Back home, the probe that could unlock secrets of solar system

A space capsule from the Stardust mission, carrying dust and comet fragments that scientists hope will unlock some of the most ancient secrets of the solar system, has landed safely in the Utah desert - the culmination of a hazardous and potentially ground-breaking seven-year mission.

Nasa scientists and researchers at the Johnson Space Center in Houston cheered as the capsule parachuted to earth on the arid salt flats west of Salt Lake City. A helicopter recovery team located the capsule shortly after its landing early yesterday morning and ferried it to a nearby military air base, where it will stay until its transfer to Nasa headquarters tomorrow.

"It's an absolutely fantastic end to the mission," Nasa scientist Carlton Allen told reporters.

Stardust was launched on its interstellar mineral collection expedition in February 1999. Five years later, it collected an estimated million or so fragments, some of them thinner than a human hair, from a comet called Wild 2 that was passing near Jupiter. The fragments, some of them from the comet, others possibly from outside the solar system, were collected in a large, tennis-racket shaped device smeared with a light, porous material called aerogel.

Scientists are excited because they believe the comet fragments may be older than the solar system and thus capable of offering vital new clues on the formation of the sun and planets around us. If, as expected, the mission collected interstellar dust carried in the comet's wake, researchers may also have their first opportunity to examine unaltered pieces of primordial matter from outside the solar system.

"The samples we collected are the same particles that went into the formation of the comet four and a half billion years ago," the Nasa mission's principal investigator, astronomy professor Donald Brownlee, told National Geographic magazine. That makes Wild 2 almost exactly the same age as the solar system.

Since comets are regarded as the building blocks of planets and suns, the residues collected from Wild 2 may be uniquely able to tell a very ancient story. Professor Brownlee likened the haul to the discovery of an ancient book we are still able to read and understand.

Wild 2 offered a unique set of circumstances - a comet close enough to earth to be reachable, and one travelling fast enough to prevent the destruction or alteration of its particles by the heat of the sun. Wild 2 entered the sun's orbit in 1974 and was then tugged in by Jupiter's gravitational pull until it came almost as close as Mars.

Stardust made its fragment collection in January 2004. The dust and other particles hit the aerogel at about six times the speed of a bullet.

Bringing the cache safely back to earth was far from a failsafe proposition. Stardust's sister spacecraft Genesis sent a capsule back in September 2004, but the parachutes failed to open as it hurtled towards the Utah desert and its harvest of solar wind particles was exposed to the elements and compromised.

Until yesterday, no mineral retrieval operation had successfully returned to earth since the unmanned Soviet Luna 24 mission in 1976, which brought back moon rocks and soil. Stardust itself travelled three billion miles and circumnavigated the sun three times. Two years ago, it beamed back 72 black-and-white photographs of the surface of Wild 2, showing broad mesas, craters, pinnacles and canyons.

Even after the successful return of the capsule, the mothership remains in orbit and is on standby for further missions to other comets or asteroids.