By WILLIAM J. BROAD It is the stuff of people, as well as cats, plants, buttons, shoes, seas, planets, comets, moons, cars, books and cell phones, not to mention the paper (or computer screen) on which these words appear: star dust.
For decades, science has known of the importance of star dust in the cosmic scheme of things, especially in matter more elaborate than the simple elements made in the primordial Big Bang.
But scientists have never had pure samples to study. Soon, though, they will, if everything goes as planned.
On Saturday, the National Aeronautics and Space Administration is to launch a probe meant to capture interplanetary and interstellar dust in some of its rawest and purest forms, including many thousands of grains from a comet.
After a seven-year trek, the craft is to return the samples to Earth, the first ever from beyond the Moon.
Appropriately enough, the mission is dubbed Stardust.
"We want to find out what this stuff is like," said Dr. Donald C. Brownlee of the University of Washington, the chief scientist of the mission. "It's the building block of planets and human bodies."
Dr. Alastair G. W. Cameron, an astrophysicist at Harvard University, said scooping up star dust would greatly expand a field that has recently undergone "a revolution" in understanding not only how stars live and die but thrive in surprising diversity.
The urge to investigate is driven by more than curiosity. Earth sits in a cosmic shooting gallery, surrounded by speeding blocks of ice and rock. The last time a really big one hit, 65 million years ago, the tumult helped do in the dinosaurs, among other creatures.
Today, scientists are seeking to learn more about comets and other clumps of star dust in case earthlings might one day want to divert one of the intruders, which can threaten more ruin than all the world's nuclear arsenals combined.
Dr. Don Yeomans, a senior scientist at the Jet Propulsion Laboratory in Pasadena, Calif., which manages the Stardust mission, said it was only prudent to find such objects, learn their structure and "keep an eye upon their future trajectories."
Most intriguing of all, scientists hope a harvest of Stardust data will yield clues to the origin of life on Earth, and perhaps elsewhere. A rain of cosmic and cometary material is widely thought to have brought early Earth not only water but the carbon-rich molecules necessary for life.
Indeed, some scientists believe the speeding mountains of dirty ice known as comets may themselves support primitive life. While highly speculative, such thinking has been bolstered ever so slightly in recent years by the discovery of earthly microbes dwelling in all manner of extreme environments from hot springs to polar ices.
The possibility of cometary life is real enough that NASA asked some of the nation's top biologists and other scientists to assess whether the returning probe might inadvertently infect Earth with a plague of microscopic aliens.
That risk was ruled out. But scientists say there is a very slim chance the craft will carry back dead extraterrestrials killed by heat during capture.
"We concluded that the collection method would not allow any organisms to live," said Dr. Clark R. Chapman, a senior scientist at the Southwest Research Institute, based in San Antonio, and a member of the review team. "But they might come back mashed and partly melted."
The backdrop for the mission was set some 12 billion to 16 billion years ago when repercussions of the cosmic explosion known as the Big Bang filled the expanding universe with hydrogen and helium, the simplest elements. They burned as stars flashed to life. The starry furnaces in turn transformed the hydrogen and helium into a variety of heavier elements, including carbon, nitrogen, oxygen, silicon and iron.
Such ashes were blown into space from parent stars by stellar winds or blasted out by stellar explosions, the great heats of which are thought to have made still heavier elements.
Over generations of stars, a mix of gas and dust formed in interstellar space that included grains of graphite (pure carbon), silicon carbide (carbon and silicon) and water ice (hydrogen and oxygen).
The mix is thought to have gotten richer still as new stars lit up nearby dust clouds with bursts of ultraviolet light, turning simple atoms and molecules into more complex ones like methane, methyl alcohol and dozens of other so-called organic compounds rich in carbon.
Such intricate dusts in turn formed new generations of stars, perhaps accompanied by planets and icy comets not unlike those of our own solar system, which is thought to have condensed out of a dusty cloud some 4.6 billion years ago.
Today, scientists know about this cosmic chemistry in part by having peered with telescopes into the icy depths of space and found the signatures of all kinds of simple and complex molecules. Joni Mitchell, it seems, got it right in her song "Woodstock" when she sang, "We are star dust." The celestial mix (much water, many organic chemicals) matches that of the human body better than Earth's general makeup does.
