I can't believe how cool this is

Dr Marc Rayman, DS1 mission director

The Deep Space 1 probe passed within 2,000 km (1,243 miles) of Comet Borrelly.

It is extremely rare for probes to get so close to comets and make detailed measurements.

A spokesman for the American space agency, Nasa, said DS1 was performing well and had already sent back pictures of the comet.

DS1 was set to take upwards of 30 black-and-white images of Borrelly's heart - a mountainous ball of ice and dust - during the fly-by. The first pictures may not be released to the media until Monday.

Long time

"I can't believe how cool this is,'' said Dr Marc Rayman, the mission director on the DS1 project, as the first data from the probe came through. "The images started coming down about [2245 GMT on Saturday], but most are still onboard the spacecraft. Still, this is incredibly cool.''

Comet Borrelly orbits the Sun every 6.9 years

It will take several days for the probe, which only has a 30-centimetre- (one-foot-) diameter antenna, to weakly transmit all the data 220 million kilometers (137 million miles) across space, he said.

"The images are coming along fantastically. We're very, very excited. We had an extremely risky and ambitious goal for the day. When the images started coming down, we could finally start breathing again after all this time.''

DS1 has also taken measurements of Borrelly's surface, measured and identified the gases it gives off, and studied the interaction of the solar wind with the comet. This is the process that creates the comet's distinctive tail.

Main mission

DS1 was launched in October 1998, to test a range of new technologies in space.

These included a long-lasting ion propulsion engine, a solar array which focused sunlight for extra power, and software that gave the craft crude artificial intelligence.

DS1 finished its main mission two years ago, and the probe will end its usefulness in November when all its fuel is consumed.

DS1 was not built specifically to encounter comets. That it has succeeded in gathering close-up information on Borrelly is therefore a tremendous fillip to Nasa and the project scientists and engineers at the Jet Propulsion Laboratory in California.