Planning for the Deep Impact mission began in 1999. It culminates on July 4, 2005, when a “fly-by” spacecraft will release a smaller “impactor” spacecraft, which will smash into comet Tempel 1 at 37,000 kilometers (22,000 miles) per hour…. The impact is expected to create a crater 100 meters in diameter and up to 30 meters deep. But A’Hearn warns that scientists know so little about comets that the cratering experts can’t even agree on what physics are relevant to the impact, and thus can’t agree on what exactly will happen. —Stefan Lovgren
—Scientists Plan ‘Deep Impact’ Crash With Comet (National Geographic News)