According to , heat given off by the plutonium-powered radioisotope thermoelectric generators which Galileo could melt ice on the surface of Europa and allow Earthern bacteria to breed when the craft's hull ruptures on impact. This would contaminate the moon and make it very difficult to separate any later-discovered extra-terrestrial bacteria from the introduced sample.
Galileo has provided us with some of and its , and the mission has developed some emotional significance for many NASA technicians. So much so, that they will be holding a wake for the probe after its impact on Saturday.
A live video webcast will be made available on the Galileo site , so you can catch all the gory details. I'm assuming this feed won't be real-time. There's also a (RealPlayer required) summarising the various milestones of the galileo mission, which is actually pretty good (if a little imperialistic).
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