If, however, the comet is a hard "ice ball," he continued, the crater will be deeper but not as wide. And finally, he added, if the comet is made of fluffy ice or other less dense material, "it will behave like Styrofoam," and the projectile will be embraced "without leaving much of a crater at all."
As spacecraft instruments and telescopes record the event, scientists will try to determine the nature of Tempel 1's interior: Is its chemical composition different from the surface composition? Is there carbon monoxide or carbon dioxide (dry) ice below a depth of 60 feet? If the interior is water ice, is it crystalline (like quartz) or amorphous (like glass)?
Deep Impact, a $267 million mission, will be launched from Cape Canaveral Air Force Station on a 263 million-mile intercept course to meet up with Tempel 1, which will be 63 million miles from Earth on July 4, hurtling through space at 64,871 mph.
The spacecraft, carrying two telescopes and the projectile, will be traveling in a slightly different direction at 46,975 miles per hour. It will release the projectile about 24 hours before impact and conduct a "deflection" maneuver to get out of the way, but remain in the vicinity to record the event.
The projectile is an 820-pound spacecraft powered by a single 250-ampere battery and equipped with thrusters and sensors to keep it pointed at the target. The projectile's leading surface is a 249-pound dead-weight copper sphere, chosen because copper, like gold and silver, is a noble metal that does not react with water.
"We wanted the impactor to be the largest we can carry on a rocket we can afford," A'Hearn said.
A camera will record the approach up to the last moment, sending data to the mother ship, which will send the data back to Earth. The playback of data and images from the impact will begin the day after the event and continue for as many as 30 days. The spacecraft will continue flying, perhaps to seek out other comets in follow-on flyby missions.
Even though the comet "could easily fit inside the city of Washington," A'Hearn said, the explosion, albeit spectacular, is not going to alter the configuration of the heavens in any appreciable -- or dangerous -- way.
"It might cause the equivalent of moving the center of Washington from the Capitol to the end of the Mall," he said.
Think of it, added Yeomans, as "a bug hitting the windshield of an 18-wheeler."