A space capsule loaded with comet dust was barreling toward Earth after being released by its mother ship following a seven-year, 2.9 billion-mile journey to recover materials formed at the dawn of the solar system.

NASA?s Stardust mission will allow scientists to study comet samples for the first time. NASA scientists and engineers at the remote military outpost were nervous but upbeat as they awaited the vessel?s return, according to one Stardust team member.

In what was likely to assuage some of those concerns, the Stardust spacecraft severed the umbilical cables between it and the capsule as scheduled. One minute later, springs on the craft pushed the capsule away, to enter the Earth?s atmosphere roughly four hours later at a speed of 28 860 miles per hour (46 440 km per hour), the fastest of any man-made object on record. The descent may be visible from the ground in northern California, Oregon and Nevada.

Stardust?s mission, which began in 1999, took it around the sun three times and halfway to Jupiter to catch particles from comet Wild 2 in January of 2004. The dust was captured by a tennis-racket-shaped space probe containing ice-cube-sized compartments lined with aerogel, a porous substance that is 99.9 percent air. The particles, most of which are expected to be a tenth as wide as a piece of human hair, became lodged in the aerogel before being shuttered inside the capsule.

Comets are thought to be leftovers from the process of planet formation, and scientists hope the dust collected by Stardust will give them clues about the origins of the solar system 4.5 billion years ago. ?Capturing particles from a comet as it whizzes by is a way of looking back in time?, Jones said. Stardust?s mission marks the first time since 1972 that any extraterrestrial solid material has been collected and brought back to Earth. In 2004, a space probe called Genesis carrying solar ions crashed to Earth when its parachute failed to open.

Shortly after landing, the capsule will be retrieved in the desert by helicopters or ground vehicles, depending on weather conditions. Jones said the team was optimistic that they would be able to use the helicopters, though he was not certain. The samples will be flown to the Johnson Space Center in Houston early next week.

Nichola GROOM