Twelve years after launch and seven years after it collected dust from comet Wild 2, NASA's Stardust probe is making a bonus Valentine's Day flyby late today. This time the probe will close in on comet Tempel 1 to find out how the icy body has changed since it was visited by another NASA spacecraft in 2005.

The renamed Stardust-New Exploration of Tempel mission--Stardust-NExT--is on track to pass within about 124 miles of the nucleus of Tempel 1 at 8:37 p.m. PT, snapping 72 high-resolution images and collecting data about the dust environment in the immediate vicinity as it races past at a relative velocity of 24,300 mph.

Because radio signals to and from the spacecraft will take about 45 minutes to cover the 418-million-mile round-trip distance between Earth and the spacecraft, Stardust-NExT will carry out the flyby autonomously under control of its onboard computer. The first images of Tempel 1 are expected to reach NASA's Jet Propulsion Laboratory in Pasadena, Calif., around midnight PT.

Comet Tempel 1 as seen from the Deep Impact spacecraft in 2005, showing areas of flow-like terrain ('a' an 'b') and the site of an impact by an instrumented probe that blasted out a crater on the surface (large arrow' photo taken before impact). The white line at lower right represents 1 kilometer for scale.

"In the few minutes around closest approach, we'll be taking the bulk of our images, we'll be taking 72 high-resolution images," said JPL Project Manager Tim Larson. "We cannot transmit those to the ground real time because of the flyby geometry. So we have to store all of those on board in the spacecraft memory.

"An hour after the flyby, we turn the spacecraft to point the high gain antenna back to Earth and at that point, we'll start relaying all the information back to Earth. It will take approximately 12 hours to get all the data back own on the ground. The first images should be hitting the ground around midnight Pacific Time on the 14th."

The $300 million Stardust mission was launched in 1999. On January 2, 2004, the spacecraft flew past comet Wild 2, using an innovative collector to capture particles from the coma, the cloud of debris surrounding the nucleus. Passing back by Earth two years later, a small re-entry capsule carrying the collected material was ejected and fell to a landing in Utah where it was recovered for detailed analysis.

In the meantime, NASA carried out the Deep Impact mission, sending another spacecraft to comet Tempel 1, a roughly potato-shaped body with a nucleus measuring 4.7 by 3 miles. During a dramatic encounter in 2005, Deep Impact released an instrumented probe that crashed into the comet, throwing up a cloud of debris from the surface. The Deep Impact spacecraft monitored the crash from a safe distance and carried out remote observations with cameras and other instruments.

But the cloud of debris, or ejecta, thrown up by the Deep Impact probe prevented scientists from seeing the crater the crash excavated.

With the Stardust probe still healthy after its successful mission to Wild 2, NASA approved a $29 million mission extension and agreed to send the spacecraft to Tempel 1 to study how the comet had changed during a full trip around the sun.

Joe Veverka, the Stardust-NExT principal investigator at Cornell University, said Tempel 1 turned out to be "unusually interesting."

"In places on Tempel 1, we see layered terrains, which probably contain information about how comet nuclei are put together, and we would like to see more of these terrains," he said, explaining why Tempel 1 was targeted for a second visit. "Deep Impact saw only about a third of the surface. We would like to see more."

Deep Impact also showed areas that appear to be smooth flow-like deposits, along with crater-like features that could be ancient vents.

A view of comet Tempel 1 as an instrumented probe crashed into its surface in 2005.

But Veverka said the most important reason to return to Tempel 1 is that "this will be an opportunity, for the first time, to see how much a comet changes between two close passages to the sun."

"Deep Impact saw the comet in 2005, we're going to be seeing it one comet year later, just after its closest passage to the sun in 2011," he said. "We know comets lose material, but the question is, how much does the surface change and where does the surface change So we'll be able to answer that question by comparing our images with those taken by Deep Impact in 2005."

One year ago, engineers carried out a major rocket firing to put Stardust-NExT on course for its Valentine's Day encounter. The trajectory was selected based on careful studies of the comet's estimated 41.9-hour rotation. The goal was to make sure Stardust NExT had a view of the Deep Impact crater as it flew past.

"That impact threw up so much ejecta that Deep Impact never saw the crater," Veverka said. "So it could never complete the experiment, to see how big the crater is and what that tells us about the mechanical properties of the comet's surface. That's important if we're ever going to go back to a comet, land a spacecraft on the surface, dig up material from the surface, bring it back to Earth...So here, we have a chance to complete the deep impact experiment."

But Steve Chesley, a co-investigator at JPL, said there were no guarantees.

"If we've aligned our light curves correctly and the comet continues to cooperate...then we'll meet our mission objectives and hopefully also have a fantastic view of the deep impact crater," he said.

"But the alternative is not so bad because then we'll get fantastic views of never before seen cometary terrain. So we'll have fantastic science no matter what."

Flight controllers will not know whether they hit the bull's-eye until the first images come in.

"We won't know until a few hours after the flyby," Chesley said. "I'll be on the edge of my seat, but I think it's important to emphasize that aspect of the mission is bonus science. We're going to find out a lot about how comets evolve."

Stardust-NExT has covered some 3.5 billion miles since launch in 1999. The spacecraft is not expected to have enough fuel left to carry out any additional encounters.