If you've ever played pool, then you know about kinetic energy, which is the energy possessed by any moving object. The kinetic energy of a struck cue ball is what gets transferred to other balls on the table. Astronomers believe the same principle could deflect an earthbound asteroid. In this case, the cue ball is an unmanned spacecraft similar to the probe used in NASA's Deep Impact mission (not to be confused with the movie). The mass of the Deep Impact vessel was only 816 pounds (370 kilograms), but it was moving really, really fast -- 5 miles (10 kilometers) per second [source: NASA].
Kinetic energy depends on both the mass and speed of an object, so a small object moving fast still has a lot of energy. When mission engineers slammed the Deep Impact probe into the surface of the Tempel 1 comet in 2005, it was slated to deliver 19 gigajoules of kinetic energy. That's the equivalent of 4.8 tons of TNT, enough to shift the comet ever so slightly in its orbit [source: NASA].
Astronomers weren't looking to alter Tempel 1's trajectory, but they know now it could be done, should an asteroid or comet set its sights on Earth. Even with a success under their belt, scientists acknowledge the enormous challenge of such a mission. It's sort of like hitting a speeding cannonball with a speeding bullet. One wrong move, and you could miss your target completely or hit it off-center, causing it to tumble or crack into pieces. In 2005, the European Space Agency came up with the Don Quijote concept to improve the odds of a kinetic impactor mission (see sidebar).
You might classify nuclear weapons or kinetic impactors as instant-gratification solutions because their success (or failure) would be immediately apparent. Many astronomers, however, prefer to take the long view when it comes to asteroid deflection.