Stardust NExT must love comets. On Valentine's Day the spacecraft will get up close and personal with its second. It's been seven years since the original Stardust danced with Wild 2 out beyond the orbit of Mars, capturing a thimbleful of comet dust in its collector. It's been five years since the craft jettisoned its sample-return capsule and its precious cargo for a landing in the Utah desert.

Next Monday the probe will make history again in a 125-mile embrace with comet Tempel 1. It will be the first time two different comets have been surveyed with the same set of scientific instruments. And Tempel 1, explored by NASA's Deep Impact mission in 2005, will be the first comet to be visited by two spacecraft. (See a NASA video "Date With a Comet.")

University of Washington astronomer Donald Brownlee, who had a ringside seat for the Wild 2 flyby as Stardust's principal investigator, is a co-investigator for Stardust NExT. The principal investigator is Joseph Veverka of Cornell University.

Brownlee has seen what the spacecraft can do, knows what it has survived and has enjoyed the phenomenal success it already has chalked up.

"Had we known at the time of the Wild 2 flyby how comets worked, we would have been even more nervous. There were jets at sonic speeds, and there were clumps of material coming out from the comet and breaking up," he said. That's scary when you know a particle larger than a centimeter across - less than half an inch - could destroy the spacecraft, along with years of planning and work.

Knowledge about comets has shifted significantly in just the last five years, thanks to the Wild 2 samples Stardust sent back to Earth.

Because comets form in the ultra-frigid region beyond Neptune, scientists expected the non-icy parts of the comet to be made up of particles that flowed into the solar system from out in the cosmos.

Instead, it turned out that nearly everything bigger than a micron (one-millionth of a meter) in diameter was formed in the inner solar system, and at very high temperatures. The material somehow was carried out beyond the planets and was incorporated into comets.

"When the solar system was forming, it was actually turning itself inside out," Brownlee said. "I think we're seeing a component that was uniformly distributed when the solar system was forming."

Stardust NExT no longer has the ability to capture samples from a comet, since its collector returned to Earth following the encounter with Wild 2. At Tempel 1 it will take photos of the crater formed during the Deep Impact mission to learn more about the interior of comets. Deep Impact couldn't gather those images because its camera's vision was obscured by a cloud of debris from the creation of the crater.

Stardust NExT also will measure the size and distribution of particles flowing from Tempel 1 and analyze the particles' composition, and the science team hopes to make detailed observations of how interaction with the sun has physically changed Tempel 1 in the six years since the Deep Impact encounter.

Getting two high-profile science missions from the Stardust spacecraft took a lot of careful planning, but there also was a measure of luck.

"It's great to have a good proposal, but it takes a lot of good fortune too," Brownlee said. "We were fortunate to be selected, and once we were selected it took a lot of hard work, but a lot of things had to go right."

In fact, things went so right that the science will never be the same again.

"Scientifically, it was a phenomenal success. People think about the formation of the solar system differently than they used to," he said. And the discoveries are likely to go on for many years, as tools are developed to do new types of research on the comet grains.

"We do things now that we couldn't do at all when the sample came back, let alone when the spacecraft was launched," Brownlee said. These include the ability to make precise isotope measurements in micron-size grains, or detect the amino acid glycine in such tiny samples.

Stardust started out with about 22 gallons of hydrazine fuel for its thrusters. Now, two missions and about 3.6 billion miles later, there is perhaps a cup of fuel left, not enough for any meaningful operations.

So, after 12 years of being guided by mission controllers at NASA's Jet Propulsion Laboratory, the office-desk-sized spacecraft will continue to orbit the sun on its own. It will still have its scientific instruments and its camera, a spare from the Voyager program.

It also still carries two microchips bearing the names, in microscopic type, of more than 1 million Earthlings who signed up before the launch. (Duplicate chips came back in the return capsule and now are on display at the Smithsonian Air and Space Museum in Washington, D.C.)

Brownlee recalls the bittersweet moment at the end of the first Stardust mission, when the spacecraft was put into hibernation until needed again, and the board in the control room flashed "LOS" - loss of signal. He knows even at the end of this mission the spacecraft will still be out there, in its long loops around the sun for perhaps a million years, until it crashes into Earth or, more likely, is ejected by gravity from the solar system.

"When it's chucked from the solar system by Jupiter, the spacecraft and its 'crew' of signatures will keep going for billions of years," he said. "The chip will probably be readable after the Earth is gone."