Moon over Eugenia

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Say this for the structure of the universe: it does tend to repeat itself. Stars orbit the pivot point at the center of galaxies, planets in turn orbit stars, and moons in turn orbit planets. Last week astronomers writing in the journal Nature announced that this cosmic reductionism goes even further. For the first time, ground-based telescopes spotted a tiny moonlet orbiting a mere asteroid in Earth's own solar system.

In most respects the asteroid that's causing the celestial stir is nothing remarkable. Known to astronomers as Eugenia, it measures about 133 miles across and is one of thousands of bits of cosmic flotsam in the great rubble stream between Mars and Jupiter. When an international team of astronomers working at the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea in Hawaii turned their attention toward Eugenia one evening last fall, however, they spotted something curious. Off on the upper-left corner of the fuzzy-looking image was another smear of light they couldn't identify. "These blobs are often artifacts of the optics," says astronomer William Merline, head of the team, "but this blob hung around. Once we saw it was moving in a pattern consistent with an orbit, we knew it was a satellite."

The Hawaii discovery did not mark the first time a moonlet had been found around an asteroid. In 1993 the Galileo spacecraft sped past the 20-mile-wide asteroid Ida and spotted a scrap of moon just under a mile wide circling it. But the only way Galileo could detect the tiny target was to fly there across many millions of miles of space and do its exploring up close. Now, thanks to new optics in the CFHT, it's possible to search for moonlets from the comfortable perch of a faraway Earth.

Light streaming in from space tends to get distorted by the planet's atmosphere, causing a star's familiar twinkle. The CFHT, however, is equipped with optical hardware that lets it calibrate itself on the light from a known star--whose degree of atmospheric distortion will generally be predictable--and then use that information to correct the distortion of other, unknown bodies. A little fiddling with the incoming image and even the blurriest picture snaps right into focus.

Already the discovery of the moonlet is paying scientific dividends. By analyzing the orbit of the satellite, astronomers are drawing surprising inferences about the composition of Eugenia itself. Most asteroids are thought to be about three times as dense as water, but Eugenia is barely 20% denser, suggesting it either is made of loosely packed rubble or is rich in ordinary ice. Further analysis could help settle the question, and more discoveries of more moonlets could shed similar light on Eugenia's asteroid-belt sisters.