Blasted by icy particles striking at 4 miles per second - six times faster than a rifle bullet - NASA's armored Stardust probe flew within 150 miles of a 3-mile-wide comet today, capturing primordial debris left over from the birth of the solar system 4.2 billion years ago.

A view of the icy nucleus of Comet Wild 2 as seen by the Stardust spacecraft today. Credit: NASA/JPL

Engineers monitoring radio signals from the spacecraft erupted in cheers and applause as the moment of closest approach came and went with little or no degradation in the stream of data flowing back to Earth. That steady signal meant Stardust had survived its unprecedented encounter with comet Wild-2, the culmination of a five-year voyage spanning more than 2 billion miles since blastoff in February 1999.

"We're a jubilant crowd!" said Tom Duxbury, Stardust project manager at the Jet Propulsion Laboratory. "Everything so far has gone by the book. ... Life is tremendously good. We've flown through the worst of it and we're still in contact with our spacecraft. ... The encounter just went off tremendously."

The spacecraft's navigation camera snapped photos of Wild-2's frozen nucleus as the spacecraft made its final approach and NASA planned to share the images with reporters at a late afternoon news conference.

"The science that's going to come out of this, that's going to tell us about the early formation of our solar system, the role that comets have played in the formation of Earth and ourselves, that will unfold over the next few years," Duxbury said. "The science that this project is returning will be unprecedented."

From this point on, it's all downhill for the Stardust team as the spacecraft falls back into the inner solar system. If all goes well, a small re-entry pod carrying the captured comet stuff will slam into Earth's atmosphere in January 2006, parachuting to a touchdown in Utah.

From there, the priceless cargo will be flown to the Johnson Space Center in Houston where eager scientists will begin chemical analyses of the captured particles. The results are expected to answer long-standing questions about the cloud of dusty debris that coalesced to form the solar system and whether comets helped seed planet Earth with water and the organic building blocks of life.

"Comets can tell us about the history of the early solar system and the early history, perhaps, of our own Earth," said co-investigator Martha Hanner of NASA's Jet Propulsion Laboratory. "Comets formed four-and-a-half billion years ago along with the planets. Whereas planet Earth and other planets have undergone lots of geological changes since that time, the comets ... we see today have been essentially parked in cold storage for much of the four-and-a-half-billion-year history of our solar system. So they're truly frozen time capsules and that's why we're so interested in them."

The Stardust flyby came on the eve of another historic milestone in planetary exploration, the planned landing of a sophisticated robot field geologist on Mars late Saturday evening. "With the momentum we have had and our success," Duxbury said today, "we hope it carries over to our MER (Mars Exploration Rover) colleagues."

The Stardust probe began its seven-your voyage Feb. 7, 1999, with a near perfect launch from the Cape Canaveral Air Station in Florida atop a Boeing Delta 2 rocket. Built by Lockheed Martin under a $91.2 million contract, Stardust was the fourth in a series of low-cost Discovery-class missions launched by NASA as part of former Administrator Daniel Goldin's "faster, cheaper, better" approach to interplanetary exploration. It also was the first of four planned U.S. and European missions that will target comets over the next decade.

An illustration of the Stardust spacecraft. Credit: NASA

Carl Pilcher, science director for solar system exploration at NASA headquarters, said missions like Stardust are important because comets "are the most fundamental building blocks of the solar system. The solar system formed from the same material, but that material is only present today in the sun. The problem is the sun is all atomic elements and ions, it doesn't preserve any of the chemistry, it doesn't preserve any of the molecules.

"But comets do," he said when Stardust was launched. "In fact, they preserve not only some of the rocks and metals but they also preserve the ices and organics, which are really fundamental. Because the Earth and the other large bodies that formed close to the sun lost all their original ices and organics (because of) a combination of heating from the sun as well as the heat from the formation of the planets themselves."

But not comets.

"Comets are the stuff that life is made of," Pilcher said. "The organics we find in meteorites, which we think are similar to the organics we would find in comets, contain lots of amino acids and amino acids are the building blocks of proteins. So comets probably delivered lots of amino acids to Earth as well as enough water to fill Earth's oceans.

"The fact that life on Earth formed very quickly after the Earth formed itself is probably due to all this material that was brought into Earth by comets," Pilcher concluded. "Comets probably delivered this same material to Venus and Mars as well, so comets are a link between our study of the formation of the solar system and our study of the biological potential of Mars."

Comet Wild-2 was a particularly attractive target for Stardust because it has spent virtually its entire life in the outer solar system. In 1974, however, the comet made a close flyby of Jupiter, which deflected it into a different orbit that has since carried it around the sun three times. Compared to other short-period comets, Wild-2 should be relatively pristine, providing an unprecedented window on the birth of the solar system.

The probe's trajectory was designed, in a sense, to let the comet run over the spacecraft from the right rear at about six times the speed of a rifle bullet. Just before closest approach, a two-sided 14-inch-wide dust collector shaped like a tennis racket was extended into the dust stream. Cells on the back side of collector were used earlier in the flight to collect interstellar dust grains. Both sides feature 132 cells filled with aerogel, an ultra low-density material invented in 1933. Sometimes referred to as frozen smoke, aerogel is made by mixing silica and a solvent.

Aerogel is "the magic material that makes this possible," said Joseph Vellinga, Lockheed Martin's Stardust program manager. "A particle hitting aerogel at hyper velocity essentially forms a little heat shield in front in the aerogel as it melts. The particles are coming in at about six times the speed of a rifle bullet, about 14,000 mph. As the kinetic energy is absorbed, you're melting the aerogel as you bring it to a stop in one to three centimeters or so. A lot of kinetic energy is absorbed in the aerogel, keeping the heating of the particle down [below 1,000 degrees Fahrenheit], preserving the organic materials."

While scientists can determine a comet's elemental composition remotely, they need actual samples to study the structure of the particles and "tell how that particle may have formed or whether it had a previous history before it was frozen into the comet," Hanner said. "What's often very telling about the history of a little particle is looking at the elements that are not very abundant, isotope ratios, or looking for traces of organic material. Those are only there in very small amounts."

"On planet Earth, we can have a very sophisticated laboratory with electron microscopes and many other instruments you couldn't possibly bundle and fly in a spacecraft," she said.

Video coverage for subscribers only:
   VIDEO: STARDUST SURVIVES CLOSEST APPROACH TO COMET QT
   VIDEO: PRE-ENCOUNTER INTERVIEW WITH PROGRAM MANAGER QT
   VIDEO: MISSION PLANNER DESCRIBES STARDUST'S CHALLENGES QT
   VIDEO: DESCRIPTION OF THE STARDUST SCIENCE PROGRAM QT
   VIDEO: REPORT ON THE STARDUST SCIENCE INSTRUMENTS QT
   VIDEO: INTERVIEW EXPLAINS STARDUST'S CAMERA SYSTEM QT
   VIDEO: LEARN MORE ABOUT THE EXOTIC AEROGEL MATERIAL QT
   VIDEO: AN INTERVIEW WITH STARDUST'S NAVIGATOR QT
   VIDEO: A LOOK AHEAD TO DEEP IMPACT COMET MISSION QT

   VIDEO: OVERVIEW OF THE SEVEN-YEAR STARDUST MISSION QT
   VIDEO: ANIMATION OF STARDUST COLLECTING COMET SAMPLES QT
   VIDEO: STARDUST RETURNS TO EARTH AND PARACHUTES TO UTAH QT
   VIDEO: ENCOUNTER DETAILED BY 39-MINUTE NEWS CONFERENCE QT
   ADDITIONAL VIDEO COVERAGE
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