A Small Rock With Pull: Asteroid Has 2 Moons

One of the thousands of asteroids orbiting the sun turns out to be a mini-system all its own.

Asteroid 87 Sylvia -- a potato-shaped rock about 175 miles long in the asteroid belt between Mars and Jupiter -- is the first found with two moons in its orbit, according to research published in the current issue of the journal Nature.

The asteroid’s larger moon, unofficially named Romulus, measures 11.3 miles across. It orbits 87 Sylvia at a distance of 860 miles, circling the asteroid every 88 hours.

The smaller moon, dubbed Remus, has a diameter of 4.4 miles. It orbits 87 Sylvia at a distance of 450 miles, completing one circle every 33 hours.

The moons travel in the same plane with nearly circular orbits.

Astronomers have long surmised the existence of such “mini planetary systems” because when asteroids collide, they often break into rubble, which can be drawn by gravity to large objects.

Scientists have identified 60 asteroids with one moon.

Researchers identified Romulus four years ago using images from the Hubble Space Telescope and the Keck II telescope atop Mauna Kea in Hawaii.

Astronomers detected Remus through observations that began last fall using the 8-meter Yepun telescope in northern Chile.

Franck Marchis, a UC Berkeley research astronomer who directed the study with colleagues at the Observatoire de Paris, said his team expected to find more multiple systems with two or more moons as spectroscopy became more sophisticated.

“Since there’s nothing stopping more than one fragment from coalescing around the object with the greatest mass, we expect our solar system has many asteroids with two or more moons,” Marchis said.