The comet’s orbit never takes it much closer to the Sun than Mars gets (about 1.5 times Earth’s distance from the Sun), but that’s close enough to vaporize some of the comet’s frozen gas to form jets, a coma (the cloud around the nucleus) and tail. According to Tom Duxbury, the mission’s Project Manager, those jets, along with tidal forces from the Sun’s slightly uneven gravitational pull on the near and far sides of the comet, are likely to affect the comet’s rotation, making it harder to time the encounter. Scientists will have to predict what effect those factors will have. “That’s not the simplest of things to do,” Duxbury said, “but they’re going to take their best shot at it.”

“We have a big effort to model the rotation rate going on at three different places,” Veverka said. “We think we can do this successfully and accurately. And when we do, it will be the first time that somebody has actually predicted the rotation state of a comet a year ahead of time.”

Stardust-NExT will also be one of the few spacecraft that have observed two different comets with the same set of instruments. “So if we see differences,” Veverka said, “no one can claim that those differences are because there are different instruments that didn’t quite work the same.”

Veverka estimates that Deep Impact imaged about 40% of the comet. “We want to see new territory,” Veverka said, noting that Deep Impact revealed some strange and unexpected surface features. “But we also want to see some of the old territory to see the changes and see the crater. Of what our coverage is going to be in 2011, 40% will be stuff we’ve seen before, including the crater, and about 60% will be new stuff.”

A series of fortunate events during the original Stardust mission made it possible to give the spacecraft this new mission. First, the weight of the Stardust spacecraft left enough margin to enable the team to completely fill its fuel tanks. The launch went perfectly, requiring no corrective maneuver. Subsequent maneuvers, including a gravity assist at Earth in 2001 to target comet Wild 2, were similarly accurate. The result is that after completing its successful sample-return mission, the spacecraft still had enough fuel—if used cleverly—to conduct a new mission.

“We hit our targets,” Cheuvront said, “so we didn’t have to use a lot of extra fuel to correct for unknowns.” He added, “The spacecraft was a very, very well-behaved spacecraft. Its performance was outstanding and predictable, which let us save fuel.”

But given the amount of fuel remaining, could the spacecraft get to an interesting new target?
Knowing that Deep Impact had been unable to view the crater, Robert Farquhar, who had been Mission Director for NEAR, MESSENGER, and New Horizons among other missions, took it upon himself to see whether the Stardust spacecraft could get to Tempel 1. Working with his colleague, David Dunham, he discovered that the Earth flyby for which the spacecraft was headed in 2009 could be used to retarget it for Tempel 1. Farquhar proposed the idea to Veverka and became a co-investigator on the mission.

So, if all goes right, Stardust-NExT will approach Earth in January 2009 at exactly the right distance, speed, and angle to change its trajectory to one that will intercept Tempel 1 on February 14, 2011. And in February 2010, depending on the mission scientists’ calculations, it will fire its thrusters to speed up or slow down just enough so that, a year later, it will zoom by the comet just after the crater rolls into view. Sounds like a particularly difficult pool shot if all the balls were moving while you were shooting. “That’s right,” said Shyam Bhaskaran, a JPL spacecraft navigator, “except we know where the balls are going.”

“We’re able to take a spacecraft that everybody had given up on and basically do a first-class mission at 10 percent of the cost of a new mission,” Veverka said. “I think that’s very exciting.”

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