At 1940 GMT on January 2, the comet Wild-2, a ball of ice and rock measuring more than five kilometers (3.1 miles) in diameter, and Stardust, five meters (16 feet) long, will come within 300 kilometers (186 miles) of one another at a relative speed of 21,960 km (13,645 miles) per hour, according to

The "comet flyby," as NASA calls the encounter, will pass the probe through the halo of dust that surrounds the nucleus of the comet.

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

The probe's dust collector is already deployed in readiness for the historic event.

NASA describes it this way: "A tennis-racket-shaped particle catcher of more than 1,000 square centimeters (160 square inches) of collection area, filled with a material called aerogel.

"Made of pure silicon dioxide, like sand and glass, aerogel is a thousand times less dense than glass because it is 99.8 percent air. The high-tech material has enough 'give' in it to slow and stop particles without altering them radically."

"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.

"After the sample has been collected," said NASA, "the collector will fold down into a return capsule, which will close like a clamshell to secure the sample for a soft landing (by parachute) at the US Air Force's Utah Test and Training Range in January 2006."

From Utah, the collector will be transported to NASA's Johnson Space Center near Houston, Texas, where the comet dust specimens will be analyzed.

From the chemical and physical data contained in these tiny particles, researchers are hoping to gain clues to the formation of the solar system, the birth of the planets and the matter from which they were formed.

Before its flyby encounter with Wild-2, Stardust has already collected a sampling of interstellar dust, the first phase of its 3.2 billion kmbillion mile) mission, using one side of the particle collector.

When Stardust comes into close proximity with the comet, the other side of the collector will trap dust and gas particles escaping from the comet's interior, said Tom Duxbury, project manager at NASA's Jet Propulsion Laboratory in Pasadena, California.

During the flyby, Stardust's optical navigational camera will also be activated to capture digital images of the dark mass that is the comet's nucleus, storing them in the probe's on-board computer which will transmit them back to Earth right after the encounter.

Wild-2 (pronounced Vilt-2), discovered in 1978, is in orbit around the Sun, making a complete revolution every 6.39 years.

Stardust, at a cost of 165 million dollars, is the first mission destined to collect extraterrestrial elements since Apollo 17 landed American astronauts on the surface of the moon and brought them back with a sampling of moon rocks in 1972.

Duxbury was enthusiastic about the probe's close comet encounter Friday.

"We are good to go," he said. "We have a great team of engineers and scientists that have trained hard for this moment, and we have a spacecraft that is in great shape."

SPACE.WIRE