The Galileo spacecraft was en route to Jupiter in 1993 when it encountered this pair of asteroids, 243 Ida is 60 km long x 25 km wide and had been seen with Earth-based telescopes. When it was photographed by Galileo, it was unexpectedly discovered to have a moon. Its tiny moon Dactyl is only about 1.6 kilometers across and was the first moon of an asteroid ever discovered.

Image Credit: NASA