Coming Home

NASA via Associated Press

In an image taken last January by the Stardust spacecraft, Wild 2 shows its pockmarked surface.

After covering 2.88 billion miles over seven years, the Stardust spacecraft is nearing home with its minute but precious cargo: samples of what are believed to be the oldest materials in the solar system.

Tucked away in what looks like a giant fly swatter of a collector is dust swooped up from a close encounter with the comet Wild 2 and an accumulation of particles picked up in three circuits of the Sun.

"This has been a fantastic opportunity to collect the most primitive material in the solar system," said Donald Brownlee of the University of Washington, the principal investigator for the mission. "We fully expect some of the comet particles to be older than the Sun."

Comets, icy bodies that normally inhabit a region near Pluto's orbit, are made of material many scientists believe is virtually unchanged since the Sun and the planets formed about 4.6 billion years ago.

Studying comets not only provides clues to how the solar system was created but could also help explain how certain materials and conditions combined to form life, researchers said.

"Comets are a library of our history," said Thomas Duxbury, project manager at the National Aeronautics and Space Administration's Jet Propulsion Laboratory in Pasadena, Calif., which is supervising the mission.

After its launching in 1999, the Stardust circled the Sun and flew by Earth for a gravity boost to rendezvous with Wild 2 (pronounced vilt 2) near Jupiter. On Jan. 2, 2004, the Stardust came within 149 miles of the comet, deploying shields to protect itself from cometary dust while extending a 160-square-inch collector filled with a material called aerogel.

This low-density silicon material, composed of 99.8 percent air, gently slowed and trapped particles without significantly altering or damaging them. Stardust also spent 195 days collecting interstellar particles that flow through the solar system.

The challenge now is to bring them home safely. If all goes as planned, a capsule bearing the space dust will dive into the atmosphere early Sunday morning and gently parachute the samples to the Utah desert.

But bringing the Stardust home will require an orchestrated sequence of events, many of them performed autonomously by the spacecraft. Adding to the tension is the fact that the Stardust return capsule is similar to one released by an earlier NASA probe called Genesis, which crashed to Earth in 2004 when its parachutes failed.

Collector plates that trapped solar particles during Genesis' two-year mission were shattered and contaminated, but scientists are trying to recover some of the science.

"We are convinced that this is not going to happen on Stardust," said Edward Hirst, the mission's systems manager. "We took the lessons learned on Genesis and looked at Stardust."

Both craft were built by Lockheed Martin and share some systems. But engineers said that the faulty switch believed to have failed in deploying parachutes on the more complex Genesis capsule passed testing on Stardust before launching.

Nonetheless, Mr. Duxbury said, NASA has prepared contingency plans to recover the samples in case the mission, which cost $212 million, ends in a crash landing. "If we have an accident and land hard, we still think we can get the science out," he said.

Stardust has begun its final preparations to come home. On Nov. 16, it performed the first of three trajectory correction maneuvers aiming it at a target area southwest of Salt Lake City. The second, Hirst said, was performed Thursday and was a textbook maneuver. "After sifting through all the post-burn data, I expect we will find ourselves right on the money," he said.

The last adjustment, scheduled for Friday, will place the craft in a re-entry corridor for a landing point within an ellipse measuring 47 by 27 miles.

Plans call for Stardust to release its 101-pound sample return capsule on Sunday at 12:57 a.m. Eastern time, when the spacecraft is 68,805 miles from Earth. About 15 minutes later, the main spacecraft is to fire thrusters that divert it from Earth into an orbit around the Sun.

Four hours after release, the three-foot-wide return capsule is to enter the Earth's atmosphere at 410,000 feet above the Pacific. At 28,860 miles an hour, this will be the fastest a human-made object has ever entered the atmosphere. At 200,000 feet, the capsule's heat shield will reach a peak temperature of 365 degrees Fahrenheit, followed 10 seconds later by peak deceleration as the capsule experiences 38 times the force of gravity.

The fireball of the descent should be visible from areas in Northern California, Southern Oregon, Northern Nevada, Southern Idaho and Western Utah, depending upon clouds and the brightness of the Moon, NASA officials said.

At about 100,000 feet, a small pilot parachute is to deploy, and the capsule will begin a vertical descent to 10,000 feet, when the large main parachute will unfurl to lower the craft to the ground at less than 10 m.p.h..

Specialists aboard helicopters or all-terrain vehicles are to converge on the capsule to secure it and document its landing area. From there the space cargo is to be transferred to a temporary, special "clean room" in a hangar at Michael Army Air Field to avoid contamination and then moved to a special laboratory at the Johnson Space Center in Houston.

Dr. Brownlee said Stardust's cargo should contain more than a million particles weighing in total less than a small fraction of an ounce, with only about 2,000 being as large as the diameter of a human hair. But because scientists will be examining them on a molecular scale, he said, they will look like "huge, giant rocks."

There should be enough samples to occupy scientists for decades without consuming them all, he said.