Images taken by Hubble Space Telescope on June 14 of a bright eruption from Tempel 1 shooting a jet 1,400 miles into space. (AFP)

Washington, June 28: A two-stage spacecraft called Deep Impact is about to make an ambitious attempt to dissect a comet by slamming into it and blowing some of its innards into space for all to see.

Launched from Florida on January 12, Nasa’s Deep Impact is nearing the end of a finely calibrated 268-million-mile journey that puts comet Tempel 1 within its sights.

An 820-pound copper-core “impactor” is to smash into the comet’s nucleus at 23,000 miles an hour in the early hours of July 4, an unprecedented event that will, if all goes well, be witnessed by its companion craft and numerous observatories in space and on Earth.

Rick Grammier, the mission’s project manager at the Jet Propulsion Laboratory in Pasadena, California, said the final part of the encounter 83 million miles away was so intricate and so fast that the twin ships would have to handle these maneouvres on their own without help from human controllers. “It’s a bullet trying to hit a second bullet with a third bullet, in the right place at the right time,” he said.

Comets are believed to be remnants of materials that formed the solar system some 4.5 billion years ago. Astronomers believe comets’ interiors have undergone little change since then and contain the pristine ice, gases, dust and other materials from which the rest of the solar system formed.

Understanding comets is a way of understanding how the solar system was born.

Another reason to study comets is that they, along with rocky asteroids, pose a threat of hitting the Earth and causing cataclysmic damage. Defending against such possibilities requires knowing more about these objects in hopes of deflecting or destroying dangerous ones, experts say.

The target of the mission, Tempel 1, discovered in 1867, is a dark-coloured comet that moves about the Sun in an elliptic orbit between Mars and Jupiter every 5.5 years.

About 24 hours before intercepting Tempel 1, the spacecraft will divide, leaving the impactor in the comet’s path while the flyby craft fires jets to veer away. At the moment of impact, the 1,300-pound flyby craft, carrying high-resolution and wide-angle telescopes, should be 5,300 miles away relaying data from both craft. Some 14 minutes later, it will make its closest approach to the comet nucleus, passing 310 miles below it.

“The whole mission is riding on what happens in about 800 seconds,” Michael A’Hearn of the University of Maryland, principal investigator of the $333-million project, said. “We’ve only got one shot at it.”