When it comes to space exploration, where scientists often measure their needs in milli-this and micro-that, Deep Impact, as its name suggests, has all the subtlety of a punch in the mouth. Barring unforeseen delays, NASA will launch on Wednesday a 1,325-pound spacecraft on a one-way trip to the comet Tempel 1. On July 3, the spacecraft will jettison an 820-pound copper projectile in the comet's path and get out of the way as comet and projectile meet at a relative speed of 23,000 mph. This, perhaps not surprisingly, will happen on July 4, and if you are somewhere in...