To read the past, NASA smacks a comet

It's a concept even kids can relate to: If you want to see what's inside a rock, whack it with a hammer. That's precisely what planetary scientists are ready to do with one of the solar system's most ancient objects - comet Tempel 1. Their hammer is loaded aboard Deep Impact, an SUV-sized spacecraft set for launch next Wednesday from the Kennedy Space Center in Florida.

If all goes well, the craft will reach Tempel 1 on July 4 and release an 820-pound "impactor" in the comet's path. With its own guidance system and motor, the projectile, as large as a washing machine, will keep the nucleus in its cross-hairs. Once the two collide at 23,000 miles per hour before impact, the impactor could carve a crater in the comet's core deep enough to hold Rome's Coliseum.

Scientists are staging this $330 million assault to get at the story comets may tell about the early solar system.

Once feared as harbingers of doom, comets have come to be valued as cosmic time capsules. Researchers hold that these "dirty snowballs" are relics from the colder outer reaches of the disk of dust and gas that gave rise to the planets some 4.6 billion years ago. To analyze pristine samples of comet material, they hold, is to open a window on a critical period in solar-system history and test ideas about where and how quickly the outer planets formed.

But while other missions past and present have yielded big lessons about comets, planetary scientists have only scratched the surface - literally.

Over the years, scientists have detected a range of ices and chemical elements in the tails of dust and gas that comets throw off. Flyby missions have passed through a comet's tail, analyzing the shrouds of dust and glowing gas. One of them, NASA's Stardust mission, is returning to Earth with samples it swept up from a comet a year ago.

All of these measurements, however come from materials thought to originate from the comets' "skin," which scientists believe to be up to 30 feet thick. The skin's material has been heavily processed by heating, thawing, and exposure to radiation in space.

This presents scientists with a "Humpty Dumpty" problem, says Michael A'Hearn, Deep Impact lead scientist. It is difficult to confidently piece these altered bits of chemical "shell" back into a coherent picture of the comet's unprocessed interior.

"We really have no idea what pristine is," says Dr. A'Hearn, an astronomy professor at the University of Maryland at College Park.

But by punching a crater deep into Tempel 1's core, researchers should be able to analyze purer material ejected from deep in the nucleus.

Even before scientists reach better conclusions about composition, they may learn something new about how comet cores are held together. The key lies in the impact crater and the pattern made by the ejected material. If the energy is spent breaking strong bonds, the crater will be shallow. If the nucleus turns out to be a crust-covered cosmic slush ball, the impactor will burrow itself a hole with little material ejected. At the moment, the international science team associated with the NASA mission is looking for something in between. But the outcome is uncertain.

"We have a prediction for the crater," A'Hearn says. But "the uncertainty is what makes this conceptually simple experiment worth doing."

Learning about such information has a practical use, says science-team member Jay Melosh, a researcher at the University of Arizona in Tucson. Scientists have gained greater understanding of how Earth and the course of life on it have been shaped and reshaped through collisions with asteroids and comets. They've also come to appreciate that, while rare, a future comet impact on Earth can't be discounted.

"If we ever have to deflect a comet, this is the kind of observation that would be needed," he says.

The impactor is expected to hit the surface of the comet - a potato-shaped object estimated at 2.5 miles across and 7 miles long - with the energy of 4.5 tons of TNT.

The event - whose effect on the comet's course researchers liken to a mosquito hitting a 747 in mid-flight - will be widely watched.

The spacecraft carries instruments that will analyze the crater and the material ejected right after impact. But the flyby lasts only 20 minutes. After the impactor strikes, the craft must quickly be reoriented so its shield protects it from the comet's dust.

Astronomers also will train the Hubble Space Telescope and its infrared counterpart, the Spitzer Space Telescope, on the comet during the impact. Large ground-based observatories also will record the spectacle. In addition, a legion of amateur astronomers has been enlisted to supply digital images of Tempel 1 before, during, and after impact. They can spend longer stretches of the calendar monitoring the comet than can big telescopes, where observing time is a precious commodity.

Once hit, the otherwise faint comet should be visible to anyone in the right locations using binoculars or a small telescope. Some astronomers suggest that depending on the amount of material the impactor kicks up, Tempel 1 might briefly brighten up enough to see with the naked eye.