Sunday, the Deep Impact probe will release a 372-kilo (820-pound) projectile -- roughly the size of a washing machine -- designed to ram into the Tempel 1 comet on Monday at some 37,100 kilometers (23,000 miles) per hour, or 6.3 miles per second.

"We will be attempting probably one of the most daring and the most risky mission that has ever been undertaken in space exploration," said Charles Elachi, director of NASA's Jet Propulsion Laboratory in Pasadena, California.

"Like any exploration, the payoff would be worth the risk, because for the first time in thousands of years instead of fearing comets we'll be able to know of what they are made of."

Scientists say the collision could gouge a crater the size of a football stadium, spewing an immense cloud of gas and dust.

"Basically, we have a bullet trying to hit another bullet with a third bullet in the right place at the right time," said Rick Grammier, who is heading the 333-million-dollar mission.

The final phase of the mission will be carried out automatically, without human intervention, Grammier noted.

Cameras and other instruments on board Deep Impact will be used to observe and analyze the mission from 500 kilometers (310 miles) away, while the Hubble Space Telescope, the Chandra X-ray Observatory and the Spitzer Space Telescope will also be used.

The projectile has also been outfitted with a camera to transmit two hours' worth of images until the time of impact.

"The long tradition of NASA is to understand how the solar system formed, how it'll change and find out if life exists elsewhere in the universe," said Andy Dantzler of the US space agency's solar system division. "Comets, the oldest celestial objects in the solar system, contain clues to answer a lot of these questions."

"Whatever we find should place constraints on whatever happened 4.6 billion years ago," when the solar system was formed, said University of Maryland astrophysicist Michael A'Hearn, one of the project's lead researchers.

Comets are believed to comprise primitive material created in the early solar system and, some experts say, may hold organic molecules that could have been the chemical building blocks for creating life on Earth.

But comets can also deliver death.

The long reign of the dinosaurs is believed to have been snuffed out some 65 million years ago when a comet smashed into modern-day Mexico, kicking up a pall of dust that obscured the Sun and chilled the climate.

A stepped-up US-led effort to scour the skies for dangerous near-Earth objects has not yielded any prospect of a hit, although several rocks could make near misses in coming decades.

But any collision would carry astronomical costs. Smaller objects measuring two or three hundred meters (yards) across could devastate a region or trigger a tsunami, while larger ones could be climate-crushers.

Tempel 1, discovered in 1867, is among the comets closest to Earth and orbits the sun every five and a half years.

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