The scientist who wrote the book on planetary impact cratering will join NASA and other university researchers at Kennedy Space Center, Fla., to watch the launch of Deep Impact this month.

If all goes as planned, Deep Impact will become the first mission to slam into a comet, giving astronomers worldwide something far better than any other fireworks show on July 4, 2005 the first look inside a comet at the most primitive material left in the solar system.

"The idea is that the best way to find what's inside a comet is to blast a hole in it," University of Arizona Regents' Professor and Deep Impact science team member H. Jay Melosh said. "Other comet-rendezvous missions have proposed sampling less than a foot into the upper surface. But that doesn't get at the ices in the interior, which scientists believe are early solar system materials that have been kept in the deep freeze for the past 4.5 billion years."

Melosh has done more than any other single scientist to explain how impact cratering has shaped the terrestrial planets, including Earth. His 1989 book on the topic is still the universal reference for scholars, expert or novice. Melosh's research interests relate to the origin and evolution of the early solar system. Deep Impact could add chapters to that story.

NASA's Deep Impact spacecraft is planned for liftoff Jan. 12, 2005, and no later than Jan. 28, to reach comet Tempel 1 beyond the orbit of Mars on July 4. During the rendezvous, Deep Impact will deploy an 820 pound (372 kilogram) copper probe into the comet at about 23,000 mph (37,000 kph), or about a hundred times faster than a bullet fired from a .22 caliber gun.

The probe carries a digital camera that will send images in real time back to the spacecraft as the projectile hurls through dust and debris toward the comet.

Powerful cameras and a spectrometer on the parent spacecraft, flying about 300 miles (500 kilometers) away, will capture what happens on impact. The instruments will follow events for about 10 minutes after the collision, until the spacecraft goes into shield mode to survive flight through the comet's dusty orbital plane, where it will be blasted by buckshot-to-gumball-sized particles.

"The impact may form a crater about the size of a football field and deep enough to swallow a 10-story building. The deeper the better, " Melosh said. "We're hoping to see the crater explosion and the ejecta plume. We'll get some idea of the strength of the interior of the comet as the crater grows and ejecta is blown out." The comet nucleus could be hard, Melosh said -- or fluffy as a bowl of cornflakes.

Impressive as the impact will be, it's only a minor hit as far as Tempel 1 is concerned, Melosh said. The mass of the copper impactor is miniscule compared the mass of the five-to-six-mile-long comet. An analogy would be a smash-up between an 18-wheeler and an armadillo crossing the road, or even a mosquito smashing into a windshield, he noted.

Scientists plan to get spectra that will tell them what molecules make up the comet ices.

"There'll be a flash of volatiles that we may see with the spectrometer in the early stage of impact, and that astronomers will see from the ground and with space-based telescopes like Hubble, Chandra and Spitzer."

A global network of professional and amateur astronomers will observe the comet for the next few days as it brightens after impact.

During mission design, Melosh carefully calculated the abundances of "critical" elements scientists might expect to see vaporize on impact. Critical elements are those which scientists want to measure because they are important in early solar system processes. Melosh also modeled how much mass of each of the different elements would be vaporized on impact so scientists can know how much vaporized material comes from the comet and how much from the spacecraft.

The Deep Impact probe is made mostly of copper because "copper is an element that no geochemist or cosmochemist trying to work out the origin of the solar system cares about. It's not characteristic of any particular solar system process," Melosh said.

University of Maryland astronomy professor Michael A'Hearn is principal investigator for Deep Impact. He leads the mission from the University of Maryland, College Park. Kitt Peak National Observatory Astronomer Emeritus Michael Belton of Belton Space Exploration Initiatives, Tucson, is another member of the Deep Impact science team.

Ball Aerospace & Technologies in Boulder, Colo., built NASA's Deep Impact spacecraft. It was shipped to Florida in October for final launch preparations. NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the Deep Impact project for the Science Mission Directorate at NASA Headquarters. Deep Impact is a mission in NASA's Discovery program of moderately priced solar system exploration missions.

More information on Deep Impact is on the Internet at http://www.nasa.gov/deepimpact