Dactyl
Dactyl is the tiny moon of asteroid (243) Ida — about 1.4 km across — and the first natural satellite of an asteroid ever discovered. It was found in NASA Galileo images of Ida taken during the 1993 flyby, and has only ever been seen by that one spacecraft, from one side, at low resolution. Its orbit around Ida is not precisely known — Galileo saw it just once, nearly edge-on — so its ~90 km separation and ~20-hour period are approximate.
Key facts
Science
Physical
- Mean radius
- 0.7 km
Orbit around (243) Ida
- Distance from (243) Ida
- 90 km
- From JPL orbital elements
- Orbital period
- 20.00 hours
- From JPL orbital elements
- Eccentricity
- 0
- From JPL orbital elements
- Inclination to (243) Ida's Laplace plane
- 8.00 °
- From JPL orbital elements · The Laplace plane is the one a moon's orbit precesses about: close in it is the planet's equatorial plane, the plane of the rings, and further out it tilts toward the planet's own orbit.
Composition
- Bulk composition
- An S-type body of silicate rock, spectrally very similar to Ida — likely a fragment from the same Koronis-family collision, not a captured object. Its surface shows slightly less space-weathering reddening than Ida's. No global surface map exists: Galileo imaged only the sunlit side it saw during the single flyby.
Radiation & temperature
- Geometric albedo
- 0.20
- Geometric (V-band) albedo — similar to Ida (Galileo).
Discovery & history
- Discovered
- February 17, 1994
- By
- Ann Harch (Galileo imaging team)
- Method
- Spacecraft Imaging (Galileo)
Galileo photographed asteroid (243) Ida on 28 August 1993, but the data were radioed to Earth slowly over the following months. On 17 February 1994, imaging-team member Ann Harch noticed a tiny second body in the frames — the first moon ever found orbiting an asteroid. Galileo had caught it in 47 images over about 5.5 hours. It was named Dactyl, after the Dactyls of Greek myth who lived on Mount Ida. Because the spacecraft passed almost through the plane of the moon's orbit, its exact path around Ida could not be determined.
Images
Images from open archives · credit shown per image