The asteroidal moon is about eight miles (13 kilometers) in diameter, Merline said. But he told SPACE.com that this estimate is uncertain because in examining such relatively small bodies so far away an astronomer can't be sure how much light the rocks reflect and whether they were viewed broadside or possibly showing a narrower profile, two key factors in determining size.

The satellite is thought to orbit the Hermione every 3 or 4 days at about 620 to 930 miles out (1,000 to 1,500 kilometers).

Asteroid pairs were unknown until 1993, when the Galileo spacecraft spotted the moonlet Dactyl orbiting asteroid Ida. Merline and his colleagues reported the second known moonlet in 1999, circling asteroid Eugenia.

Merline said the growing number of known asteroid pairs over the past three years and the diversity of configurations -- object of roughly equal size circling each other, as well as tiny "moonlets orbiting large rocks -- mean that more than one formation mechanism will be needed to explain them all. In general, however, most are probably the result of collisions.

He added that while the Hermiones are not a threat to Earth, learning about binary asteroids in general would help scientists decide what to do if a pair of space rocks were ever found to be headed our way.

The new pair is the ninth known to populate the main Asteroid Belt, Merline said. Some 14 pairs inhabit space closer to our planet and are classified as Near Earth Objects (NEOs) -- things that merit investigation to make sure they're not on a collision course with Earth. None pose an imminent threat, based on present knowledge.

Seven asteroid pairs have been found beyond Neptune, in a vast sea of frozen Trans-Neptunian Objects (TNOs); these are sometimes called Kuiper Belt Objects. Finally, one dancing duo is part of a special class of "Trojan" asteroids locked into orbits ahead of or behind Jupiter.

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