Aristotle believed that comets erupted from the Earth into the heavens, while American astronomer Fred Whipple suggested in 1950 that comets resembled dirty snowballs composed of rocks, frozen water, and other compounds made up of carbon and hydrogen.
The impact, projected to leave a crater that could swallow a football stadium, is intended to scrape away part of Tempel 1's surface and expose what's underneath.
"We know that the crust--the outside shell of a comet and the stuff that comes off a comet--is changed by the solar wind," said Randii Wessen, a scientist at NASA's Jet Propulsion Laboratory. "One of the things that we're curious about is, some people will tell you that comets actually produce organic compounds...We want to see if that's inside."
A NASA spacecraft called Deep Impact, which launched in January, is scheduled to release an "impactor" module at 11:07 p.m. PDT on Friday. Barring technical glitches, the 820-pound module will collide with Tempel 1 while Deep Impact stays at a safe distance to take photographs and analyze the plume of dust and rocks.
Dozens of observatories will have their telescopes trained on the patch of sky where the rendezvous should happen--about 83 million miles away from Earth--and the impact should even be visible to some lucky earthbound observers.
The University of Maryland is offering tips on spotting the event, which will appear to take place in the Virgo constellation. U.S. watchers on the West Coast should be able to witness the impact, which can be seen with a telescope and perhaps glimpsed with the naked eye, at about 25 degrees above the horizon.
As Deep Impact closes in on the comet, its cameras have been firing photographs back to earth that show the bulk of Tempel 1 drawing nearer. (A photo library is available on NASA's Web site.)
The cost of the mission to taxpayers is expected to be at least $267 million, not counting the price of the launch vehicle.
Government scientists say the price tag is worth it. "One, we'll learn about comets," said NASA's Wessen. "Two, we'll learn about how that applies to the Earth, whether it brought organic material to the Earth...We can even learn, if a comet was coming our way, what it would take to deflect one of those things."
If successful, the Deep Impact mission could help answer some of the questions that have been nagging astronomers ever since Edmond Halley used Newtonian mechanics to predict cometary orbits--including the once-every-75-years comet that bears his name today.
One popular theory is that comets, suspected to be about 50 percent water, were responsible for delivering much of Earth's water, which in turn led to the emergence of life on this planet. On the other hand, a comet is also believed to be the most likely source for the mass extinction of dinosaurs about 65 million years ago.
As Deep Impact's impactor prepares to head to its final destination, NASA is moving to allay fears about the mission resulting in the comet breaking apart and a chunk spiraling into the planet. In a statement, NASA mission scientist Don Yeomans said: "In the world of science, this is the astronomical equivalent of a 767 airliner running into a mosquito. It simply will not appreciably modify the comet's orbital path. Comet Tempel 1 poses no threat to Earth now or in the foreseeable future."