[meteorite-list] Asteroids Spin at YORP Speed, Thanks To The Effects of Sunlight

[Ron Baalke]

http://www.news.cornell.edu/stories/March07/margot.yorp.html

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

March 7, 2007

By Lauren Gold (LG34 at cornell.edu)
Cornell University

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, is an effect 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, said Margot, 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."

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