My problem was answered a funny way *** SPOILER***

If you remember, when they were approaching the target site on the moon, they started to make modification on the bomb to convert it.

So I was asking myself, while watching the movie, how will they do since obviously it's going to take some time to make such difficult and precise work? I saw the graphic earlier that they were not going to get in orbit (So not drop the bomb on second orbit), and they were already almost there, So no time to modify the bomb and place it in time....

Well, they answered my question by having the shuttle hovering inexplicably in "mid space" (how do you say "mid-air" when there isn't air?), invalidating a 400 years old theory from Newton, over the target site...

Seriously, the writers must have benn shocked when they learned, while making their scientific research on their cereal box, that the earth is actually sperical, and not flat... A discovery so shocking to them they had to include it in their film.