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NSF PR 00-82 - October 26, 2000
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Astronomers Find Surprising Double Asteroid and a
New Asteroid Moon
Astronomers announced today that they have found a
large, double asteroid in our solar system. The configuration
is a surprise to astronomers, who once thought asteroids
were lone objects.
An international team led by William Merline of the
Boulder, Colo., office of the Southwest Research Institute
and supported by the National Science Foundation (NSF)
and NASA found the asteroid pair. The team used the
Keck telescope on Mauna Kea, Hawaii, outfitted with
adaptive optics, which allow astronomers to examine
asteroids and other celestial objects with unprecedented
clarity.
Each asteroid in the pair is about 50 miles across.
They are separated by about 100 miles, mutually orbiting
a spot in space. The asteroid pair was once assumed
to be a single body, called Antiope, orbiting the
sun in the outer parts of the asteroid belt between
the orbits of Mars and Jupiter.
The team also found a small moon orbiting the large
asteroid Pulcova, using adaptive optics on the Canada-France-Hawaii
Telescope on Mauna Kea. Pulcova was the third asteroid
observed to have a moon. The first was found in 1993
by the Galileo spacecraft, which observed a one-mile-wide
moonlet around the 19 mile-diameter asteroid Ida.
The Merline team reported the second moonlet a year
ago, circling the 135-mile-diameter asteroid Eugenia.
"Preliminary study of about 200 asteroids has turned
up only two asteroids with moons (Eugenia and Pulcova)
and just one double (Antiope)," Merline said. "It
is possible that a few more moonlets might emerge
from more sophisticated analysis of the data we have
collected." The astronomers expect to find still more
configurations and surprises as the survey continues.
"It's getting to be kind of bewildering," said team
member Christophe Dumas of the Jet Propulsion Laboratory.
"Asteroids were once thought to be single, mountain-like
chunks of material, perhaps smashed into 'flying rubble
piles' by occasional collisions among themselves."
Asteroidal companions provide vital information about
asteroids that has been difficult to obtain. Until
now, the best measurements of asteroid mass and density
came from deflections of spacecraft flying past an
asteroid. Such spacecraft encounters are rare, and
deflections of more distant objects (such as other
asteroids or planets) by an asteroid's gravity are
weak and difficult to measure. But an asteroidal satellite,
or twin, is a body whose trajectory is deflected by
the asteroid's gravity and forced to orbit around
it. The revolution time provides a measure of the
body's mass, hence density. Using these techniques,
Merline's team earlier found that Eugenia, Pulcova,
and Antiope are light bodies, with less density than
rocks, even though their sufaces appear dark like
rock.
Adaptive optics enable ground-based telescopes to observe
asteroids and other small points of light with the
same clarity as the Hubble Space Telescope. Until
recently, such observations were hindered by distortions
caused by the earth's atmosphere, in much the same
way water distorts the view of an underwater object.
With the new technique, optical and electronic elements
within the telescope sense the distortions and adjust
the telescope's output.
The scientists announced the discoveries at the 32nd
annual meeting of the American Astronomical Society's
Division for Planetary Sciences in Pasadena, California.
Editors: Images will be available at 11:00 a.m.
EDT October 26 see: http://www.boulder.swri.edu/merline/press.
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