An Asteroid with Two Moons

Art of Sylvia 87 and two moons
An artist depicts 87 Sylvia and its two known moons. The main-belt asteroid is the first minor planet known to possess two satellites. The exact shapes of the three bodies are unknown, but 87 Sylvia is elongated.
Courtesy European Southern Observatory.
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.

87 Sylvia and two moons
This image of 87 Sylvia, taken on August 9, 2004, with the 8.2-meter Yepun reflector, shows 87 Sylvia and its two satellites. The overexposed central blob is the primary asteroid. The two moons are to its left, with the fainter object being the newly discovered body. The image, obtained with the benefit of adaptive optics, is actually a combination of four 2-second exposures added together. The moons are 0.4 arcsecond from the primary.
Courtesy Franck Marchis, et al. / ESO / Nature.

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).

Orbital motion of 87 Sylvia moons
These images show the orbital motion of the 87 Sylvia's two moons over the course of 9 nights. The very faint dotted lines represent their orbits. The inset image shows the approximate shape of the main body.
Courtesy Franck Marchis, et al. / ESO / Nature.

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.