By Dr David Whitehouse
BBC News Online science editor
The Genesis probe is a week away from the dramatic conclusion to its mission: returning to Earth a sample of the solar wind, particles from the Sun.
The dot is Genesis as tracked by the 2.24m telescope at Mauna Kea in Hawaii
Astronomers have been tracking the tiny craft, measuring its trajectory and planning a final course correction before its capsule descends to Utah, US
To preserve the delicate cargo, it will be captured mid-air using helicopters flown by Hollywood stunt pilots.
It is the first return of material to be retrieved from beyond the Moon.
A piece of the Sun
On 8 September, the drama will unfold over the skies of central Utah, over the US Air Force's test and training range, when the spacecraft's sample-return capsule will be snagged by helicopter
"The Genesis mission - to capture a piece of the Sun and return it to the Earth - is truly in the Nasa spirit: a bold, inspiring mission that makes a fundamental contribution to scientific knowledge," says Genesis's Steven Brody.
"What a prize Genesis will be," says principal investigator Don Burnett of the California Institute of Technology. "Our spacecraft has logged almost 27 months far beyond the Moon's orbit, collecting atoms from the Sun.
The pilots have been practising their catching technique
"With it we should be able to say what the Sun is composed of at a level of precision that has never been seen before."
Genesis was launched on 8 August 2001. Then, on 3 December 2001, it unfurled its collecting panels and started an 884-day period gathering solar particles at the L1 gravitational balance point between the Earth and the Sun.
On 2 April 2004, the panels were stowed in Genesis's sample-return capsule and the spacecraft fired its thrusters for home.
But if the capsule were to descend all the way to the ground, some of its precious cargo would be destroyed. Hence, the mid-air retrieval by helicopter.
Spy satellite technique
"These guys fly in some of Hollywood's biggest movies," says Genesis project manager Don Sweetnam, "but this time, the Genesis capsule will be the star."
Although catching a payload returning from space this way is a first for a civilian scientific mission, it is a technique that was used routinely by the US military between 1960 and the mid-1980s.
Canisters containing film from spy satellites were grabbed this way during the Cold War. It is estimated US Air Force pilots may have done it 300 times.
The return of the Genesis cargo will be a trailblazer for other sample-return missions. On 15 January 2006, the Stardust probe will return with its cometary samples over the same Utah test range. A Japanese probe may return material from an asteroid in 2007.
Genesis was launched in 2001
The incoming Genesis probe will be of particular interest to other scientists as well.
Those studying the problem of finding and tracking asteroids that may prove a threat to Earth will be watching it closely.
They plan an experiment to measure its position in space and determine at what point it will be possible to say it will definitely strike the Earth.
The researchers say the experiment may be tricky as Genesis will make its own course-corrections but they do think it will be a valuable exercise.