Well past their expiration dates, NASA's two Mars rovers are still sifting through the far-off dust for signs of water, and for the first time since the mission began six months ago, both stand on the brink of discovery.
The success of Opportunity on the dark, pockmarked plains of Meridiani is well documented - first finding evidence of past liquid water, then discovering that its site was once the shore of a long- vanished sea. Spirit, however, has suffered from one near-fatal technical glitch and a neighborhood of dust and rocks only slightly more exotic than a Wal-Mart parking lot.
That should change this week. As Opportunity tips its wheels into a stadium-size crater in a Martian kamikaze mission, Spirit will arrive at an intriguing range of hills two miles from where it landed.
Each rover promises potential breakthroughs, and both look healthy enough to continue working into September, if all goes well.
For Opportunity, which could become trapped in its slick-stoned crater, the chance to look deeper into Meridiani's watery past - and see if conditions were favorable for life - would be a fitting epitaph to a remarkable mission. For Spirit, after months of frustration, the ancient rocks of the highlands offer a last lifeline for scientists to answer the question that led them there: Was this basin once a vast Martian ocean?
"Two of the biggest events of the mission are happening almost at the same time," says Steve Squyres, lead scientist for the rovers at the Jet Propulsion Laboratory in Pasadena, Calif. "It's like the mission is starting all over again."
Ask fellow scientist Jim Rice which mission excites him more, and he doesn't hesitate: Spirit's trek to the so-called Columbia Hills. It's not that Opportunity's job is less interesting. It's just that Spirit's journey has had something of the epic about it - failures, disappointments, doubts, and at long last a pilgrimage to a distant destination that tests the very circuits and steel of the machine.
For six months, Spirit has been plodding along in anonymity, spinning its odometer in the vain search for an interesting stone to sniff. All the rocks that Spirit stumbled across in the Gusev basin - a Connecticut-size crater that includes the 300-foot-tall Columbia Hills - were volcanic splinters left over from relatively recent times. If liquid water was ever at Gusev, it was gone long before these rocks formed. Spirit craved something older.
Halfway around the planet at Meridiani, Opportunity had the good fortune to land in a small crater where the newer skin of the planet had been broken, and an ancient outcrop was exposed. That's what allowed geologists to prove the past presence of water.
Surely, the Columbia Hills - ancient sentinels of rugged rock too stubborn to be worn away - were made of older stuff than the surrounding plains. But engineers doubted if Spirit could cover the distance. The rovers, after all, were designed to last only three months and travel just more than one-third of a mile.
By the end of this week, though, Spirit should arrive at its promised land: the westernmost spur of the hills. "We really don't know what we're going to find," says Dr. Rice. "Mars has a story to tell and our job is to unravel that story."
From space, it looks fairly convincing that the story of Gusev Crater involves water. Signs of erosion and flooding suggest that large amounts of liquid water once reshaped the landscape.
That would make the Columbia Hills islands in a Martian ocean that spread to the bend of each horizon - and their flanks might show evidence. When Spirit arrives, it will look for an outcrop or a rock that fell from the highlands, and begin its most eagerly anticipated work of the mission. Later, the rover might scoot up the side of the hills, yielding a panorama, but scientists don't expect to summit the peaks.
That Spirit is even still going is something of a marvel. Spirit has now traveled six times its expected distance, "and we've got a fair amount of rover left," says Dr. Squyres. The performance has been so robust, in fact, that the scientists are considering shutting the rovers down at the height of the Martian winter (September) and then starting them back up in the spring when they can draw more solar power from longer days.
For its part, Opportunity has a broken heater that will eventually ruin one of its noncritical sensors. But the biggest problem might not be power or heaters, but basic traction. At the moment, it stands at the edge of a 60-foot-deep crater called Endurance. Ahead lies a trove of science - and the possibility of getting stuck.
Scientists will proceed slowly down a slope that would challenge an intermediate skier, testing Opportunity's traction. But there's the chance that Opportunity will never emerge.
"The worst thing that could happen is that we spend the rest of the mission in the candy store," says Squyres, adding that the science at Endurance "is much more of a sure thing" than in the Columbia Hills.
That is because, scientists have already filled in part of Meridiani's story with the research completed at the small landing crater. It's just a question of going deeper into the geological record. From the bedrock in the landing crater, which is less than 10 feet deep, scientists already know that there was water at some time in the past. Now, the deeper and older bedrock at Endurance will tell them what was there before.
It is a profound question. If the rocks show no water, they might offer clues about Martian climate change. If scientists find further evidence of water, though, it suggests that water stayed for a long time, raising the likelihood of life. Says Bruce Betts of the Planetary Society: "The duration is a very significant question for organic life."