Ida and Dactyl: Galileo's second asteroid — and the first asteroid moon

Two years after Gaspra, Galileo flew past Ida and found something no one had confirmed before: a moon.

Published 2026-10-23

Galileo's encounter with Gaspra in 1991 was historic.^[3] Two years later, it did something even more surprising.

The second asteroid

On August 28, 1993, Galileo flew past 243 Ida, a member of the Koronis family about 31 km across, at about 12.4 km/s. It was the second asteroid visited by a spacecraft.^[5] The first images came back in September 1993, and the rest followed in February 1994, when Galileo was close enough to Earth for faster transmissions.^[5]

A moon!

On February 17, 1994, Galileo team member Ann Harch spotted a tiny companion while examining the delayed image downloads.^[6] The moon, named Dactyl after mythical beings who lived on Mount Ida, is only about 1.4 km across. It was the first confirmed moon of an asteroid.^[5]

Why a moon matters

A moon's orbit reveals the mass of what it orbits. Gaspra's mass remains unknown because Galileo could not measure it, but Dactyl allowed scientists to constrain Ida's density to less than 3.2 g/cm³.^[3][5]

Solving the S-type puzzle

Before Galileo, scientists debated whether S-type asteroids were related to ordinary chondrite meteorites or to stony-irons. Ida's images showed space weathering: older surfaces reddened over time, while freshly exposed areas looked like ordinary chondrites. The flyby pointed to S-types as a source of ordinary chondrites, the most common meteorites on Earth.^[5]

The birth of asteroid geology

The Gaspra and Ida flybys, together with the later NEAR Shoemaker mission, allowed the first real study of asteroid geology.^[5]

Numbered citations link to the sources list on the Gaspra overview page.