When NASA's Galileo spacecraft flew by the asteroid 243 Ida in 1993, it stumbled upon the tiny moon Dactyl — proving that asteroids can have their own moons. Since then, astronomers have found dozens of satellites around small bodies, including near-Earth asteroids, main-belt asteroids, and Kuiper Belt objects. But each of these moons were like only children. Now, for the first time, astronomers have identified an asteroid with two moons.
Using the 8.2-meter Yepun reflector of the European Southern Observatory's Very Large Telescope, and adaptive optics to counter atmospheric turbulence, Franck Marchis (University of California, Berkeley) and three colleagues found a second, inner moon orbiting the 280-kilometer-wide (175 mile) main-belt asteroid 87 Sylvia. As reported in the August 11th Nature, the group imaged the newfound companion in 12 of 27 individual observations over a span of two months, which enabled the astronomers to compute its orbit. The moon is about 7 km across and orbits Sylvia every 33 hours at a distance of 710 km. The other moon, which was discovered by Mike Brown (Caltech) and Jean-Luc Margot (now at Cornell University) in 2001, is about 18 km across and orbits every 87.6 hours at a range of 1,360 km.
The International Astronomical Union has approved Marchis's proposed names for the two moons: Romulus (the outer satellite) and Remus. Because 87 Sylvia was named for Rhea Silvia — the mythical mother of the founders of Rome — Marchis proposed naming the two moons after Rome's founders.
Both satellites orbit 87 Sylvia in circular, equatorial, and prograde orbits, so Marchis's group proposes a common origin for the system. In this picture, an impactor slammed into the parent asteroid, breaking it apart into numerous fragments. Most of these chunks reassembled into a rubble pile that is now the primary body. The two small satellites gravitationally accreted from remaining debris.
This scenario is consistent with 87 Sylvia's mean density: 1.2 times that of liquid water. With such a low density, the asteroid is clearly a rubble pile of rock and water ice. "It could be up to 60 percent empty space," says codiscoverer Daniel Hestroffer (Paris Observatory, France).
Daniel D. Durda (Southwest Research Institute), who models the formation of asteroid satellites on computers, agrees with the impact explanation. "These satellites probably formed from the swarms of impact debris in the days and weeks after the impact," he says. Systems with just one satellite have greater orbital stability, but the collision process must occasionally allow for the formation of two or more satellites. Durda says he won't be surprised if ongoing searches turn up additional asteroids with multiple moons.