Asteroid Moons

When a big asteroid zipped past Earth a few months ago, astronomers discovered a companion tagging along with it like a puppy: a small moon. Radar images showed it as a bright sliver spinning around the darker asteroid.

The discovery isn’t all that surprising, though. About 150 asteroids are known to have moons, including a few that have two moons. And some estimates say that about one of every six large asteroids should have a moon.

The first asteroid moon was discovered 20 years ago today. It was found as the Galileo spacecraft flew close to Ida, a large asteroid that orbits between Mars and Jupiter. Galileo’s pictures revealed a moon that was less than a mile long, compared to a length of about 35 miles for Ida. The moon was named Dactyl, for a group of monsters associated with Ida in Greek mythology.

An asteroid moon may form when its parent asteroid is hit by another asteroid, sending a chip flying off into space. The collision has to be at the right speed and angle, though. If