The comet is a leftover chunk of the primordial mass that evolved into the solar system. The particles grabbed by Stardust are believed to be 4.5 billion years old.

Researchers on Tuesday released new images documenting the space encounter that occurred Friday at the speed of 22,000 kilometers (13,670 miles) an hour. They show the core of the comet made up of rock and ice emitting plumes of gas.

Stardust was launched into space on February 9, 1999, and on Friday it passed within 240 kilometers (150 miles) of the Wild-2 comet to collect hundreds of samples.

"We can say over 10 million particles hit the shields," said Benton Clark, chief scientist for solar system exploration at Lockheed Martin Space Systems, which built the space craft for NASA.

During its encounter, the four small rocket engines fired more than 1,000 times automatically, about 300 times a piece, to keep the vehicle straight and safe from comet particles that were hitting the craft at a speed six times the speed of a gunshot, Clark said.

"We have collected hundreds of thousands of particles, enough to exceed all expectations," said Tom Economou, a professor from the University of Chicago in charge of a team responsible for monitoring an instrument aboard the craft that collects dust.

In preparation for the encounter, the five-meter (16-foot) long probe deployed its tennis-racket-shaped particle catcher.

The catcher is filled with a material called aerogel, a silicon dioxide substance that is 99.8 percent air and has enough 'give' to slow and stop the particles without altering them radically.

Once it captured samples, the collector folded up inside the return capsule like a shell fish.

"The capsule has been sealed up and will not be open until it lands" on Earth, said Stardust project manager Tom Duxbury at NASA's Jet Propulsion Laboratory here in Pasadena.

A capsule containing the comet dust is scheduled for a parachute-assisted landing at the US Air Force Test and Training Range in the western state of Utah in mid-January 2006, NASA said.

The specimens will then be sent to NASA's Johnson Space Center near Houston, Texas, for analysis.

The Wild-2 (pronounced Vilt-2) comet, discovered in 1978, fully orbits the Sun every 6.39 years.

At the end of its mission in two years, Stardust will have covered some four billion kilometers (2.5 billion miles), all at a cost of 165 million dollars.

From the chemical and physical data contained in the comet's miniscule 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.

Stardust is the first mission aimed at collecting outer space material since US astronauts aboard the Apollo 17 mission brought back rocks from the moon in 1972.

SPACE.WIRE