June 28, 2005: 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.
Back on Earth, amateur astronomers will be watching, too. The comet glows like a 10th magnitude star and can be seen through backyard telescopes. It should brighten considerably when Deep Impact strikes. The impact plume will reflect sunlight, boosting the visibility of the comet to 5th or 6th magnitude, making it a faint naked-eye object. The Pacific side of Earth will be facing the comet at the moment of impact (0552 UT on July 4th; 10:52 pm PDT on July 3rd); observers in Hawaii, Mexico and the US southwest are favored. Click here and here for observing tips.
Rest of article at http://science.nasa.gov/headlines/y2005/28jun_deepimpact.htm?list761981
