Galileo spacecraft discovers first asteroid moon during 1993 Ida flyby

On August 28, 1993, NASA’s Galileo spacecraft performed a flyby of the main-belt asteroid 243 Ida while en route to Jupiter. During the encounter, the spacecraft passed within approximately 1,500 miles (2,400 kilometers) of the asteroid's surface at speeds of nearly 28,000 mph.

Although an antenna failure limited data transmission, the imaging team was able to identify a small object alongside the asteroid. This led to the discovery of Dactyl, the first confirmed natural satellite of an asteroid. The discovery proved that asteroid moons, previously thought to be uncommon, might be more frequent in the solar system than astronomers had anticipated.

243 Ida is an S-type asteroid composed primarily of silicate rock. It is believed to be a Koronis asteroid, consisting of debris from an ancient collision between two larger bodies.