Armageddon: How did the scientists knew that upon nuclear explosion in the asteroid it will explode into two halves?

I saw this movie Armageddon(1998). What I could not understand was how the NASA scientists knew that after the nuclear explosion in the asteroid, it will split into two halves which will take a trajectory circumventing the earth. In reality a blast can create any amount of varying size particles. Some of which could have landed upon earth. In my opinion hence it was best to try to deflect the asteroid when it was far away. Even a very-very small deviation would have prevented it from hitting the earth.