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The solar system's first triple asteroid system has been found, boosting predictions that such systems result from collisions between asteroids.
About 20 "binary" asteroids - composed of two asteroids orbiting one other - have been found in the main asteroid belt between Mars and Jupiter. Now, a third object has been spotted around one of these pairs, which includes one of the largest asteroids in the belt.
Called 87 Sylvia, the 280-kilometre-wide, potato-shaped asteroid lies about 3.5 times further from the Sun than the Earth does. Astronomers discovered an 18-km-wide moon orbiting it at a distance of about 1360 km in 2001. The newly found moon lies about twice as close to Sylvia, at a distance of 710 km, and measures just 7 km across.
Researchers led by Franck Marchis of the University of California in Berkeley, US, found the little moon using an infrared camera and adaptive optics on one of the four 8-metre telescopes making up the Very Large Telescope in Chile. Adaptive optics is a technique used to correct for Earth’s atmospheric turbulence.
"People have been looking for multiple asteroid systems for a long time because binary asteroid systems in the main belt seem to be common," Marchis says. "I couldn't believe we’d found one."
Romulus and Remus
They dubbed the new moon Remus and the known moon Romulus, after the mythological twins who founded ancient Rome. 87 Sylvia had already been named for Rhea Sylvia, the mother of the twins.
The moons' orbits allowed the team to calculate Sylvia's mass and density, which is about 20% greater than that of water. It is probably made up of a mixture of water ice and rock and appears to be between 25% to 60% empty space.
That suggests Sylvia is a loose rubble pile made up of debris that fell together after an asteroid collision. The two moons circle Sylvia in the same plane and in the same direction, suggesting they are probably also detritus from the collision. They may have settled into their orbits after Sylvia coalesced - a scenario previously predicted by theorists and seen in computer models.
"Most main-belt asteroids with companions have a rubble-pile structure," says Marchis. "Because of the scenarios of their formation, we expect to see more multiple asteroids like this."
The team hopes to image the triple system with the powerful Keck and Gemini telescopes in Hawaii to observe the system's evolution over time. They have already seen the moons' orbits precess, or wobble, due to Sylvia's non-spherical shape.
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