Just as 17th century astronomer Galileo Galilei was first to see moons around another planet, the 20th century spacecraft named in his honor was the first to discover a moon orbiting an asteroid.
Most scientists imagined asteroids as isolated mountains or rock piles tumbling alone through space. But while en route to Jupiter in 1993, NASA's Galileo spacecraft flew by a 19-mile-wide asteroid called Ida and discovered that it has its own little moon. Just one mile wide, the moon was named Dactyl. Ida and Dactyl were the first binary -- or double -- asteroids.
It was six years before another asteroid-moon system was found, but a small avalanche of discoveries followed:
- In 1999, astronomers using Earth-based telescopes found that 135-mile-wide Eugenia has an eight-mile-diameter moon, which they dubbed Petit-Prince.
- In 2000, 90-mile-wide Pulcova was discovered to have its own moon, about nine miles wide.
- In 2001, scientists found Linus orbiting Kalliope, and another moon around asteroid Sylvia.
Dozens of binary asteroids have been confirmed in the main asteroid belt and among Near-Earth Objects, including some in which the moon is much closer in size to the main asteroid. Some Trans-Neptunian Objects (beyond the orbit of Neptune) are also binaries.
It seems likely that most asteroid moonlets are fragments from past collisions. It's also possible that some loose-rubble asteroids passed close enough to a planet at some point for gravity to pull them apart and create natural satellites (the formal name for a moon).
Astronomers used radar to observe some of the closer asteroid-moon pairs. Most of the others were discovered in visible light, using ground-based telescopes with adaptive optics. (These systems use computer-controlled deformable mirrors to compensate for the blurring effects of Earth's atmosphere, creating sharper images.) Furthermore, scientists are able to calculate an asteroid's mass and density by observing the moon orbiting the asteroid.