NASA probe hits bullseye in comet experiment

IT sounded like science fiction NASA scientists using a space probe to chase down a speeding comet 83 million miles away and slamming it into the frozen ball of dirty ice and debris in a mission to learn how the solar system was formed.

Yesterday, scientists in charge of the Deep Impact mission released an 820-pound copper and aluminium missile the size of a washing machine into the path of comet Tempel 1, which was hurtling through space at 23,000mph.

Mission controllers at the American space agency NASA's Jet Propulsion Laboratory in Pasadena, California, clapped, cheered and hugged each other when the impactor struck.

"What a way to kick off America's Independence Day," said project manager Rick Grammier.

Hitting the Tempel 1 "bullseye" was an incredible technical achievement which involved targeting a fast-moving object less than 3.7 miles wide from a distance of 537,000 miles.

The blast, equivalent to the energy released by five tons of dynamite, was so bright that scientists expected it to be visible to the naked eye from Earth.

Rough images from the mothership that released the probe on its suicide mission 24 hours earlier showed a bright white flash from the comet upon impact, which hurled a cloud of debris into space. When the dust settles, scientists hope to peek inside the comet's frozen core a composite of ice and rock left over from the early solar system.

The camera of the Deep Impact probe temporarily blacked out twice, probably from being sandblasted by comet debris, NASA said. Still, the probe, on battery power and using thrusters to get a perfect aim, took pictures right up to the final moments, revealing crater-like features. The last image was taken three seconds before impact.

Scientists had compared the barrel-shaped probe's journey to a "mosquito hitting a 747 passenger jet".

The crater left by the smash could be as small as a house or as large as a football stadium. Much depends on the comet's composition. Early indications are that the comet's outer crust is surprisingly frail. They say there is no risk of Tempel 1 being deflected and endangering the Earth. But data from the mission may help scientists work out ways to stop a comet that does pose a threat.

Soon after the crash on the comet's sunlit side, the mothership prepared to approach Tempel 1 to peer into the crater site and send more data back to Earth.

Comets are frozen balls of dirty ice, rock and dust that orbit the sun. A giant cloud of gas and dust collapsed to create the sun and planets about 4.5 billion years ago and comets formed from the leftover building blocks of the solar system.

Deep Impact launched on January 12 from Cape Canaveral, Florida, on its 268-million-mile voyage.