The need to defend against a large asteroid is slim, but it's not out of the realm of possibility. NASA scientists estimate that an object larger than 400 meters (1,312 feet) in diameter could collide with Earth about once every 160,000 years [source: Graham].
NASA's Asteroid Defense Plan
So a nuclear bomb would be essentially useless at disintegrating an asteroid several miles wide, but scientists at NASA think that a nuclear weapon could be used in a different way to defend the planet.
In 2005, U.S. Congress asked NASA to develop plans for preventing an asteroid-Earth collision. In 2007, the space agency presented its ideas at the Planetary Defense Conference in Washington, D.C. (which sounds like something out of a sci-fi flick). In its report, NASA outlined several options, a few of which involved using nuclear explosives to deflect the asteroid away from Earth. The force from the explosions would (hopefully) provide enough momentum to nudge the asteroid in a different direction, preventing disaster.
In the explosions category, NASA discovered that nuclear explosives are way more effective for asteroid deflection than non-nuclear explosives, due to the sheer amount of energy they produce. NASA tested four nuclear scenarios: a surface explosion, a delayed surface explosion, a subsurface explosion and a standoff explosion (where the bomb doesn't come into contact with the asteroid). The surface and subsurface explosions are the most effective, but there's a good chance of splitting the asteroid. In the end, the space agency determined that a series of standoff nuclear explosions would be the most effective way to deflect an asteroid headed for Earth.
The best option in the non-nuclear category is a kinetic impact (a nice way of saying they would ram objects into the asteroid), but doing so would require detailed knowledge of what the surface of the asteroid is like. Some other non-nuclear options that NASA considered include using a laser or a giant mirror to focus energy on a spot on the asteroid and "boil off" parts of it, or using a spacecraft to tug the asteroid in a different direction.
So, will we ever know in our lifetime if it's possible to deflect an asteroid? Maybe. In December 2009, the director of the Russian Federal Space Agency, Anatoly Perminov, announced that Russia was considering making plans to deflect the 270-meter (885-foot) asteroid Apophis from its possible collision course with Earth. While NASA claims that the chances of Apophis colliding with Earth are only about 1 in 250,000, it does demonstrate that shooting objects at asteroids to knock them out of the way is a definite possibility [source: Discovery News].
For more information about bombs, asteroids and bombs blowing up asteroids, visit the links on the next page.