Dactyl (asteroid)

Dactyl, discovered in 1993, is a tiny asteroid (diameter 1.4 km) that orbits asteroid 243 Ida. The origins of Dactyl are unclear. The two main theories are that it and Ida formed at the same time, and that it was knocked loose by a later impact. Both theories have problems and cannot explain the situation satisfactorily.

Dactyl, found by the Galileo probe, was the first asteroid moon discovered.

Ida (left) and Dactyl (right), as photographed by Galileo. Click here to see a larger version