For the past four years, scientists at NASA's Jet Propulsion Laboratory have known they wouldn't be doing anything terribly romantic on Valentine's Day this year. That's because the Stardust spacecraft, which had already paid a call on Comet Wild 2 in 2006, would be making a second visit to Comet Tempel-1 a 3.7- mile-diameter (6 km) ball of rock and ice, which at the moment might be the most interesting ball of rock and ice in the solar system. On Valentine's night, precisely on schedule, Stardust flew past Tempel-1 at a distance of just 110 miles (175 km), bringing to a close a mission that was as much of a navigational triumph as it was a scientific one.
NASA had barnstormed Tempel-1 once before, back in 2005, when the Deep Impact spacecraft flew by and, true to its name, fired an 820-lb. (370 kg) copper cannonball into the flank of the comet. The purpose of that bit of cosmic skeet shooting was to blast out a plume of cometary dust, ice and other debris that the spacecraft could scan and analyze.
About a year before that, Stardust had had its own encounter with Comet Wild 2, flying through its plume and collecting a sample of its dust. Stardust then circled back around toward Earth and, in 2006, dropped a re-entry canister off in the atmosphere. Then it simply flew on, its mission complete or ostensibly complete. But Stardust was a tough ship, its systems were in good shape and it still had some fuel sloshing around in its tank. It seemed a shame to let a good vehicle go to waste, and mission planners thus decided not to. Instead, they sent Stardust in pursuit of Tempel-1. There were good reasons to pay the comet a second visit.
Comets are among the oldest cosmic artifacts around bits of debris left over from the earliest days of the solar system. Astronomers have always known that the deeper they peek inside a cometary nucleus, the farther they peer back in celestial time. The fresh wound the Deep Impact probe had gouged in Tempel-1's side could mean a direct line of sight deep into the interior. What's more, in the time it would take Stardust to catch the comet, both would have made a complete circuit of the sun. Comets are volatile, with at least some of their ice steaming away each time their elliptical orbits carry them near the solar fires. That means that the surface of Tempel-1 would look different to Stardust from the way it looked to Deep Impact just a few years earlier. The extent of that difference would provide more clues to the comet's composition.
It was a nice plan, but the logistics were daunting. The ship had fired its thrusters and engines hundreds of thousands of time in the eight years it had been in space from its launch in 1999 to its retasking with Tempel-1 in 2007, and it would need hundreds of thousands more tweaks and adjustments to complete that mission. That meant it would have to make the most of whatever fuel it had left.
"Nobody has invented an entirely reliable fuel gauge for a spacecraft," said Stardust project manager Tim Larson. The best estimates he and his team could come up with did not give them much of a margin for error: by the time of the Valentine's Day rendezvous, they would have about 3% of a tank left.
Just as worrisome was the question of temperature. For purposes of providing a frame of reference, NASA likes to point out that Stardust has been flying since Bill Clinton was in the White House, and throughout that time, its periodic sweeps close to the sun and deep into space have sent it through a constant thermal cycling from blazingly hot to paralyzingly cold often at the same time. (Since there's no atmosphere in space, temperature doesn't average out around an object. Instead, the side facing the sun can be scalding while the side facing away is frigid.) Try putting a complicated piece of home electronics through that torture and imagine how it would come out.
"All this mission's challenges are just that," Larson said, "challenges."
Finally, there was the question of cometary randomness. Tempel-1 is irregularly shaped and its periodic outgassing causes it to tumble and spin on its axes as it flies through its orbits. If one of its narrow edges happened to be facing the sun when Stardust flew past, it would reflect very little light back to the ship, making photographic conditions poor. If its wide sides caught the sun, the conditions would be much better. Whether its new crater would drift into view at all was impossible to say.
With all that could go wrong, the ultimate encounter went fantastically if fleetingly right. The ship began closing in on the comet and snapping pictures at 11:36 p.m. E.T. on Valentine's night and moving at a speed of about 4 miles (6.5 km) per second made its closest approach and swept by in just minutes. On its way, it passed through the comet's surrounding halo of dust and micro-debris, which struck its side like antiaircraft flak and which an onboard audio system picked up and beamed home. "So aren't you glad you weren't aboard the spacecraft?" one NASA official asked at a press conference the next day after playing the rat-a-tat recording.
The six dozen pictures the spacecraft took, which have only begun to be analyzed, turned out to be just what the scientists had hoped for. The comet showed its broad, luminous side, and the crater indeed accommodated the lens, allowing for a few quick portraits before the two moved out of alignment. The little bit of comparison forensics the scientists have been able to do so far lining up new images of the comet's surface with those taken six years earlier showed fresh erosion features nearly 100 ft. (30 m) long. That's an awful lot in a very short time.
NASA will spend months analyzing the images and the other data Stardust has sent back, even as the spacecraft itself at last enters its long-delayed and well-earned retirement. "When Stardust ended its [first] mission in 2006, it was in an orbit that could possibly reach two comet targets," said Joe Veverka, principal investigator of the mission. "I chose Tempel-1 because it is a fascinating place." And Stardust provided a fascinating look at it.