But scientists don't know what will happen when the projectile hits the comet, because no one knows how hard its outer layers are. The impact could make a small dent or a crater the size of a football stadium. According to one long-shot theory, the comet may even disintegrate.
"We expect to be surprised," Melosh said.
The flyby spacecraft, which carries the most powerful telescope ever sent into deep space, will observe and record data about the impact. A camera on the impactor itself will snap pictures as it hurtles toward the comet.
The comet may become nearly a hundred times brighter as a result of the impact, scientists speculate, and the smash-up could create a fireworks display visible in a dark sky by the naked eye.
Deep Freeze
Comets are composed of ice, gas, and dust—primitive debris from the time of the early solar system—and they could answer basic questions about how the solar system was created.
"[The comets] have spent most of their lives in the deep freeze of the outer solar system, [so] they likely retain the original ices and materials that were present during the outer solar system formation process that began 4.6 billion years ago," Yeomans said.
Tempel 1, discovered in 1867 by German astronomer Ernst Tempel, was probably formed beyond the planet Neptune in the Kuiper Belt, a disk-shaped region that is the source of most of the so-called short-period comets.
Tempel 1 is probably three times as long as it is wide. It has been in its present orbit a long time and has made many passages through the inner solar system.
But scientists know little of the comet's nucleus.
"We still need to determine its brightness, orientation, and shape," Grammier said.
Scientists hope the mission will help them understand the composition and interior structure of the heart of Tempel 1.
"Like a field geologist using a hammer to probe a rock's strength and density, the Deep Impact spacecraft will probe the comet's strength and density by observing the reaction of the nucleus to a controlled impact," Yeomans. said
Nuking Comets
Deep Impact is expected to barely alter the comet's orbital path around the sun, and will not put either the comet or a chunk of it on a collision course with Earth.
However, the mission may help scientists devise ways to deflect any other comet that threatens to collide with Earth in the future.
"The kind of information we could get out of this—how porous or how strong the comet is—is just the sort of thing we need to evaluate any potential deflection schemes," Melosh said. "We're completely ignorant about that now."
Possible deflection options range from landing a spacecraft on the surface of a comet and trying to push it in a different direction to using nuclear weapons to blow it up.
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