After traveling more than 3 billion kilometers for nearly five years, US spacecraft Stardust on Friday had its closest encounter with its main target -- a comet named Wild 2 -- to collect samples of microscopic dust from the ball of dirty ice and rock.

The flyby occurred at 2:40 p.m. EST (1740 GMT) when Stardust hurtled past Wild 2 at a distance of about 300 kilometers and at a relative speed of 21,960 kilometers per hour, over six times faster than a speeding bullet.

"All indications are that the Stardust had a successful flyby of Comet Wild 2 today. Data playback is in progress," US space agency NASA said in a brief statement shortly after the encounter.

"Stardust marks the first time that we have ever collected samples from a comet and brought them back to Earth for study," Dr. Don Brownlee of the University of Washington, principal investigator for the mission, said earlier.

As the spacecraft sailed through the hailstorm of comet debris, a tennis- racket-shaped collector would capture and store particles coming off the comet's nucleus.

The optical navigation camera aboard Stardust was also activated during the flyby, and was expected to snap images of the dark mass of the comet's nucleus.

"The samples we will collect are extremely small, 10 to 300 microns in diameter, and can only be adequately studied in laboratories with sophisticated analytical instruments," said Brownlee.

Mission accomplished, the collector will fold down into a return capsule, which will close like a clamshell to secure the sample for a soft landing at the US Air Force's Utah Test and Training Range in January 2006.

Comets are leftover materials that formed the planets and the Sun more than 4.5 billion years ago and contain many of the organic materials thought to be essential for the origin of life.

Scientists believe in-depth analysis of cometary samples will reveal a great deal not only about comets but also the earliest history of the solar system.

Stardust, launched on February 7, 1999, is the first US mission dedicated to exploring a comet and also the first US unmanned probe to obtain samples in deep space and return them to Earth.

Wild 2, named after Swiss astronomer Paul Wild who discovered the comet in 1978, was picked as the target because by the time Stardust encountered it, the comet has made only five trips around the sun with most of its dust and gases still being kept in relatively pristine conditions, making it an ideal choice for study.

(Xinhua News Agency January 3, 2004)