Now, with all the drama inherent in a comet on a near approach to the earth, and given that in this day and age it's actually possible to get an education, learn some bloody facts, why do scriptwriters do crap like this.
The comet has changed course and is heading for Earth at 3000 miles per second.
I paraphrase ever so slightly.
Anonymous Military Drone (watching screen): The Chinese have launched their missiles. (see missile traces on screen)
AMD 2: 'We're launching our Type 2's" (JFC, at least go to the trouble of googling some real missile names) (More missile tracks on screen)
AMD 1: There goes the French and the Russians!
Cut to outside and view of bright light in sky: Missile trails heading towards it.
AMD 1: Impact in two minutes.
Cut to outside, whole lot of bangs and flahes.
AMD 2: No effect.
Cut to shot of bright light sweeping round the moon and heading to Earth

Whisky Tango Foxtrot?
Listen morons. Giant spacecraft or comets on a collision course for Earth, are, by their very nature, dramatic events. You don't damn well need to ignore the laws of physics (and if I need to explain why the above scenario is 97 or so different shades of intelligence insulting crap, walk away quietly now

If you can't then you should be offering coke with that, not doing something most of us here would give at least somebody's eye teeth for. And do it better.
And while i'm ranting, surely it is possible to write a story of this type (Taken, I'm looking at you here too!) without some creepy wise-ass little girl in it?