PASADENA, Calif. -- It began with two comets (Wild 2 and Tempel 1), two spacecraft (Stardust and Deep Impact) and two encounters. But before the four objects orbit their separate ways, their dance may get more intricate -- with two recombining to form a new couple; one pushing forward toward an unfamiliar match, and one left to finish solo.

Led by Cornell astronomy professor Joe Veverka, a team of astronomers and engineers made a final pitch on May 15 to lead the proposed Stardust New Exploration of Tempel 1 (Stardust NExT) mission. Their presentation came at an all-day site review at NASA's Jet Propulsion Lab (JPL) in Pasadena.

If the mission gets the go-ahead, Stardust NExT will use the still-healthy Stardust spacecraft -- which dropped off a capsule of dust from the comet Wild 2 in the Utah desert last year -- to observe the comet Tempel 1 two years after a probe released by the Deep Impact spacecraft carved a crater into that comet's surface.

The mission could provide the first good look at the crater, which was obscured by a plume of debris immediately after the collision. The crater's size and the amount of ejected material that fell back to the surface post-impact offer clues about how porous or cohesive the material is -- important considerations for future missions, and also for contingency plans in case a rogue comet were to head toward Earth.

The new Stardust mission could also offer a unique opportunity to compare particle analysis from two comets (Wild 2 and Tempel 1) taken with the same instruments -- and to compare two observations of a single comet (Tempel 1) taken before and after a single orbit around the sun. The latter could help scientists work backward to reconstruct what the comet may have looked like just after it was formed.

"Every time that comets come close to the sun, material sublimates and the surface changes," said Veverka, who chairs Cornell's astronomy department. "But what actually happens on the surface -- how does the surface change -- that's never been documented because no one's ever looked at a comet before and after."

And Stardust NExT is also likely to see new terrain on Tempel 1 -- a comet whose surface features, as seen by Deep Impact's cameras, already have astronomers puzzled. The images show a pattern of layers, as if snow and slush had flowed down the sides of a hill in waves -- surprising because particles on the surface experience almost no gravity and therefore no sense of downhill.

"These are things that have never been seen on comets," said Veverka. "But the hope is, if you see a little bit more of it -- if you can see around the corner -- you might get some important clue."

Meanwhile, the Deep Impact spacecraft may also get a second life. Michael A'Hearn, Deep Impact's principal investigator and University of Maryland astronomer, is proposing a mission called Extrasolar Planet Observation and Deep Impact Extended Investigation, or EPOXI. It would send the spacecraft to visit the comet Boethin and also use it to study planets outside the solar system. (The proposal is the melding-together of two other submissions, nicknamed DIXI and EPOCh. Hence the geekily clever glue allusion.)

The EPOXI and Stardust NExT teams are both slightly reshuffled and added-to versions of the original Stardust and Deep Impact scientists. Stardust's principal investigator, University of Washington astronomer Don Brownlee, is on the NExT team. Veverka, leading NExT, would also play a role in EPOXI.

If one or both missions goes forward, it wouldn't be the first time a NASA spacecraft has been granted a new life after completing a primary goal. The two Voyager spacecraft, for example, are still functional three decades after they were launched on a four-year mission to study Jupiter and Saturn. Now the farthest man-made objects from Earth, they are busy exploring the edges of the solar system and are expected to venture into interstellar space.

A final decision from NASA officials is expected for both missions this summer. In the meantime, the NExT team is planning a maneuver for later this year that would steer the spacecraft toward its new target. Stardust would then fly by Earth in January 2009 and arrive at Tempel 1 around Valentine's Day 2011.