NASA Gears Up for Fourth of July 'Fireworks' in Space

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."