In a few days, a capsule containing wisps of material scooped from a comet's tail will plunge into Earth's atmosphere 10 times faster than a speeding bullet.
The container, carrying dusty detritus from Comet Wild 2, should then parachute into the Utah desert, giving researchers a chance to study its precious cargo: a milligram of dust left over from the solar system's birth 4.5 billion years ago.
US scientists have admitted the fate of their Stardust mission is hanging in the balance. An attempt to retrieve a similar capsule -- from NASA's Genesis mission -- crashed into the desert in September 2004 after its parachutes failed to open.
Engineers fear the Stardust probe, launched in 1999, could suffer a similar fate. It, too, is due to parachute into Utah.
"Watching the landing is going to be a very, very nervy experience," one Stardust team member said last week.
NASA's Stardust probe has travelled almost 4.8 billion kilometers on its journey past Wild 2. It made its closest approach on Jan. 2, 2004, when the probe swept over the comet at a height of 240km and sent back a series of breathtaking images of its surface.
"We saw incredible features -- steep cliffs, overhanging cliffs, spires and many other things we had never seen on other solar system bodies," principal investigator Don Brownlee said. His team saw jets of gas carrying dust and debris into space.
Comets are large balls of dirty ice that swarm at the edge of the solar system. Occasionally one gets dislodged from this distant orbit and swings towards the Sun. At its closest approach, materials on its surface start to boil off, producing the vast, gaseous tails that make these celestial objects so conspicuous.
Scientists are particularly interested in comets because they believe they are rubble left over from making the solar system, which later played a profound role in the development of Earth. They probably delivered most of the water for Earth's oceans and bombarded our planet with complex organic compounds that could have been crucial to the evolution of life here.
For these reasons, researchers have sought a source of comet molecules and designed Stardust to provide it.
"This is a history project. We are going to the edge of the solar system, collecting the original building blocks of the planets and bringing them back to our labs," Brownlee said.
"This [mission] gives us an opportunity to find out if our long-held suspicions are right, that comets played a major role in the origin of life," he said.
When Stardust reached Wild 2, the probe swept through the comet's tail, using a collector shaped like a tennis racket to bag fragments from its dusty plume.
The secret of this collecting "racket" lies with the colored gaseous jelly it carries. This is aerogel, the lightest material on Earth, known as "frozen smoke" by scientists and designed to trap the dust molecules the probe swept up as it passed through the comet's tail.
However, scientists still do not know if any particles have been trapped in this way. They will have to wait until Jan. 15 when Stardust reaches Earth. It will then send the capsule -- containing the "racket," its aerogel, and, hopefully, grains of comet embedded in it -- plunging into the atmosphere at almost 13km per second.
First a drogue parachute will open about 30km above the ground. Then, at 3km, the main parachute will open and slow the craft so it hits the ground at about 4.6m per second.
"There is always some residual risk that something could go wrong on return, but we think that is very low at the moment," said Ed Hirst, Stardust's mission system manager.
In fact, the Stardust capsule could survive an impact even if its parachute does not open, its designers claimed.
"We know we can finish our science," said the project manager, Tom Duxbury.
Brownlee was equally confident. "It's a charmed mission," he said.
After landing, the capsule will be taken to the Stardust Laboratory at the Johnson Space Center in Texas, where its aerogel package will be broken open. Inside scientists expect to find about 1,000 grains from Wild 2. In total, the US$430 million probe will return about 1 milligram of comet matter, less than a thimbleful.
"Some scientific secrets will come out almost immediately," Brownlee said.
"Other things will take years. But we expect to get a significant scientific return within the first couple of months of investigation," he said.