main-belt asteroid
Ida
The main-belt asteroid where Galileo spotted Dactyl in 1993 – the first moon ever found orbiting an asteroid.
- Distance from Earth today
- 520.1 million km3.476 AU · light: 28 min 55 s
- Distance from the Sun today
- 425.6 million km2.845 AU
- As of
- 2 November 2026position from NASA/JPL orbital elements
Where it is today
Top view of the plane of Earth’s orbit, compressed distance scale. Open on the big map with the time slider →
Facts
- Diameter
- 32 km
- Dimensions
- 59.8 × 25.4 × 18.6 km
- Mass
- 0.0412 × 10¹⁸ kg
- Density
- 2,600 kg/m³
- Rotation period
- 4.6 h
- Semi-major axis
- 2.862 AU (428.1 million km)
- Perihelion
- 2.730 AU
- Aphelion
- 2.994 AU
- Orbital period
- 4.84 years
- Orbital eccentricity
- 0.046
- Orbital inclination
- 1.1°
- Last perihelion
- 4 October 2025
- Next perihelion
- 8 August 2030
- Minimum orbit distance from Earth’s orbit (MOID)
- 1.746 AU (261.2 million km)
- Albedo
- 0.262
Source of the figures: JPL Small-Body Database (public domain).
Discovery: 1884 – Johann Palisa
Description
Ida was discovered by Johann Palisa in Vienna in 1884 and belongs to the Koronis family, a group of fragments from an ancient collision. In August 1993 the Galileo spacecraft, on its way to Jupiter, flew past Ida and revealed an elongated, heavily cratered body. The images held a surprise: a tiny companion, later named Dactyl, was orbiting Ida, the first confirmed moon of an asteroid. The discovery showed that asteroid satellites exist, and many more have since been found.