Deep Impact, a NASA Discovery Mission, is the first space mission to probe beneath the surface of a comet and reveal the secrets of its interior.

After a voyage of 173 days and 268 million miles, a hyper-speed impact between space-borne iceberg and copper-fortified probe will take place. The potentially spectacular collision will be observed by the Deep Impact spacecraft, ground and space-based observatories at approximately 1:52 AM Eastern Time on July 4th.

During the early morning hours of July 3, the Deep Impact spacecraft will deploy a 39-inch cubic shaped impactor into the path of the comet, which is about one-half the size of Manhattan Island, N.Y. Over the next 22 hours, Deep Impact navigators and mission members, more than 83 million miles away at JPL, will steer both craft towards the comet.

The impactor will steer into the comet and the flyby craft will pass approximately 310 miles below. Tempel 1 is hurtling through space at approximately 6.3 miles per second. At that speed you could travel from New York to Los Angeles in less than 6.5 minutes. Two hours before impact, when mission events will be happening so fast and so far away, the impactor will kick into autonomous navigation mode. It must perform its own navigational solutions and thruster firings to make contact with the comet.

The crater produced by the impact could range in size from a large house up to a football stadium from two to 14 stories deep. Ice and dust debris will be ejected from the crater, revealing the material beneath. The flyby spacecraft has approximately 13 minutes to take images and spectra of the collision and its result before it must endure a potential blizzard of particles from the nucleus of the comet.

"In the world of science, this is the astronomical equivalent of a 767 airliner running into a mosquito," said Don Yeomans, a Deep Impact mission scientist at JPL. "The impact simply will not appreciably modify the comet's orbital path. Comet Tempel 1 poses no threat to the Earth now or in the foreseeable future."

Deep Impact will provide a glimpse beneath the surface of a comet, where material from the solar system's formation remains relatively unchanged. Mission scientists expect the project will answer basic questions about the formation of the solar system, by offering a better look at the nature and composition of the frozen celestial travelers we call comets.