Dactyl (asteroid)

243 Ida (left) and Dactyl (right), as photographed by Galileo.

243 (1) Dactyl, is a tiny asteroid moon (diameter 1.4 km) that orbits asteroid 243 Ida with a period of 1.54 days at an average distance of 108 km, with an inclination of 9° to Ida's equator. The orbit is not very accurately known because the Galileo probe coincidentally passed very nearly in its plane whilst taking the images. It was imaged by the Galileo probe on August 28, 1993; Galileo mission member Ann Harch, while examining the delayed image downloads, discovered it on February 17, 1994. It was provisionally designated as S/1993 (243) 1.

The origins of Dactyl are unclear. The two main hypotheses are that it and Ida formed at the same time, and that it was knocked loose by a later impact. Both hypotheses have problems and cannot explain the situation satisfactorily.

Dactyl, found by the Galileo probe, was the first asteroid moon discovered. The discovery settled the long debate over the existence of asteroid moons.

See also: List of geological features on 243 Ida and Dactyl.