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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 7 Mar 2007 10:40:39 -0800 (PST)
Message-ID: <200703071840.l27Ieei09727_at_zagami.jpl.nasa.gov>


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

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

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."

Received on Wed 07 Mar 2007 01:40:39 PM PST

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