The whole missiles-fired-at-comet part is wrong: the missiles are shown to impact their target mere seconds (or at most minutes, if we give continuity some slack) after launch. Later, we see the comet fly past the Moon towards Earth. It would take even the fastest rockets at least hours, and more realistically 1-2 days to get there, and standard ballistic missiles can't do it at all. Additionally, the comet is stated to travel at 3,000 km/s, which would bring it to earth in less than two minutes instead of the stated half hour. It would also be very, very hard to hit by missile at all at that speed.