The Stardust probe is approaching its rendezvous with Comet Wild-2.
By Christine McGourty
BBC science correspondent in Pasadena, California
The craft has been racing through space for five years to get itself into a position on Friday where it will be within 300 km of the "dirty ice-ball".
Stardust will photograph the object and grab particles streaming away from its nucleus for return to Earth in 2006.
Scientists believe the samples will reveal a great deal not only about the construction of comets but also about the early history of the Solar System.
The rendezvous is set for 1940 GMT. At this time the comet - which is 5.4 kilometres (3.3 miles) across - will sail past the US space agency (Nasa) spacecraft at a speed of 21,960 km per hour (13,650 miles per hour).
"In recent decades, spacecraft have passed fairly close to comets and provided us with excellent data," said Dr Don Brownlee, of the University of Washington, principal investigator for the Stardust mission.
"Stardust, however, marks the first time that we have ever collected samples from a comet and brought them back to Earth for study," he added.
Dr Simon Green of the Open University, part of the British team involved in the mission, said it had been agreed just this week that the encounter distance was as close as the scientists felt it was safe to go without risking destruction of the spacecraft.
The mission is being operated from Nasa's Jet Propulsion Laboratory in Pasadena, where researchers will be holding their breath during the crucial approach.
Dr Don Brownlee told BBC News Online that these would be tense moments for the Stardust team: "We're very excited. We've been in space for five years. We've travelled three billion km, and all for this day, to collect this sample.
The Stardust sample return capsule will touch down in the Utah desert
"There's an element of risk here. If we get hit by a rock it may cause problems. We think we're well protected from it but we'll be much more relaxed a couple of minutes after the closest approach."
British scientists in Pasadena for the mission will be analysing the results from the spacecraft's dust monitor immediately after the encounter, to glean instant information on the particles collected.
Dr Neil McBride explained the ultimate aim: "We're attempting to collect small cometary dust particles from the coma, from near the nucleus, and bringing them back to Earth.
"This is done with a sensor, which is made of aerogel - a very low density glass.
"As a cometary particle impacts into it, it gets slowed down and held in the aerogel."
Faster than a bullet
Tom Morgan, Nasa Stardust programme scientist, said the onboard instruments would provide interesting data during the fly-by, but the most interesting results will come in two years' time when the capsule containing the samples collected completes its long journey back to Earth and parachutes down to the desert in Utah.
"We'll have a chance to look directly at the most primitive material in the Solar System, the material from which the planets were formed," he said.
MISSION TO CATCH COMET DUST
Stardust launched in Feb 1999; return to Earth in Jan 2006
Probe will come within 260 km of the comet nucleus
It is 4.8 metres (15.9 feet) long, including solar panels
Its total weight is around 385 kg (848 lbs)
Comet dust will be caught in an aerogel collector (above)
The project manager for Stardust, Tom Duxbury, described the spacecraft as a "wonderful machine" and said it would face some risks in the coming hours.
"In order to collect these particles we have to put our spacecraft in harm's way.
"We have to fly through the coma of Comet Wild-2. This is a tremendously severe environment, where we're going through a storm of dust particles hitting us at over six times the speed of a bullet.
"It's leading edge planetary exploration."
Special shields have been designed to protect the spacecraft from the dust. These will be pointed in the direction of the incoming particles.
The dust collector, on a robotic arm that's been described as something like a tennis racket, has been deployed in the last few days, and is ready to absorb the samples, which will become embedded in the aerogel.
During the flyby the spacecraft is under its own control. It will turn itself to point the onboard camera at the nucleus during the encounter.
Tom Duxbury said this would be "the most tense period of the mission". Until that point, the spacecraft will be sending high rate telemetry data to Earth.
Few close-up images of comets have been obtained
"After it turns itself to point the camera we'll just be getting a signal to tell us that it's okay," he said. "A few minutes after the flyby the spacecraft will automatically turn itself back to Earth, point its high gain antenna back to Earth and start dumping data again.
"That's when our white knuckles turn back to normal colour and we start breathing normally. It's going to be quite exciting."
His colleague Dr Simon Green said the results could provide a new window into the distant past.
"Comets are made of ice and are very cold and have been very cold since they were formed.
"That protects the material of which they were made from any process of heating, so they haven't been changed since they were formed, right at the beginning of the formation of the Solar System. So we can have almost a little time capsule of what things were like 4.5 billion years ago."
Europe's daring mission to land on a comet - the Rosetta mission - is set for launch in February.