The big, grown-up boys on the NASA (search) team can hardly wait. Next Fourth of July, they get to bust up a comet, Hollywood-style.

"Blow things up? I'm there. Yeah, I don't have any issue with that," says Richard Grammier, manager of the project for Jet Propulsion Laboratory (search). (And, oh yeah, he used to work with explosives in the military.)

The spacecraft is called Deep Impact (search) just like the 1998 movie about a comet headed straight for Earth. NASA's goal is to blast a crater into Comet Tempel 1 (search) and analyze the ice, dust and other primordial stuff hurled out of the pit.

Mission planners say the energy produced will be like 4.5 tons of TNT going off — producing a fireworks display for the world's observatories.

Scientists know little about comets and even less about their nuclei, or cores. They believe that penetrating the interior for observations by space and ground telescopes is the next best thing to actually landing, scooping up samples and delivering them to Earth.

"A sample return would be the ultimate, but this is one exciting mission because for the first time we're actually reaching out and we're going to create our own crater," says Donald Yeomans, a senior research scientist at JPL in California — and an adviser on the movie.

"We'll understand how the comet is put together, its density, its porosity, whether it has a surface crust and underlying ices, whether it's layered ice, whether it's a wimpy comet or whether it's a rock-hard ice ball. All of these things will become apparent after we smack it."

Astronomers are counting on Deep Impact to live up to its Hollywood name on July 4, six months after its mid-January launch.

This is one spacecraft NASA wants to smash and trash.

"It would be like it's standing in the middle of the road and this huge semi coming down at it at 23,000 mph, you know, just bam!" Grammier says.

If all goes well, Deep Impact will be the first spacecraft to touch the surface of a comet. NASA's Stardust spacecraft — on its way back to Earth with dust from Comet Wild 2 (search) — flew through the coma, or dusty gas cloud.

Deep Impact will have traveled 268 million miles from the time it is launched aboard an unmanned rocket until it intersects with Comet Tempel 1 just beyond the orbit of Mars, at a point more than 80 million miles from Earth.

Liftoff is targeted for Jan. 12, two weeks late because of software and rocket problems. NASA has until Jan. 28 to launch Deep Impact. After that, Tempel 1 will be beyond rocket reach and scientists will have to pick another comet and swallow a lengthy delay.

That's what happened to the European Space Agency's Rosetta spacecraft, which will attempt a controlled landing on a comet, but not until 2014.

Deep Impact, by contrast, will provide "instant gratification," says Grammier. The entire $330 million mission should be wrapped up a month after impact.

Comet Tempel 1 is ideal from a scientific and demolition perspective.

It's a typical comet — all the better for scientific analysis — yet has a large nucleus and weak coma, all the easier for the impactor to survive the dusty obstacle course and to nail the nucleus.

Grammier says the latest calculations put the chance of the impactor missing its target at less than 1 percent. The automatic navigation software has already been tested in space; this will be a fancier version of what successfully flew on NASA's Deep Space 1, a testbed spacecraft launched in 1998, and Stardust, the earlier comet spacecraft.

"We all feel pretty comfortable with that (the odds), but as we've all said before, we're doing something we haven't done before," Grammier says.

No matter what, fans of the 1998 disaster film can rest easy. (Coincidentally, the movie and spacecraft people hit on the same name independent of each another, at about the same time.)

NASA guarantees that no matter how powerful the punch or how big the crater, Deep Impact will barely alter the comet's orbital path around the sun and will not — repeat, not — put the comet or any part of it on a collision course with Earth.

Yeomans calculates that to move Tempel 1 or a piece of it into an Earth-intersecting orbit, the impactor would have to be 6,000 times more massive than what will shoot out of the mothership on July 3. The very next day, the 820-pound impactor will strike at the heart of the comet, creating one awesome Fourth of July display.

By celestial standards, the crater that is formed — anywhere from the size of a house to Rome's Coliseum, and from two to 14 stories deep — should be just a dent. Besides, comets get bombarded with stuff all the time; they're pockmarked with craters and cliffs.

"You've got an object the size of a bushel basket running into an object that's 9 miles in length, so we're not going to do any real damage to the comet," Yeomans says.

Some scientists, however, contend the comet will shatter into several pieces. Others hypothesize that Deep Impact will create a crater but shove everything in, with hardly anything or nothing ejected.

"It is the uncertainty in the predictions — or the wide range of predictions — that make it particularly important to do this conceptually very simple experiment," says the University of Maryland's Michael A'Hearn, the mission's chief scientist.

Whatever the outcome, scientists expect to learn something about deflecting a killer comet — or possibly an asteroid — if one ever happens Earth's way. Comets, after all, have hit Earth before and are thought to have brought water with them.

Another practical benefit of the mission: By knowing what's inside comets, NASA would be better able to use them in the future as watering holes and fueling stations. Robots or astronauts, for instance, could break the comet's water down into its basic elements, hydrogen and oxygen, the ingredients for rocket fuel.

Then there is all the scientific knowledge to be gained from studying comets, essentially giant dirty snowballs circling the sun.

Formed the same time as the planets 4.5 billion years ago, comets are considered the leftover building blocks of the solar system. When the comets periodically swing close by the sun, their surfaces heat up and change, and so only their interiors preserve cosmic-origin clues.

The impactor — composed mainly of a 317-pound solid copper disk — will maneuver itself in the oncoming path of the comet and, in essence, get run over by the comet. The relative speed at the moment of the collision will be 23,000 mph, enough to vaporize the impactor.

Copper was chosen because, like gold and silver, it does not react with water and will not taint the observations, and it is much cheaper.

A camera on the impactor will photograph the comet and beam back the pictures, almost all the way up until the moment of destruction. A pair of cameras on the mothership — flying by at a safe 300 miles — will document the actual strike and the ensuing eruption and crater, and send back all the images.

"We expect to provide great fireworks for all our observatories," Grammier says, "and that's exciting to do it on July Fourth."