NASA has set up an Independence Day traffic accident in space, deliberately crashing an 820-pound spacecraft into an onrushing comet. The resulting impact should provide scientists with their best glimpse yet of what the solar system was made of when it was formed 4.5 billion years ago.

At 1:52 a.m. Eastern time tomorrow, barring a last-minute mishap, comet Tempel 1, a lumpy, Manhattan Island-size potato hurtling through space at 66,000 mph, will overtake the 820-pound "impactor" dropped off in its path by a NASA spacecraft poking along at 43,000 mph.

The mission is fittingly dubbed "Deep Impact." Think of it as a bee smacking into the windshield of an 18-wheeler. Or a prairie dog being trampled by a herd of stampeding buffalo. Or a seagull walloped by the leading edge of a hurricane.

This cataclysmic rendezvous will produce an explosive force equivalent to that of 4.5 tons of dynamite, causing the impactor to punch a crater in the comet -- maybe as big as a football stadium, maybe much smaller, maybe shallow, maybe deep. A cloud of dust and ice will fly into the heavens.

The spacecraft -- stationed a prudent 310 miles away -- will watch and photograph the fireworks, as will NASA's space-based Hubble, Chandra and Spitzer telescopes, a network of ground-based telescopes and thousands of amateur astronomers around the world. The European Space Agency's Rosetta spacecraft, en route to its own 2014 encounter with a comet, will also be watching.

"We don't know what to expect," said University of Maryland astronomer Michael F. A'Hearn, who leads the Deep Impact mission. "It's possible that the change will be so small you can only see it with a four-meter telescope. Or you could see it with binoculars. Those are the two extremes."

For any stargazer west of the Mississippi River, Deep Impact could be quite a show. The flash will occur in the southeastern sky to the left of Jupiter and a few degrees above Spica in the constellation Virgo. Alas, it will take place below the horizon for would-be observers in the D.C. area.

The $333 million mission, directed from NASA's Jet Propulsion Laboratory, was launched Jan. 12 on a course to intercept Tempel 1 about 83 million miles away -- just about on the line that Mars follows as it orbits the sun.

The 1,325-pound spacecraft, built by Ball Aerospace & Technologies Corp. of Boulder, Colo., has performed almost flawlessly. Engineers detected a blurriness in one of its cameras early in the mission, but they compensated by mathematically sharpening its images. "In fact, we get somewhat better quality than before," said JPL's Rick Grammier, the Deep Impact project manager.

More recently, and potentially more problematic for the mission's outcome, the comet on June 14 and June 22 had sudden outbursts, brightening dramatically as clouds of gas and dust exploded from it.

A'Hearn said the light dissipated rapidly, suggesting that most of the ejected material was water ice and other frozen material that vaporized almost immediately in the sun's rays, along with dark-colored dust that did not reflect light.

"The real question is what causes the outbursts," A'Hearn said in a telephone interview. "My hunch is that there are pockets of unusually volatile ices inside the comet, and when the heat gets down below the surface, the gas suddenly vaporizes and blows a hole, ejecting the gas and whatever's above it."

This phenomenon would simply be a fascinating curiosity, except that the impactor is supposed to home in on its target with a camera tracking the brightest spot -- ordinarily the leading edge of the comet itself.

"If we're unlucky enough to have an outburst in the final two hours, it will change the solution" to intercept, because the impactor's navigation system will steer toward the burst of light, said Monte Henderson, deputy project manager for Ball Aerospace.

"But not enough," Henderson continued. "The most brightly lit area of the comet should [still] be the nucleus" of the comet itself.

Scientists believe comets date from the dawn of the solar system, formed from dust and gas that spread out from the young sun into distant orbits beyond the planets. Tempel 1, described by JPL astronomer and Deep Impact team member Donald K. Yeomans as a "jet black, pickle-shaped dirt ball," is a "short period" comet that migrated in from beyond Neptune, perhaps because its original orbit was perturbed by a passing star.

Because they have been so cold for so long, comets are largely the same as they were 4.5 billion years ago. Unlike planets and moons, comets are unbesmirched by volcanoes or meteors or erosion by wind, water or other weather.

But to get to the pristine content, scientists must have a way to punch through a surface that is cooking away in the heat of the sun, producing a vapor "coma" that surrounds it and trailing a "tail" of dust. Deep Impact is that tool.

The impactor, about the size of a washing machine, is capped by a 249-pound copper sphere -- the bullet that will penetrate the comet. The impactor has a camera that functions both as a navigation aid and as an imager that may be able to transmit photos until perhaps the last two seconds before the collision.

The mother spacecraft, about as big as a Volkswagen Beetle, will be positioned below the impactor to watch the crash. It carries high-resolution and medium-resolution cameras with filters that allow scientists to take pictures in various parts of the spectrum.

With all the help that the spacecraft will be getting from telescopes in space and on the ground, researchers are hoping for a river of data that will enable them to determine not only the comet's chemical composition, but also how it is put together.

If Tempel 1 is a "rubble pile" of rocky grains, dust and ices held together principally by gravity, the crater made by Deep Impact will likely be about the size and depth of a football stadium. If it has solid material, or a solid core, the impact burst will be "quick and small," Yeomans said, with the impactor's energy dissipating on the hard surface and leaving a shallower crater.

If the core is softer, "the impactor will act like a meteor," Yeomans continued, plunging into the guts of the comet until friction explodes it "like a Roman candle," producing a narrow but deep crater. Or the comet could simply swallow the impactor, engulfing it like a pillow "with almost no crater at all," he said.

In all, the Deep Impact team figures it will have 800 seconds to take images of the event from the time the impactor begins its final approach until Tempel zooms out of range of the sentinel spacecraft. NASA officials say the first images will be available well before dawn tomorrow, and a preliminary report on its success will come later in the day.