Galileo's 1993 asteroid flyby uncovered the first known moon orbiting an asteroid
On Aug. 28, 1993, NASA's Galileo spacecraft flew past asteroid 243 Ida while en route to Jupiter, coming within 1,500 miles of the asteroid. The encounter revealed a tiny moon, later named Dactyl, marking the first confirmed satellite of an asteroid. This discovery proved that asteroids can have moons, a theory previously unconfirmed.
The discovery of Dactyl came months after the flyby itself, when imaging team member Ann Harch spotted the tiny moon while reviewing Galileo's data in February 1994. The moon's name derives from mythological beings said to have lived on Mount Ida, the same sacred mountain that lent the asteroid its name. Ida, an S-type asteroid composed largely of silicate rock, is believed to be a fragment from an ancient collision between two much larger bodies, placing it among the Koronis family of asteroids.
Galileo's encounter with Ida was a secondary objective on its long journey to Jupiter, following an earlier flyby of asteroid Gaspra in 1991. The spacecraft continued on after the brief encounter, reaching Jupiter in December 1995 and orbiting the planet for nearly eight years before being deliberately crashed into it in 2003. The Ida flyby remains significant because it confirmed what had previously been only a theory about asteroid satellites.
This discovery could reshape public understanding of the solar system's complexity, showing that even small rocky bodies can host companions. For planetary scientists, the confirmation may encourage further study of binary asteroid systems, which could inform future asteroid deflection strategies and deepen knowledge of how planets and moons form. The finding may also inspire broader public interest in asteroid exploration, as missions like Lucy and Hayabusa2 continue to reveal unexpected structures among these ancient objects.