"For the next class of asteroids less than 6 miles wide, scientists have proposed firing a series of nuclear explosives, each equivalent to 30 million tons of TNT, within a few hundred feet of the asteroid. The weapons would then release high-speed gasses that would give the asteroid a 'kick' away from its original path.

Asteroids bigger than that are virtually impossible to stop and are labeled 'great extinctors' by scientists because of the likelihood that they would trigger devastating climate changes, such as a major freeze."
—"No protection from killer asteroids," The San Francisco Examiner