Galileo
The first close look at an asteroid, and the discovery that asteroids can have moons.
Key facts
- Agency
- NASA (United States)
- Type of mission
- Flybys and Jupiter orbiter
- Target
- Asteroids Gaspra and Ida; Jupiter
- Launched
- 18 October 1989
- Status
- Ended 2003
Galileo was built to orbit Jupiter, but its long, looping route through the inner Solar System took it across the asteroid belt twice. Mission planners used those crossings to make the first ever close flybys of asteroids.
Gaspra
On 29 October 1991 Galileo passed the asteroid 951 Gaspra, an irregular, potato-shaped body about 18 kilometres long. The pictures showed a surface covered with small craters and grooves. Gaspra is an S-type asteroid: its reflected light shows the minerals olivine and pyroxene, as in stony meteorites.
Ida and Dactyl
On 28 August 1993 Galileo flew past 243 Ida, about 60 kilometres long. When the images were sent home months later, they revealed a tiny moon, about 1.6 kilometres across, circling Ida. It was named Dactyl. It was the first moon of an asteroid ever confirmed. Many more have been found since, and they are valuable, because a moon's orbit reveals the mass of the asteroid it circles, and so its density.
Comet Shoemaker–Levy 9
In July 1994, on its way to Jupiter, Galileo had a direct view of fragments of the comet Shoemaker–Levy 9 slamming into the far side of the planet, which could not be seen from Earth. It was the first collision of two Solar System bodies ever observed.
Why it matters to meteorites
Gaspra and Ida were the first chance to see the kinds of bodies that ordinary chondrites come from. Galileo arrived at Jupiter in December 1995 and was steered into Jupiter's atmosphere in 2003.
Key dates
| Date | What happened |
|---|---|
| 18 October 1989 | Galileo launched from the Space Shuttle Atlantis. |
| 29 October 1991 | The first close flyby of an asteroid: 951 Gaspra. |
| 28 August 1993 | Flyby of 243 Ida, which turned out to have a moon, Dactyl. |
| July 1994 | Watched fragments of comet Shoemaker–Levy 9 strike Jupiter. |
| 7 December 1995 | Went into orbit around Jupiter. |
| 21 September 2003 | Ended its mission by plunging into Jupiter. |
See where this mission fits among all the others in the mission timeline.
Did you know?
Galileo's main antenna failed to open, so all its pictures had to be sent home through a small backup antenna, very slowly.
Dactyl, named after mythical beings who lived on Mount Ida, was the first asteroid moon ever found.
Related pages
Comets connected with this mission: Comet Shoemaker–Levy 9. See all comets.