Astronomers have discovered a unique set of triplets: an asteroid with two moons orbiting it. Described in a report published today in the journal Nature, the rare combination is the first of its kind ever observed.

The asteroid is a 280-kilometer-wide body called 87 Sylvia, which was first sighted in 1866 and lies between Mars and Jupiter. Four years ago, scientists spotted a moon associated with the asteroid. Now Franck Marchis of the University of California at Berkeley and his colleagues have spied a second satellite circling 87 Sylvia using Yepun, one of the telescopes in the European Southern Observatory's Very Large Telescope array. "Since double asteroids seem to be common, people have been looking for multiple asteroids systems for a long time," Marchis says. "I couldn't believe we found one."

The newest moon, dubbed Remus, measures seven kilometers across and travels around 87 Slyvia once every 33 hours in an orbit about 710 kilometers from the asteroid. Researchers named the second 18-kilometer-wide moon, which takes almost 88 hours to circle Sylvia at a distance of 1,360 kilometers, Romulus. Detailed observations of the paths of the two smaller asteroids around Sylvia allowed the team to calculate its mass and density, which is only 20 percent higher than that of water. "It could be up to 60 percent empty space," posits study co-author Daniel Hestroffer of the Paris Observatory. The results suggest that Sylvia is a so-called rubble-pile asteroid, a patchwork of fragments created from a collision that later joined together. The small moons, they add, are most likely debris from the same collision that were later captured by the bigger body's gravitational pull.