NASA bombs a comet - for science

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.

Indeed, astronomers aren't sure what will happen. Dr. Schultz has spent part of his time the past few years shooting marble-size projectiles into washtub-like containers of material selected to model different ideas of comet composition. Using a vertical gas gun at NASA's Ames Research Center, he has accumulated a wealth of information that will help him interpret what Deep Impact sees, earning him the title "Master Blaster" among his more daring graduate students.

"There's a debate right now; some folks think we'll get a much bigger crater than we expected. Others think it will be much smaller," he says. The range most often given is from a house-size crater to one the size of a football stadium on Tempel 1, which itself if about half the size of Manhattan. But he adds, "Nature throws us curves."

The size, shape, and structure of the crater and the plume of ejected material will go a long way toward solving the riddle of how densely or loosely the nucleus is packed and whether the material inside is relatively fine-grained like sand or a loose aggregation of larger rubble.

The Deep Impact mission was launched Jan. 12 and has proceeded almost flawlessly. Researches believe they have solved an earlier problem with one of the telescopes.

Reaching the comet has been a technical challenge which Deep Impact project manager Rick Grammier likens to one bullet trying to hit a second bullet with a third bullet 38 million miles from Earth. The main craft carries two telescopes to take images and spectrographic data of the comet The impactor also carries cameras.

The comet, impactor, and main spacecraft move so quickly that the main craft's cameras will have only a blink-and-you-miss-it opportunity to record the collision and any initial cratering. Yet the crater itself could take as long a five minutes to form fully, Dr. Schultz says. The whole show is expected to last about five minutes.

The event is drawing a large crowd of other spectators - professional, as well as amateur. Three of NASA's orbiting observatories - the Hubble Space Telescope, the Chandra X-Ray Observatory, and the Spitzer Space Telescope - will be among the space-based craft watching the event. In addition, some 100 astronomers in 20 countries will monitor the comet long after Deep Impact's main craft has turned off its cameras.

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