Comet Targeted in Cosmic Drive-by Shooting

US, British and European scientists are training their sights for an unprecedented demonstration of long-distance marksmanship: a direct hit on a comet 83m miles away, moving at a speed of 23,000mph.

At precisely 6.52am on Monday, a copper bullet the size of a washing machine dropped from a Nasa spacecraft called Deep Impact will collide with a 1bn tonne lump of ice, dust and chemicals called Tempel 1. The smash, at more than six miles a second, should blast a hole at least as big as a house, and possibly as big as a football stadium, in the fabric of the comet.

Showers of cometary shrapnel will fly out into space to be recorded by Deep Impact, programmed to pass within 310 miles of the cosmic drive-by shooting. For Americans, it will be the ultimate July 4 fireworks display.

The drama will be monitored by the Hubble telescope, other spacecraft, a battery of Earth-based optical telescopes from Arizona to Hawaii, and by thousands of amateurs, for whom Tempel 1 is little more than a faint dot in the night sky.

The collision will be the first probe into the interior of a comet, a ghostly lump of builder's rubble left over from the making of the solar system 4.5bn years ago. The eruption of dust and ice could reveal some of the secrets of the planets and the Earth's oceans, and details of the original organic chemistry from which life must have been formed. But the operation will be a tricky one. "We really are threading the needle with this one," said Rick Grammier, the mission's project manager at Nasa's Jet Propulsion Laboratory in Pasadena. "In our quest of a great scientific payoff, we are attempting something never done before, at speeds and distances that are truly out of this world."

Comet Tempel 1 is about half the size of the island of Manhattan. Deep Impact's copper projectile is a 99cm (39in) cube.

On Sunday - 172 days after its launch, and after a journey of more than 260m miles around the sun - Deep Impact will release its bullet directly into the path of the comet. Over the next 22 hours, engineers on Earth will steer the two spacecraft: one of them designed to miss the collision but watch the fun; the other designed to slam into the huge, dusty iceberg. And then, two hours before the impact, an autonomous navigator on board the mothership will take charge.

"The autonav is like we have a little astronaut on board. It has to navigate and fire thrusters three times to steer the wine cask-sized impactor into the mountain-sized comet nucleus closing at 23,000mph," Mr Grammier said. Michael A'Hearn of the University of Maryland, the mission's principal scientist, said: "The last 24 hours of the impactor's life should provide the most spectacular data in the history of cometary science. "With the information we receive after the impact, it will be a whole new ballgame. We know so little about the structure of cometary nuclei that almost every moment we expect to learn something new."

A high resolution camera on the Hubble telescope has already spotted a spontaneous jet of dust streaming from the nucleus of Tempel 1. The nucleus is a potato-shaped lump of matter less than nine miles long. To spot it across a distance of 75m miles is like standing in Brentford and spotting a potato in Bologna. British schoolchildren will see images from the two-metre Faulkes telescope based in Hawaii. What they see will be over swiftly. The collision will turn the 372kg (820lb) projectile into a puff of hot atoms, and excavate a huge crater in the comet.

But it will neither destroy the monster, nor even deflect it by an inch. Researchers have compared the experiment to slamming a mosquito into a the path of a Boeing aircraft. The collision will provide the first direct evidence of the fabric of a comet. In January 2004 a space mission called Stardust sped through the coma of a comet, collecting its dust, but the evidence will not get back to Earth until 2006. A European mission called Rosetta will meet a comet in 2014, ride with it, and plant a lander on it.

"Rosetta will be in the most privileged position to observe the event from space," said Iwan Williams of Queen Mary, University of London. "Rosetta's instruments will measure the composition of the crater and its ejected material - a cloud of dust and gas that is expected to expand and reach its maximum brightness about 10 hours after impact."

But Rosetta has a long way to go. The spacecraft is still relatively close to Earth, 50m miles behind Tempel 1.