I was watching Deep Impact the other night and I was struck by the fact that the Messiah mission takes eight nukes to the comet, but only has the manpower to plant four of them. Not only that, they only take with them the arming codes for four of them - the remaining nukes appear to be little more than ballast. So why take them at all?
Is this just a plot hole, conveniently leaving four nukes to destroy the comet at the end of the film? I could just about accept that they took four in reserve in case any of the 'live' nukes developed a fault during the journey, (though I don't recall any mention of this in the script) but then they still can't arm them without the codes.