Nasa's Stardust space probe, launched nearly five years ago, on Friday is due to capture the first-ever samples of comet dust, bringing them back to Earth in 2006.
On January 2, the comet Wild-2, a ball of ice and rock measuring more than five kilometres (3.1 miles) in diameter, and Stardust, five meters (16 feet) long, will come within 300 kilometres (186 miles) of one another at a relative speed of 21,960 km (13,645 miles) per hour, according to Nasa.
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 centimetres (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 analysed.
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 km (2 billion mile) mission, using one side of the particle collector.