This one wasn't as "cheesey" as some others, but it never was a Blockbuster for some reason. At the end, all nations that had nuclear tipped missiles launched one on a tightly controlled sequence to head for the incoming asteroid and explode simultaneously in a great big "we are humanity, hugs all around, feel good" explosion........Never mind the fact that ICBMs aren't made to leave earth orbit.........
If I remember correctly, the U.S. and U.S.S.R. saved the world by launching their missiles from super-secret orbiting space weapons platforms. I forgot the name of the U.S. system, but the Soviets called theirs "Peter the Great," which I thought was uber cool at the time (when I was a 10-year-old kid).