The data returned from the Galileo flybys of Gaspra and Ida, and the later NEAR Shoemaker asteroid mission, permitted the first study of asteroid geology.
Ida's relatively large surface exhibited a diverse range of geological features.
The discovery of Ida's moon Dactyl, the first confirmed satellite of an asteroid, provided additional insights into Ida's composition.

Ida is classified as an S-type asteroid based on ground-based spectroscopic measurements.
The composition of S-types was uncertain before the Galileo flybys, but was interpreted to be either of two minerals found in meteorites that had fallen to the Earth: ordinary chondrite (OC) and stony-iron.
Estimates of Ida's density are constrained to less than 3.2 g/cm³ by the long-term stability of Dactyl's orbit.
This all but rules out a stony-iron composition; were Ida made of 5 g/cm³ iron- and nickel-rich material, it would have to contain more than 40% empty space.

The Galileo images also led to the discovery that space weathering was taking place on Ida, a process which causes older regions to become more red in color over time.
The same process affects both Ida and its moon, although Dactyl shows a lesser change.
The weathering of Ida's surface revealed another detail about its composition: the reflection spectra of freshly exposed parts of the surface resembled that of OC meteorites, but the older regions matched the spectra of S-type asteroids.

Both of these discoveries—the space weathering effects and the low density—led to a new understanding about the relationship between S-type asteroids and OC meteorites.
S-types are the most numerous kind of asteroid in the inner part of the asteroid belt.
OC meteorites are, likewise, the most common type of meteorite found on the Earth's surface.
The reflection spectra measured by remote observations of S-type asteroids, however, did not match that of OC meteorites.
The Galileo flyby of Ida found that some S-types, particularly the Koronis family, could be the source of these meteorites.
The data returned from the Galileo flybys of Gaspra and Ida, and the later NEAR Shoemaker asteroid mission, permitted the first study of asteroid geology. Ida's relatively large surface exhibited a diverse range of geological features. The discovery of Ida's moon Dactyl, the first confirmed satellite of an asteroid, provided additional insights into Ida's composition.
Ida is classified as an S-type asteroid based on ground-based spectroscopic measurements. The composition of S-types was uncertain before the Galileo flybys, but was interpreted to be either of two minerals found in meteorites that had fallen to the Earth: ordinary chondrite (OC) and stony-iron. Estimates of Ida's density are constrained to less than 3.2 g/cm³ by the long-term stability of Dactyl's orbit. This all but rules out a stony-iron composition; were Ida made of 5 g/cm³ iron- and nickel-rich material, it would have to contain more than 40% empty space.
The Galileo images also led to the discovery that space weathering was taking place on Ida, a process which causes older regions to become more red in color over time. The same process affects both Ida and its moon, although Dactyl shows a lesser change. The weathering of Ida's surface revealed another detail about its composition: the reflection spectra of freshly exposed parts of the surface resembled that of OC meteorites, but the older regions matched the spectra of S-type asteroids.
Both of these discoveries—the space weathering effects and the low density—led to a new understanding about the relationship between S-type asteroids and OC meteorites. S-types are the most numerous kind of asteroid in the inner part of the asteroid belt. OC meteorites are, likewise, the most common type of meteorite found on the Earth's surface. The reflection spectra measured by remote observations of S-type asteroids, however, did not match that of OC meteorites. The Galileo flyby of Ida found that some S-types, particularly the Koronis family, could be the source of these meteorites.
243 Ida