The NASA probe was launched 12 January 2005 and arrived at comet P/Tempel 1 on 4 July 2005. This was the first spacecraft to impact the surface of a comet and the first mission to investigate beneath the surface of a comet's nucleus. Data is still being analysed.

The impact was recorded by cameras on the deployed impactor and fly-by spacecraft, and broadcast on television. The spacecraft's copper mass impactor, weighing 370 kg, probably created a crater 25 m deep and 100 m in diameter on the nucleus, which is 6 km in diameter.

The mission's science objectives are to observe how the crater forms; measure its depth and diameter, the composition of its interior, and its ejected material; and determine the changes produced by the impact on the comet's natural outflow of gases.

http://deepimpact.jpl.nasa.gov/ Well-designed site about the Deep Impact mission to the comet P/Tempel 1, profiling the mission's scientific objectives, mission plan, and hardware. There are views of the spacecraft and animations depicting the comet encounter and impactor deployment.