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On the Fourth of July 2005, NASA successfully executed what may be their coolest mission ever: to shoot the comet 9P/Tempel 1 with a camera-equipped cannonball. The aptly-named “Deep Impact” mission was far neater than the movie, because it was real.
Officially, the main probe was the “flyby” and the projectile was the “impactor”. Released a day early, the 370 kg (827 lbs) rocket-propelled impactor got up to 37 000 kph ( 23 000 mph) before it annihilated itself against the celestial body. The resulting explosion was observed not only by the probe’s intact half, but by space- and ground-based telescopes everywhere.
The purpose of shooting the comet was essentially a form of quick excavation; it got material from the inside of the comet to come out where spectrophotometers and other instruments could get a look at it. The physical response of the comet also yielded information.
NASA also had another admited purpose. Although this impact came nowhere near to breaking the comet or significantly changing its orbit, such target practice could be valuable practice for heading off a big comet on a collision course with Earth.
Deep Impact Website
Bullet’s-Eye Point-of-View Movie
Special issue of Science Magazine (Cover above) 14 October 2005 (Online requires a subscription, but libraries are still free).