In our solar system, most of the original cloud has vanished. It was pulled into the Sun, blown into deep space or turned into planets where internal heats and actions altered the materials still further.
But some of the original cloud survives, mainly in the form of billions of comets. Other primal matter falls to Earth in the form of rocky meteorites or as a steady rain of cosmic and cometary dust.
"That's the irony," said Dr. Brownlee of the University of Washington, who pioneered catching such dust in high-flying jets. "We have cometary particles all over us. Anybody who has been outside today has a cometary particle on them. But you can't find them or where they came from."
The reason for the uncertainty is that the particles are mixed up amid earthly and celestial contamination and have often been altered by heat upon hitting Earth's atmosphere.
Despite its ambitious goals, Stardust is one of NASA's cheaper space probes, costing $166 million. Contributing to the low price is the recycling of spare parts from earlier craft like Voyager, Galileo and Cassini.
After launching from Florida, Stardust will head for deep space. On approaching interstellar dust streams and the target comet, known as Wild-2, it will deploy, on command from Earth, a particle catcher shaped like a tennis racket.
The catcher is covered on both sides with an extremely low-density foam known as aerogel. This semitransparent material will slow and stop particles without altering them too much. One side will be used for the comet; the other for the interstellar dust.
In 2000 and 2002, the craft will gather dust in the regions between Mars and Jupiter. Recently, other probes moving through these areas have found a strong flow of particles from roughly the direction of the constellation Sagittarius, toward which the Sun is moving.
In January 2004, Stardust is to encounter Wild-2 -- a glob of dirty ice a few miles wide. It is considered an ideal target because it only recently has been deflected by Jupiter's gravity from a distant orbit into the inner solar system, so its outer layers have undergone relatively little solar heating.
Wild-2 now travels in a looping path from just outside Jupiter's orbit to just inside that of Mars, where it makes its closest approach to the Sun and reaches peak activity.
The spacecraft, 97 days after that peak, is to zoom past the comet. The rendezvous site lies beyond the red planet. Still, the Sun will be able to coax the ice ball into shedding dense swarms of particles and vapors.
At 13,600 miles per hour, the spacecraft is to swoop through the comet's coma, the globular cloudlike mass that makes up the head, passing within 100 or so miles of the core. It will capture dust and photograph the nucleus. Particles will hit the dust catcher at up to six times the speed of a bullet fired from a high-powered rifle, NASA estimates.
The craft's front end and solar panels are shielded with armor plates to protect Stardust from the storm of icy particles.
Scientists say the particles will range from roughly 100 microns (twice the width of a typical human hair) to less than a micron (one-fiftieth of a human hair). A few, they judge, will be large and a million or so, smaller than a micron.
After the collecting is done, in January 2006, the craft's 32-inch-wide return pod is to descend by parachute toward the United States Air Force Test and Training range in Utah. The planned landing site is 100 or so miles southwest of Salt Lake City in the desert.
Then, scientists will begin analyzing its samples, working in particular to establish as firm a baseline as possible for the makeup of cosmic dust.
"It will give us ground truth," Dr. Brownlee said.
Scientists also hope to address a number of mysteries in the analysis of dust particles captured by jets and retrieved from meteorites, especially odd ratios in them of rare isotopes, which have the same chemical properties of parent elements but different atomic weights.
In theory, studies of the samples could illuminate the evolutionary history of stars and establish a firm link between interstellar dust, the state of the solar system's early cloud, the nature of comets and the materials that today make up Earth and its inhabitants.
NASA discounts the possibility that any living alien creatures will hitch a ride to Earth.
"Comets are extremely unlikely places for life," Dr. John Rummel, the agency's planetary protection officer, said at a recent press conference.
Nevertheless, NASA had the National Academy of Sciences, the Congressionally chartered group of leading scientists that advises the Government, go over the mission plan carefully. A 10-member panel appointed by the Academy's National Research Council concluded in a report last year that cometary life was unlikely but that "the possibility cannot be completely ruled out."
In addressing the alien issue, Dr. Brownlee noted that though most of the cosmic particles that fall naturally to Earth are heated, some are not. Therefore, in theory, cometary life may already have arrived on the planet long ago.
"Nature," he noted, "is always full of surprises."
