A space capsule loaded with comet dust completed a seven-year mission on Sunday, touching down safely in the Utah desert before dawn to the jubilation of cheering NASA scientists.

The landing capped a 4.7-billion-kilometre journey by NASA’s 45-kilogram Stardust spacecraft, which whizzed past a comet in 2004 to capture minute dust particles and store them in the capsule.

“It’s an absolutely fantastic end to the mission,” said Carlton Allen, a scientist with NASA’s Johnson Space Center.

Researchers believe about a million samples of comet and interstellar dust — most which are expected to be a tenth as wide as a piece of human air — are locked inside the capsule.

The dust grains are believed to be leftovers from the process of planet formation, with some of the particles thought to be older than the sun.

Scientists hope the pristine particles collected will give them clues about their chemical makeup and the origins of the early universe.

The samples were gathered in 2004 from the comet Wild 2, a frozen body of ice and dust believed to have been formed billions of years ago.

The particles were snagged by a tennis racket-shaped collector mitt containing ice-cube-sized compartments lined with aerogel, a porous substance that is 99.9 per cent air.

The samples became lodged in the aerogel before being sealed inside the capsule.

Early Sunday, four hours after leaving the probe, the capsule entered the Earth’s atmosphere 125 km over the Pacific Ocean.

It reached record speeds of 46,440 km/h, the fastest return for any man-made probe, and was visible from parts of the American northwest as a bright orange fireball.

As it neared the desert, the first parachute opened at a 32-km altitude, followed by a larger chute at about three km that guided it to a landing on the salt flats.

The capsule bounced three times before coming to rest on its side at the U.S. Air Force Utah test and training range, two minutes ahead of schedule at 3:10 a.m. local time, NASA officials reported.

“All stations, we have touchdown,” an announcer announced to an exultant control room that burst into cheers and applause.

A helicopter recovery team located the capsule on the desert floor and was transferring it to the nearby Michael Army Air Field.

The capsule will be flown to the Johnson Space Center in Houston early next week where scientists will unlock the canister containing the cosmic particles.

“Inside this thing is our treasure,'’ principal mission scientist Don Brownlee of the University of Washington told The Associated Press.

The successful landing was a relief for scientists after the 2004 Genesis mission, when the returning craft carrying solar wind particles crashed to Earth when its parachute failed to deploy, and cracked open, exposing the solar atoms to contamination.

The incident prompted the Stardust team to spend six months reviewing the spacecraft design as a precaution, and NASA officials said they were prepared for a hard landing.

Those concerns, however, were unfounded, as the capsule’s return went as planned.

“It’s most like a proud parent at the graduation of a magna cum laude student,” Ken Atkins, a former Stardust project manager who is now retired, told Reuters of the smooth landing.

Genesis and Stardust were the first robotic retrievals of extraterrestrial material since 1976, when the unmanned Soviet Luna 24 mission brought back lunar rocks and soil.

Stardust’s mission, which began in 1999, took it around the sun three times and halfway to Jupiter to catch particles from Wild 2 in January, 2004.

The mission, managed by NASA’s Jet Propulsion Laboratory, cost $212 million US, and it isn’t over.

The mothership, which severed the cables connecting itself and the capsule, is in permanent orbit around the sun, and may be used in future missions to study planets, asteroids or comets.