"It's a big event in the sense that it's a mission milestone," said Donald Brownlee, a University of Washington astronomy professor and the mission's principle investigator, or chief scientist. "We don't have to do anything during the flyby. It's all celestial mechanics."

The Earth-gravity-assist phase of the mission actually began on Nov. 14, as the desk-sized craft cruised toward Earth after traveling beyond the orbit of Mars.

Engineers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., have made course changes and other adjustments to ensure that Stardust gains the proper amount of energy from Earth's gravity.

The gravity boost will lengthen the spacecraft's orbit to about 2-1/2 years from the current two years, setting up its rendezvous with Wild 2 in early 2004. Stardust will capture particles being boiled off the comet's surface by solar heating.

Before 1974, Wild 2 traveled the Solar System outside the orbit of Jupiter, shielding it from the sun's heat. But a close encounter with Jupiter that year altered the comet's trajectory, bringing it close enough for particles to be loosened by solar radiation and close enough to make the mission feasible with current technology.

During the gravity-assist phase, the closest Earth approach was at 3:20 a.m. PST on Jan. 15, as the spacecraft flew just southeast of the southern tip of Africa. Its closest approach was 3,700 miles from the Earth's surface while moving at about 22,400 miles per hour.

"The science of the mission is basically ahead of us," Brownlee said. "We've dealt with a number of problems, but they have been fewer than most spacecraft experience."

Perhaps the most heart-stopping problem came last Nov. 9-10, when Stardust was bombarded by photons from a solar flare some 100,000 times larger than normal.

The energy overwhelmed the navigation camera, which is used to help pilot Stardust by focusing on stars and planets and then comparing that image with a star map in its memory.

Normally the camera sees only a few stars at a time, but in this case its electronic imaging chip interpreted the solar flare's photon flashes as thousands of stars.

The spacecraft went into safe mode, with its solar panels pointed toward the sun. Eventually the photon flashes faded and ground controllers were able to reset the star camera.

There also has been some intriguing science. The Cometary and Interstellar Dust Analyzer, operated by Germany's Max-Planck-Institut f�r extraterrestrische Physik, came up with an unexpected analysis of interstellar particles the spacecraft encountered.

"The surprise is that they were high-molecular-weight materials, probably large organic molecules," Brownlee said. "It would be something analogous to tar or coal."

Stardust was launched from Cape Canaveral, Fla., on Feb. 7, 1999, and this is its first return to its home planet. The next encounter with Earth comes in January 2006, when the return capsule will separate from the spacecraft and parachute into the Utah desert.

The capsule will carry comet material and interstellar dust particles, captured in a wispy material called aerogel. The particles will be sent to laboratories around the world for analysis. It is expected the samples will yield clues to the origins of the Solar System and possibly life itself.

The spacecraft's encounter with the comet will occur just outside the orbit of Mars, 242 million miles from Earth. It would be the first mission since Apollo 17 in 1972 to return extraterrestrial samples to Earth, and the first to bring back samples from beyond the moon's orbit.

Mission collaborators are the UW, the National Aeronautics and Space Administration, NASA's Jet Propulsion Laboratory at the California Institute of Technology, and Lockheed Martin Astronautics in Denver. Other key team members include The Boeing Co., The Max-Planck-Institut, NASA Ames Research Center and the University of Chicago.

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