Not one of us

Alien risk: Deadly infections could be brought back by expeditions as reality mirrors science fiction, says former Nasa expert
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The fear that aliens may not be good for us has a long history, forming the stuff of science fiction down the years - from The Quatermass Experiment in the Sixties, in which one of the British crew of an orbiting rocket is transfigured into what looks like a molten tree on legs, through The Andromeda Strain in the Seventies, when a microscopic organism clots the blood instantaneously, causing immediate death, to the forthcoming movie Independence Day, about an all-out attack by aliens on our world.

When the aliens are friendly, the standard move is to depict authoritarian bodies, desperate to keep things secret. In ET and Close Encounters of the Third Kind, the aliens were friendly, and so were their bacteria. But the signs are that, if we do encounter life, we would do better to adopt a cautious approach, rather than take it immediately to our leader.