The impact is expected to create a crater 100 meters in diameter and up to 30 meters deep. But A'Hearn warns that scientists know so little about comets that the cratering experts can't even agree on what physics are relevant to the impact, and thus can't agree on what exactly will happen.
After releasing the impactor, the fly-by spacecraft will observe and record data about the impact, ejected material 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.
"The impact should be easy to see from Earth with binoculars, if you know where to look," said A'Hearn.
Origin of Life
Comets are composed of ice, gas and dust—primitive 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 the comet's interior remain relatively unchanged from the time they were formed.
[NASA recently embarked on a multimillion-dollar research program, involving an interdisciplinary team of scientists from around the world, to study 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 about four billion years ago.]
The target comet, Tempel 1, was discovered in 1867 by astronomer Ernst Tempel. It 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.
The comet is probably three times as long as it is wide—about the size of comet Borrelly but smaller than comet Halley. It's been in its present orbit a long time, and has made many passages through the inner solar system.
"It's a pretty average periodic comet," said A'Hearn. "There's nothing unusual about it, which is what we want."
Along for the Ride
The sign-up campaign, meanwhile, is part of NASA's effort to publicize the Deep Impact mission. A CD containing the names of those who signed on board for the one-way trip will be obliterated along with the copper-tipped impactor.
"This is an opportunity to become part of an extraordinary space mission," said Don Yeomans, an astronomer at the Jet Propulsion Laboratory in Pasadena, California, and a member of the Deep Impact science team. "[People] can hitch along for the ride and be part of what may be the best space fireworks show in history."
While the experiment will not throw Tempel 1 off its course, it will help scientists devise ways of how to deflect a comet in the unlikely event that one threatens Earth, a doomsday scenario depicted in movies such as Armageddon and Deep Impact.
"If you want to deflect a comet it's very important to understand how it will react to what you do to it," said A'Hearn "This mission will tell us directly what would be the best methods to use."
A'Hearn says the Deep Impact mission is unique because it's an active experiment. "This mission takes us from the passive, look-and-see-what's-there, which is traditional of all astronomy and planetary science, into the realm of real experimentation."
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