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EXTRATERRESTRIAL life has so far been the stuff of science fiction. Yet NASA, the American space agency, is taking the idea so seriously that it is pondering how to dispose of its Galileo spacecraft, at present orbiting Jupiter, without harming possible alien life-forms on one of Jupiter's moons, Europa. NASA is worried that Galileo might blunder into Europa, contaminating it with earthly organisms. The best way to stop this happening, the agency believes, is to crash Galileo deliberately into Jupiter.