Peter N Spotts

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. 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. 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 rubble 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. The size, shape, and structure of the crater and the plu-me 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 in Jan. 12 and has proceeded almost flawlessly. 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 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. The whole show is expected to last about five minutes. Some 100 astronomers in 20 countries will monitor the comet long after Deep Impact’s main craft has turned off its cameras. — The Christian Science Monitor