Many planets enjoy the company of a moon or two, but asteroids usually travel in solitude. Now a high-tech telescopic search has revealed that the asteroid Antiope is actually two bodies of similar size circling each other. Although other asteroid pairs have been spotted before, the new formation has astronomers scratching their heads.

The first discovery of a satellite pair didn't give theoreticians such trouble. In 1994, the Galileo spacecraft found little 1.5-kilometer diameter Dactyl orbiting 56-kilometer Ida, and the most obvious explanation was a large collision. Ida belongs to an asteroid "family" of large fragments that must have formed when a collision shattered a much larger ancestral asteroid. Dactyl could be just a small relic of the same event that Ida happened to capture. The discovery in 1999 of 13-kilometer Petit-Prince circling 214-kilometer Eugenia proved more difficult to explain because Eugenia is the largest of its family; any candidate satellites among the debris would have been blasted out of its gravitational grasp before it could capture the debris.

Now, in a meeting abstract newly posted to the Web (see link below), astronomer William Merline of the Southwest Research Institute (SwRI) in Boulder, Colorado, and colleagues report the discovery of two more asteroid pairs. Using adaptive optics that undo the atmosphere's blurring effects, they imaged a small companion of the asteroid Pulcova. More surprisingly, they also split the supposed 120-kilometer Antiope--a member of the Themis asteroid family--into two equal-size bodies separated by 170 kilometers.

"I'm stunned and astonished," says planetary physicist Jay Melosh of the University of Arizona, Tucson. "It's not anything that was expected. "Theoreticians contacted by Science are at a loss to explain the twinning of Antiope. Planetary dynamicist William Bottke of SwRI, Boulder (who is not a co-discoverer), does hazard a guess. Many small collisions may have reduced an ancestral Antiope to a collection of rubble, he speculates. If so, a glancing blow by another asteroid might have spun Antiope like a top, causing it to fly apart into two equal-size rubble piles still orbiting about their center of mass. But a lot of computer modeling will be needed to support such speculation. As Melosh says, "We have a very interesting new puzzle in the solar system."

--RICHARD A. KERR

Related site

Abstract of the report detailing double asteroids