Interplanetary environmentalism

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.

An icy and apparently barren Jovian moon might not, at first sight, seem the best candidate for an alien life-support machine. But recent observations of Europa by Galileo itself tell a different story. It looks as though the moon is encased in an icy shell between 10km and 170km thick, with an ocean of water flowing deep beneath it. There is a good chance that this freezing shell cracks occasionally, forcing liquid water up to the surface. Europa may also once have frothed with volcanoes, creating hot springs rich in minerals. These three things-water, heat, minerals-are the perfect ingredients for life, at least as it is known on earth. And since extraterrestrial life has never been found, earth is the only reference-point that researchers have for deciding whether or not another celestial body might also harbour life.

NASA believes that the potential for life on Europa is great enough that Galileo should not be allowed to spoil the goods. The craft arrived at Jupiter in December 1995 for a mission originally intended to last two years. The pickings were so rich, however, that the agency decided to extend the mission for another two years, giving it the chance to take a closer look at Europa and another of Jupiter's moons, Io. But although Galileo has withstood Jupiter's harsh radiation remarkably well, and none of its essential systems has failed, it cannot last forever. So if something is to be done with it, that something needs to be done soon, otherwise control will be lost and the spacecraft could eventually collide with Europa, infecting that moon with any hardy terrestrial hitchhikers that are still aboard.

An independent team of experts (COMPLEX , the Committee on Planetary and Lunar Exploration) was therefore asked to assess the options. Since Jupiter's boiling atmosphere is the least likely place in the Jovian system to support life, COMPLEX recommended that this should become Galileo's graveyard. That decision is a sign of how serious the debate has become on the possibility of extraterrestrial life and how to handle it. The idea of protecting other planets from earthly organisms is not new-the Viking Mars landers of the 1970s were assembled in clean rooms and baked in an oven for two hours to sterilise them-but planetary-protection considerations have never before impinged so heavily on the way spacecraft missions themselves are conducted.

In the case of exploratory missions, such as Galileo, the point-aside from ethical considerations about the rights of alien beings and the preservation of pristine environments-is to protect other planets from terrestrial organisms that might jeopardise the scientific validity of any experiments conducted there in the future. If Europa, for instance, were contaminated with earthly life, scientists would never know for sure whether organisms found on it were indigenous or not.

In the case of sample-return missions, for example from Mars, there is a second consideration-protecting the earth's biosphere from whatever living organisms might be brought back. The risk of damage to terrestrial ecosystems by putative Martian or Europan microbes is slim, but it is not zero. NASA 's approach is sensibly cautious: don't let it loose until you know what it is. The agency has even separated the functions of astrobiologist and planetary-protection officer so there is no apparent conflict of interest.

And there is another question to be considered. If extraterrestrial life were found, for example on Mars, would this stop people from ever setting foot there? Michael Meyer, the head of NASA 's astrobiology programme, believes that it should at least delay a manned mission until Martian life was fully understood. But if Martian life were beneath, rather than on, the surface (not unlikely, given how harsh surface conditions are there) the search for it might actually require human presence. In that case, the astronauts doing the searching would have to be hermetically isolated from their environment so as to prevent biological contamination. NASA will hold a workshop next year to try to find ways of doing just that.

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