By Kathy Sawyer
Washington Post Staff Writer
Monday, February 1, 1999; Page A09

On Dec. 13, 1972, Apollo astronauts Eugene Cernan and Harrison Schmitt took what turned out to be the century's final walk on the moon, bagging the last chunks of another world that would be returned to Earth by human design before the millennium.

Nobody knows when humans will again set foot on the moon. But NASA's Stardust mission, scheduled for launch on Saturday, will mark a revival in the business of sending out emissaries to grab up bits and pieces of the solar system and bring them back for study. This time, however, no people need apply.

The tiny robotic Stardust spacecraft is designed to fly closer to a comet than any machine in history. Its assignment is to plunge into the vast and dazzling vapor cloud that surrounds the comet's hard core and, for the first time, capture grains of the primordial stuff from which nature constructed the sun, planets and all known life. The 101-pound capsule carrying this trove is scheduled to streak back to Earth and parachute into the Utah salt flats for pickup by eager scientists on Jan. 15, 2006.

Comets are thought to be the oldest, most primitive bodies in our solar system, storehouses of uniquely well-preserved remnants from the swirling nebula of "stardust" and gas that formed the sun and planets about 4.6 billion years ago. And as the sun's family evolved, comet impacts helped shape the destinies of the planets, perhaps single-handedly delivering enough water to fill Earth's oceans.

These fragile bergs of ice, rock and organic (carbon-based) molecules, sometimes described as dirty snowballs, "are the most fundamental building blocks of the solar system," said Carl Pilcher, NASA's director of solar system exploration. "It seems most likely that the comets were the source of the water and organics that ultimately . . . enabled life to form on Earth."

Some comets have become famous for their flamboyant intrusions into Earth's skies. But they spend the bulk of their lives in the deep freeze beyond the orbit of Pluto. Only when their elongated, egg-shaped orbits swing them in close to the sun do their surfaces heat up and vaporize into a glowing halo of fuzz, sprouting tails of luminous debris (and less visible gases) that extend millions of miles into space.

Stardust's target is a comet called Wild-2 (pronounced "Vilt-2"), whose path was only recently diverted closer to the sun and Earth by a 1974 close encounter with Jupiter's powerful gravity. Since Wild-2 has rarely ventured into this warmth, presumably little of its chemical treasure has been cooked away.

"The scientific goal of Stardust is to collect the building blocks of planets--not only in our solar system but in other planetary systems as well, and put them under the microscope and other instruments where we can study them at phenomenal resolution, down to the single atom," said lead Stardust scientist Donald E. Brownlee of the University of Washington.

Following launch from Cape Canaveral atop a Delta II rocket, the spacecraft faces a 2.3 billion-mile journey that includes a loop back around Earth to get a boost from the planet's gravity. Stardust will position itself essentially to be "run over" by the comet on Jan. 2, 2004, some 242 million miles from Earth at a relative velocity roughly "six times the speed of a rifle bullet," said project manager Kenneth Atkins of the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. This is a relatively gentle encounter, by comet standards, presumably positioning the craft to capture intact particles by the tens of thousands.

Catching hypervelocity stardust without damage is tricky, however. For this, the mission adapted a technique developed by JPL's Peter Tsou. The key is aerogel, a substance resembling what Brownlee called "frozen smoke" that was used as an insulator on the Mars Pathfinder's Sojourner rover.

As the comet overtakes Stardust, the craft will flip up a kind of catcher's mitt filled with this glass foam in an array that looks like ice cube trays. The aerogel has enough "give" to stop a grain of dust with minimum damage, and it preserves a carrot-shaped track that serves as a pointer to the particle's resting place. (The researchers noted that, although they expect no bacteria or viruses on a comet, the captured particles will be heated to killing temperatures at impact and the aerogel will instantly seal them in glass.)

Equipped with bumpers to shield its vital workings, the 5.6-foot-tall Stardust is aiming to pass within about 93 miles of the comet's hard nucleus--presumably far enough away to avoid damaging collisions (although the scientists acknowledged that this is unknown territory) but close enough to sample the freshest possible material as it erupts or vaporizes off the surface.

The craft will record its 10-hour passage through the Wild-2 blast with a camera and other instruments. The images should provide 10 times as much detail on the comet's nucleus as the previous best, from an international armada of craft that scrutinized Halley's comet in 1986, Brownlee said.

Along its route, Stardust also will collect particles from a storm of interstellar dust flowing through our solar system from the space between the stars. As in the proverbial "dust to dust," the team wrote, this represents "the ultimate in recycled material; it is the stuff from which all solid objects in the universe are made and the state to which everything eventually returns."

The $200 million Stardust mission is one of several missions planned in coming years to return the first samples ever snatched from beyond the orbit of the moon. The Genesis mission, scheduled for launch in 2001, is to return specimens from the "wind" of energetic particles that flows constantly from the sun. A 2003 mission called Deep Space 4 is to demonstrate sample-collecting technologies on the surface of Comet Tempel 1.

Perhaps the most eagerly awaited samples are those anticipated from a 2006 mission to Mars. Robot geologists are to pick up rocks and soils that could help scientists determine whether microscopic life ever developed there--life whose ingredients may have been delivered, like Earth's, in an ancient comet barrage.