Ida
Key facts
- What it is
- Main-belt asteroid with a moon
- Size
- About 60 km long
- Discovered
- 1884, by Johann Palisa
- Spacecraft
- Galileo fly-by, 1993
- Orbit group
- Main-belt Asteroid
- Average distance from the Sun
- 2.86 au (about 430 million km)
- Time to go round the Sun
- 4.8 years
- Closest and farthest from the Sun
- 2.73 to 3 au
- Tilt of its orbit
- 1.1°
- Spins once every
- 4.6 hours
An au (astronomical unit) is Earth's average distance from the Sun, about 150 million km. Orbit figures are from open tracking data, figures last updated 1 Nov 2026.
Ida is famous for its moon. When NASA's Galileo spacecraft flew past it on 28 August 1993, the pictures were stored on board and sent back slowly over the following months. In early 1994 scientists noticed a tiny companion in the images: a moon about 1.4 kilometres across, later named Dactyl. It was the first moon ever found orbiting an asteroid.
Before then nobody knew whether asteroids could have moons at all. Since Dactyl, hundreds of asteroid moons and double asteroids have been discovered, and they turn out to be common, especially among small asteroids.
Ida itself is an elongated S-type asteroid about 60 kilometres long, heavily cratered and old. It belongs to the Koronis family, a group of asteroids that share similar orbits because they are fragments of one larger body that broke apart long ago.
Dactyl's orbit was very valuable to scientists. The speed at which a moon circles its parent depends on the parent's mass, so Dactyl allowed Ida's mass to be estimated, and therefore its density. The result, somewhere around 2.6 grams per cubic centimetre, is a little lower than that of solid stony meteorites, which suggests that Ida has some empty space inside, probably cracks and gaps left by impacts.
Ida was discovered in 1884 by the Austrian astronomer Johann Palisa and named after a nymph in Greek mythology; Dactyl's name comes from the beings said to have lived on Mount Ida. Galileo's fly-by turned Ida from a dot of light into a world with a companion.