Researchers hope impact will provide vital clues to the solar system's history.
Hours before July 4th skyrockets burst into sparkling showers, NASA plans a celestial display of its own.
Around 1:52 a.m. East Coast time, a dishwasher-size projectile from the agency's Deep Impact spacecraft is slated to burrow into the core of comet Tempel 1 with the explosive force of 4.5 tons of TNT.
From the comet's perspective, the impact has been likened to a Boeing 767 running into a mosquito. But from astronomers' viewpoints, the fountain of debris and fleeting images of an expected new crater will open a unique window on the conditions and material present during the solar system's birth 4.6 billion years ago. The event marks the first time scientists have crashed a device into a comet.
Comets' reputations have improved markedly since the days when monarchs viewed them as harbingers of disaster and trembled at their appearance. Beyond their role as cosmic time capsules, comets also are thought to have seeded Earth with chemical foundation for the emergence of life, as collisions brought water and organic compounds to a young planet.
Astronomers have gleaned a great deal of information about comets from telescopes and a handful of fly-by missions. Photos from missions since 1985 have revealed a small menagerie of oddly shaped nuclei with bizarre surfaces. Comet Wild 2, for example, hosts craters 500 feet deep, spires more than 300 feet tall, and miniature mesas - all on an object about 3 miles across.
Yet researchers are still groping for hard information on some of comets' fundamental properties. Information on their chemical composition comes largely from the tails of dust and gas that they cast off and the halo of dust and gas that surrounds their rubbly cores of ice and rock.
But the dust and gas have been heavily modified during repeated freezing and thawing as comets orbit the sun from the outer reaches of the solar system. And the models astronomers have constructed of comets' physical properties are many, notes Deep Impact's lead investigator, Michael A'Hearn, an astronomy professor at the University of Maryland at College Park. "The wide range of models makes it hard to interpret the dust and gas we see," he says.
Deep Impact's projectile will act as a remote-control geologist's hammer, allowing scientists to look at what they expect to be pristine material.
"This is heady stuff," says Peter Schultz, a professor at Brown University whose expertise on cratering earned him a spot on the mission's team.