter) 1862 Apollo received enough of a push from sunlight over the last 40 years that it now performs one additional spin — or an extra asteroid “day” — per orbit around the sun. The research is detailed online journal Nature.

“The change is fairly large and clearly visible in photometric light curves,” the researchers wrote.

Kaasalainen’s team used archived and new photometric light curve measurements of 1862 Apollo between 1980 and 2005 to monitor the YORP effect’s influence on the space rock. A 2005 NASA report found that 1862 Apollo completed a single spin about ones every three hours.

Spinning out of control
Kaasalainen and his colleagues believe 1862 Apollo may have reached its peak spin rate boost from the YORP effect, which may have led the object to shed some mass and form a tiny satellite to break free. Observations have found 1862 Apollo’s small companion is less than one-tenth the size its counterpart, much too small to account for the spin rate changes attributed to the YORP effect, researchers said.

Lowry said the YORP effect could ultimately spin 2005 PH5 so fast that one “day” on the asteroid could run as short as 20 seconds. The YORP effect could also lead to the asteroid’s destruction, forcing it to spin so fast it sheds mass or loses cohesion.

"Who knows, perhaps we could witness one of these objects spontaneously breaking apart,” Lowry said. “That would be quite a show!”