Americans Just Love to Blow Things Up

NASA's Deep Impact probe has been launched and sent on a collision course with Comet Tempel1:

When it arrives in six months time, it will send its payload off on a collision course with the comet. The impact of the 37,000kph projectile will release energy equivalent to 4.5 tons of TNT, and could blast a hole the size of the Colosseum in the comet's surface.

NASA scientists hope this hole will reveal more about the composition of the comet, so shedding light on the formation of our solar system. Comets are preserved pieces of our solar system's primordial days; leftover pieces of the original matter that went on to form the planets. Some scientists think that organic molecules needed to form life, and even speculate that much of our planet's water was carried to Earth by comets.

The fly-by section of the craft will watch the collision and will send data back to Earth. NASA's Hubble, Spitzer and Chandra space telescopes, and other terrestrial 'scopes, will also observe the crash.

So does this mean that if scientists find a comet or meteor heading straight for Earth that they can blow it up before it gets here? Or at least alter its course?