Nov. 3, 2004 — A spacecraft intended to blast a hole into a comet so scientists can see what lies beneath is being prepared for launch at the end of the year.

Researchers first got the idea for the mission, called Deep Impact, after a study of Comet Halley 25 years ago yielded a startling find: While comets, which are sometimes called "dirty snowballs," are commonly believed to be icy remains from the deep-freeze sector of the solar system, Comet Halley turned out to be blacker than coal.

"We asked ourselves, 'How could this happen?' said Alan Delamere, a project consultant formerly with spacecraft designer Ball Aerospace & Technology Corp. of Boulder, Colo.

The team became more and more curious about how the black layer accumulated and in 1996 pitched a mission to NASA to explore the interior of a dead comet. A review panel, however, had technical concerns about the project's viability and turned it down.

Undeterred, the team changed targets to a live comet, refined their targeting and landing instruments and submitted a new proposal as part of the agency's low-cost Discovery science missions. The project, expected to cost about $311 million, was funded in 1998.

Deep Impact consists of two spacecraft: a flyby to take pictures and collect scientific information for relay back to Earth, and a 372-kilogram (820-pound) copper impactor that is designed to strike the surface of Comet Tempel 1 at about 23,000 mph. The one by one-meter (39 by 39-inch) projectile is expected to excavate a crater as large as a football field.

"We're going to hit it and see what happens," said Deep Impact principal investigator Michael A'Hearn, an astronomer at the University of Maryland.

Learning about comets' structure may yield more than scientific knowledge. The data could be vital for future missions intended to shift the path of an asteroid or comet on a collision course with Earth .

"The properties of (Tempel 1's nucleus) ... are probably representative of the really dark near-Earth objects, which are likely dormant comets," A'Hearn said.

Scientists say the biggest challenge of the mission is to make sure the flyby spacecraft is stable and able to track the impactor as it breaks into the comet.

"We'll have 800 seconds or so to gather high-fidelity images and data. All this makes the flyby spacecraft critical because it will be traveling through a hazardous area filled with cometary material," Delamere said.

The two spacecraft will separate 24 hours prior to impact, which is targeted for July 4, 2005. At the time, the comet will be about 134 million kilometers, or 83 million miles, from Earth. The flyby spacecraft will slow down and slightly shift direction so that it will miss the comet by 500 kilometers, or about 311 miles. The impact also will be followed by ground-based telescope observations.

If successful, the experiment will produce the highest-resolution images of a comet nucleus to date, and the first glimpse into a comet's interior composition and structure.

The primary goals of the mission are to observe how the crater forms; measure the crater depth and diameter; and measure the composition of the interior of the crater and the material that is ejected out into space from the impact.

Deep Impact left its manufacturing plant last week and arrived at Astrotech Space Operations' payload processing facility near the Kennedy Space Center in Florida. Launch aboard a Delta 2 rocket is targeted for Dec. 30.

"First to Florida, then to space, and then to the comet itself," said project manager Rick Grammier, with NASA's Jet Propulsion Laboratory in Pasadena, Calif. "It will be quite a journey."