How likely is it that Earth will be hit by an asteroid? Scientists have offered differing odds. Astronomer Steven Ostro, who you see in this episode of Frontiers, says there is a 1 in 1,000 chance that an asteroid 1 km in diameter or larger would collide with Earth in the next century. In the May 1998 issue of Scientific American, Philip Yam writes, "The odds that an 'extinctor,' an object two to five kilometers wide (about twice that of Asteroid 1997 XF11) will strike the planet this century range from about 1 in 1,000 to 1 in 10,000."

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.

And they note the horror of the worst-case scenario, a comet or an asteroid the size of Mount Everest exploding into Earth with such power that the planet is consumed with fire and those left alive starve in a dusty, smoke-filled darkness. Livermore scientists call that asteroid the "Great Extinctor" — something like the one believed to have eliminated the dinosaurs.