We've got nine more encounters in the next 14 months. None of the researchers seemed to mind going days without sleep. After a six-year journey, the Galileo spacecraft has begun beaming images from Jupiter's largest moon, Ganymede. Largest moon in the solar system, bigger than the planet Mercury, has surface of ice. Bulk of moon is half water ice and half rock.
Volcanoes expel streams of snow.
Surface ice shows signs of tectonic activity. Underground ocean of water apparently creates a baby magnetic field, surprising scientists. Approved by Congress 20 years ago, Galileo was launched aboard the space shuttle Atlantis on Oct. 18, 1989. Photo taken by Galileo spacecraft shows large impact crater on Ganymede, one of Jupiter's moons. Ancient craters mark Jupiter's moon Ganymede.