Old spacecraft, those soldiering instruments of space exploration, don't just fade away sometimes. Take a few recent cases. At the conclusion of the NASA Lunar Prospector's mission on July 31, 1999, the craft was programmed for a purposeful death plunge into the surface of the Moon to chance if this controlled crash could stir up any evidence of water there. And when a gyroscope on NASA's Compton Gamma Ray Observatory failed, the space agency on June 4, 2000 cautiously guided the instrument to a controlled reentry and watery landing in the Pacific Ocean away from populated areas, rather than risk it becoming an out-of-control satellite.
With the NASA Galileo spacecraft's mission through the Jovian system winding down, some scientists are concerned about a different kind of planetary protection. Instead of protecting people, scientists in this case want to ensure that Galileo—which entered orbit around Jupiter in December 1995, and whose mission already has been extended several times—will pose minimal possibility of contaminating any planetary bodies in the Jovian system with living organisms from the Earth. The scientists are mindful of obligations to avoid contamination that are outlined in the United Nations' 1967 Outer Space Treaty, and of evidence suggesting that Jupiter's moon, Europa, may harbor a water ocean beneath its icy surface.