By Helen Briggs
BBC News science reporter
A capsule containing comet particles and interstellar dust is plunging towards Earth after being released by an orbiting spacecraft.
The Stardust mission cost $212 million (£176 million)
The material was gathered by the US Stardust probe after a seven-year mission to fetch fragments of a comet.
Scientists believe the pristine samples will yield clues to the origin of the Solar System.
The capsule is expected to land in the Utah desert at 0312 (1012 GMT) after a fiery descent slowed by parachute.
Mission controllers at the US space agency (Nasa) Jet Propulsion Laboratory in Pasadena, California, clapped as the capsule was released at 0557 GMT by its "mothership" 111,020km (69,000 miles) above the Earth.
"We are extremely excited that the sample return capsule has separated from the main spacecraft as planned and we are looking forward to a successful entry and landing on time," former Stardust project manager Ken Atkins, told the BBC News website.
"We will be waiting at the station to collect the luggage."
About four hours after leaving the probe, the 101lb (45kg) capsule entered the Earth's atmosphere 125km (410,000ft) over the Pacific Ocean.
It reached speeds of 46,660kph (29,000mph) - the fastest re-entry of any manmade probe - and was visible from parts of the American northwest as a streak of light in the sky.
At about 32km altitude (105,000 feet), the capsule released a small parachute to slow its descent.
The main parachute is designed to open at about 3km (10,000ft), and bring the capsule down to a soft landing on a military base southwest of Salt Lake City.
If all goes to plan, the samples will be picked up by helicopter and flown to an army building, then to a special lab at Nasa's Johnson Space Center in Houston, Texas.
Meanwhile the mothership probe will go into permanent orbit around the Sun, after travelling almost 4.7 billion km (3 billion miles) around the Solar System.
Stardust was launched on its mission to capture dust and debris surrounding a comet in 1999.
It swept up particles from Comet Wild 2 in January 2004, as it flew within 240km (149 miles) of the frozen body of ice and dust.
As part of its trip, the probe also captured a sprinkling of dust that would have originated in distant stars.
It did this by extending a retractable device containing cells filled with aerogel, a porous substance designed to trap dust molecules.
One side of the collector was used for chasing the comet, the other for interstellar dust.
Comets are thought to be cosmic "time capsules", containing material unchanged since the formation of the Sun and planets.
Some even think they may have seeded Earth with the chemical building blocks required for life.
"The goal of Stardust is to collect the original building blocks of the Sun, the planets and even ourselves," Don Brownlee, Stardust principal investigator at the University of Washington, Seattle, said last week.
"When I look at the picture of a comet, what I see is a library that has picked up records of the Solar System and has been storing them away from the Sun for 4.5 billion years."
Hundreds of scientists around the world are waiting to analyse the precious samples, including planetary experts at the Open University, UK.
Dr Ian Franchi, of the Planetary and Space Science Research Institute, said it was a fantastic opportunity.
"This is the first planetary material brought back from beyond low-Earth orbit for 30 years - since the Apollo and Russian lunar samples," he told the BBC News website.
"The stuff from the comet is as it was 4.5 billion years ago when everything in our Solar System formed.
"This will help us understand how the Sun and the planets formed, and what they formed from."
Dr Franchi's research group has been trying to recover samples from the ill-fated Genesis mission, which collected particles that had been blown off the Sun.
The Genesis capsule crashed into the Utah desert in September 2004 when its parachute system failed to open.
The mission was not a complete failure, however, as researchers have been able to extract precious atoms from the smashed remains of the sample chamber.
Pre-launch tests of Stardust's capsule suggest it does not suffer from the same flaw that afflicted Genesis.
"The spacecraft has been checked out to the nth degree after the Genesis crash," said Dr Franchi.
"Assuming that all goes well, it is a much simpler landing scenario than the Genesis mission, the weather conditions are favourable and we are hopeful of getting some fantastic samples to work with in the next few weeks."