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News Release 00-082

Astronomers Find Surprising Double Asteroid and a New Asteroid Moon

October 26, 2000

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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.

Images are available at:


Media Contacts
Amber L. Jones, NSF, (703) 292-7740, email:
Don Savage, NASA, (202) 358-1753, email:

Program Contacts
Vernon L. Pankonin, NSF, (703) 292-4902, email:

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