“On the 4th of July, a NASA spacecraft will blast a hole in Comet Tempel 1. For the last five billion years of our planet’s violent history, Earth has been walloped by comets. These small bodies and their asteroid cousins whacked Earth often in its early years, knocking the stuffing out of our young world. As the solar system matured, impacts happened less often—but they have never ceased. Earth bears its scars in the form of weathered craters and extinct species.

This 4th of July is payback time. For the first time in history, Earth gets to strike back. The weapon: a NASA spacecraft named Deep Impact. The target: a 10-mile wide comet named Tempel 1.

Deep Impact is going to shoot an 820-pound projectile into the rocky, icy nucleus of Comet Tempel 1. The 23,000 mph collision will form a big crater, and Deep impact will observe the stages of its development, how deep it gets and how wide it becomes. Researchers expect a plume of gas and dust to spray out of the crater. Deep Impact will measure its composition and record what the billowing plume does to the comet’s atmosphere. In all, Deep Impact should be able to peer into the new crater for almost 15 minutes before the craft speeds away, continuing, like its cometary quarry, to orbit forever around the Sun.” (NASA)

“When Comet Shoemaker-Levy 9 split after being catastrophically disrupted in July 1992, it remained pretty bright for several months, fading considerably about a year after the split. This impact will not be nearly as disruptive, so my guess is that it will continue to brighten for a day or so, then fade over the next several weeks. But since this type of experiment has never been done, we really do not know.”