Now might have seemed the perfect time for an epitaph. One year ago this week, the first of NASA's two Martian rovers came to rest on the desolate fields of the Red Planet, the clock already ticking on a life span guaranteed to last only 90 days.
But since that moment, Spirit and its later-arriving twin, Opportunity, have written in soil and stone the scenes that scientists since the Renaissance had only been able to imagine. While one spun its way across two miles of Martian desert and then 200 feet up a rock-strewn hillside in an unprecedented feat of interplanetary engineering, the other looked into the telltale squiggles of ancient rocks and proved that a lake of liquid water once covered the wide, dark expanse of the Meridiani plain.
The rovers have exceeded not only the greatest hopes of the scientific world, but also their own life expectancy - and they are still going. Somewhat unexpectedly, on this first anniversary, there is as much reason to look forward as backward.
As Spirit scrambles toward the top of its barren hilltop, there are rocks that tell a still-misunderstood story, perhaps hinting at an ancient Martian flood. And half a world away, beyond plains that stretch featureless and gray to every horizon, a bizarre landscape of bare rock and corduroy dunes awaits Opportunity.
"We've mentally adapted ourselves to the idea that these rovers can go on and on," says Joy Crisp, a project scientist at the Jet Propulsion Laboratory in Pasadena, Calif., which runs the mission. "Potentially, it could be many months."
No one knows for sure. In truth, none of the scientists expected wheels and cameras to start falling off after 90 days. But that was the longest they were able to promise that the rovers would work. Now, it's anyone's guess. They've made it through the Martian winter, meaning that the coldest days are behind and the longest days are ahead - bringing more solar power. "It could go another year," says mission manager Leo Bister.
Then again, he adds: "We have to plan the mission with the belief that it could end any day, because it could."
That creates the slightest sense of urgency as Spirit crawls up the Martian hills. By many measures, the mission has already been a success. When Spirit first emerged from its cocoon and imaged the world around it, the so-called Columbia Hills were a distant fancy on the far edge of the possible.
"When [the lead scientist] pointed out the hills as a far-off goal, none of us were really confident we would get there," says Dr. Bister. Today, Spirit stands only a few days from cresting one of the range's highest ridges and peering down the other side into an as-yet unseen landscape.
"I don't think anyone would have expected that we would have driven as far as we have driven," he says.
Yet the greatest science might still be in front of them. Spirit's trek across the plains yielded little, and its climb up the Columbia Hills has shown only fragments of a tantalizing tale. In short, researchers don't quite know what to make of the rocks in the Columbia Hills. They could well be volcanic. How they came to rest in a 300-foot-tall heap on the floor of a crater the size of Connecticut, however, is more of a mystery.
Perhaps they landed there in an eruption billions of years ago. But preliminary evidence also suggests the possibility that they might have been carried there in a flow of liquid water. It's the sort of evidence that scientists have been looking for since they chose the Spirit site, which appears to sit at the mouth of a long-dry riverbed, according to satellite photos.
"We're interested in trying to get a better understanding of how the rocks in the hills were deposited," says Dr. Crisp. "We're very much looking for other clues."
Opportunity, meanwhile, is seemingly headed off to a different world. During the past year, Opportunity has revealed with astonishing clarity the past of the land in which it arrived. Mere feet from where the rover came to rest 49 weeks ago, clues from a rocky outcrop painted a picture that Mars enthusiasts have dreamed of for centuries. Billions of years ago, a shallow lake spread across this plain, its briny shores repeatedly receding and swelling in concert with unknown events in the Martian past.
Perhaps it was covered by ice. Perhaps a warmer and wetter Mars kept its surface ice-free. This rover can't know, at least from the evidence here. Far away lies Victoria Crater - a crater 10 times larger than the stadium-size crater in which Opportunity has spent the past several months. In between lies the Etched Terrain, a labyrinth of small ridges and bare rock that satellite photos suggest is made from materials that Opportunity has not yet encountered.
Whether or not Opportunity makes it, it has already joined the line of unmanned missions - from Voyager to Galileo - that have returned some of NASA's best science while running beyond their expiration date. Says NASA historian Alex Roland: "It's an example of what we can do with automated spacecraft."