Tempel 1 is probably three times as long as it is widesmaller than comet Halley. It's been in its present orbit a long time, and has made many passages through the inner solar system.
Comets are composed of ice, gas, and dustprimitive debris from the formation of the solar system about 4.5 billion years ago. The Deep Impact mission could answer basic questions about how the solar system was created, because scientists believe the material in comet interiors remains relatively unchanged from the time they were formed.
(One NASA research program, involving an interdisciplinary team of scientists from around the world, studies whether comets supplied the raw material to form life on Earth. Scientists believe that Earth suffered a prolonged series of cometary impacts at its formation billions of years ago.)
"There are no data on the interior [of a comet], and that's what we hope to solve with Deep Impact," the mission's principal investigator, Mike A'Hearn, said at a December news conference. A'Hearn is an astronomy professor at the University of Maryland in College Park.
Each time the comet passes close to the sun, it heats up, changing the surface layer of the comet. Thus, only the interior of the comet preserves what scientists anticipate are clues to the formation of the solar system.
The impact is expected to create a crater 100 meters (330 feet) in diameter and up to 30 meters (100 feet) deep. But scientists know so little about comets that cratering experts can't even agree on what physics are relevant to the impact, and so can't agree on what exactly will happen.
"Some people think the nuclei are strong [and will stay in place] others think we will fracture the nucleus into several pieces. Other people think we may just compress material downward," A'Hearn said.
After releasing the impactor, the flyby spacecraft will observe and record data about the impact, the material ejected from the crater, and the structure and composition of the crater's interior. Professional and amateur astronomers and telescopes on Earth will also observe the impact and its aftermath, and the results will be broadcast on the Internet.
"This is a unique mission, in that much of the scientific return is going to come from an Earth-based observation campaign, both from Earth orbit and from the ground," said Karen Meech, an astronomer at the University of Hawaii and a Deep Impact co-investigator.
For people who know where to look, the impact should even be easy to see from Earth with binoculars, scientists say.
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