In the waking days of 2004, when the Spirit rover had just rolled onto Martian soil, the mission's lead scientist gazed at a photograph of the nearby terrain and professed an almost foolhardy hope. More than a mile away rose the salmon crest of the Columbia Hills, and in time, he confessed, he hoped to look out from atop them.
Today, Spirit is blinking its signal back to Earth from that very summit, 350 feet above the Martian plains. With its twin, Opportunity, still navigating the dark and wind-swept dunes on the other side of the planet, the rovers represent a remarkable engineering feat - now nearly 1-1/2 years past their expiration date.
Yet the performance of the rovers has again given Steve Squyres a fresh and equally fantastic hope: that his rovers aren't even halfway done. For both Spirit and Opportunity, perhaps the greatest troves of science lie just ahead, tantalizingly close but separated by a gauntlet of natural obstacles.
The difficulty of the way ahead "may be comparable to everything we've done this far," says Dr. Squyres.
Spirit's path will take it down the other side of the hills to a curious, light-hued mesa called Home Plate. Since Spirit landed in Gusev Crater, it has unsuccessfully sought to confirm that the crater was once a vast lake. Home Plate may or may not provide those answers, but "it doesn't look like anything we've seen before," says Squyres.
The way down, however, is so steep in places that scientists can't see the hillside. On the Meridiani plains halfway around Mars, meanwhile, Opportunity recently spent five weeks stuck in a drift that swallowed its wheels like quicksand.
Now free but proceeding gingerly, it is pausing by one crater on its way toward the massive Victoria Crater more than a mile ahead - a site that could provide the fullest picture yet of Mars's water history.
Already, Opportunity found evidence of a watery past in one 15-inch-tall outcrop of bedrock. Not far away, a 20-foot outcrop provided more information, leading scientists to speculate that Meridiani was once covered by a necklace of dune-bound lakes formed by seeping ground water. But deeper outcrops allow scientists to look further into the past, since older layers are always lower - and Victoria is 120 feet deep.
How much longer the rovers can hold out in an environment where temperatures rise and fall more than 200 degrees F. a day is a mystery. So far, Opportunity has driven more than 3-1/2 miles; Spirit has driven three miles. They've lasted this long only because Martian windstorms have unexpectedly wiped away dust accumulating on the solar panels. Yet Squyres knows that no matter when the end comes, there will always be more to do.
"I thought we would be able to say, 'Scientifically speaking, we're done,' " he says. "I'm having to come to grips with the fact that that will not happen."