JUPITER

    NASA's Galileo spacecraft has been orbiting Jupiter since December 1995, providing a stream of discoveries about the planet, its four largest moons and the surrounding environment. The moon Europa shows evidence of a deep ocean of melted water under an icy crust. Closer to Jupiter, the moon Io is continuously resurfacing itself with fresh volcanic eruptions. Io's internal heat comes from tidal flexing due to Jupiter's gravitational pull. Galileo also dropped a probe into Jupiter's atmosphere to make the first in-place studies of the planet's clouds and winds. Galileo was preceded by NASA's Pioneer and Voyager spacecraft, which flew past Jupiter in the 1970s. Future missions that may follow up on Galileo's discoveries about Europa, which make that moon a tempting target for investigating the possibility of extraterrestrial life.