|Dactyl and Ida|
Many planets have moons. But guess what. So do some asteroids.
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.
In 1993 while on its way to Jupiter, NASA’s Galileo spacecraft flew right by an asteroid called Ida. It’s 19 miles wide and has a small one mile wide moon orbiting it called Dactyl. These were the first binary asteroids discovered.
Since then more asteroids with a satellite (formal name for a moon) have been discovered:
• 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.
Many more binary asteroids have been confirmed as Near-Earth Objects, in the asteroid belt between Mars and Jupiter, and in the trans-Neptunian area, or beyond the farthest planet (sorry Pluto) from our sun.
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.) Scientists are able to calculate an asteroid's mass and density by observing the moon orbiting the asteroid. Reference