The Deep Impact probe will fire the missile at the Tempel 1 comet in the early hours of July 3. The next morning, astronomers will see what the explosion reveals about its make-up.

“Every year I look forward to the Fourth of July, but this year promises to be extra special,” Andrew Dantzler, the director of Nasa’s solar system division, said yesterday.

A direct hit from the 800lb projectile could blow a crater the size of a football stadium into the 3.7-mile comet nucleus, throwing out clouds of debris. The mother ship will watch the collision from 300 miles away as its instruments analyse the cosmic smash for insights into the structure and composition of comets.

This could reveal details of how the early solar system was shaped, as comets are thought to have been formed in its earliest days an estimated 4.6 billion years ago. A better knowledge of how comets are put together could improve scientists’ understanding of how the planets came to be.

Deep Impact’s eyes will not be the only ones trained on the collision. The Hubble, Chandra and Spitzer space telescopes will be observing as the mater-ial is blown away, and hundreds of professional and amateur astronomers will also be watching. Scientists said that they could not predict how bright the impact would be, or whether backyard astronomers would be able to see it, but the best conditions are likely to be in the western United States. It is unlikely to be visible in British skies, although British astronomers will be watching images relayed from the Faulkes Telescope in Hawaii.

The Tempel 1 comet is hurtling at approximately 23,000mph, a speed at which one could travel from New York to Los Angeles in 6.5mins.

“We are really threading the needle with this one,” Rick Grammier, the Deep Impact project manager at the Nasa Jet Propulsion Laboratory in Pasadena, California, said. “We are attempting something never done before at speeds and distances that are truly out of this world.”

There is no danger that the impact will affect the comet’s trajectory in any significant way, let alone nudge it into an orbit that could threaten a collision with Earth.

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