For the last year and a half, Steve Squyres has been living on Mars -- breathing its cold, thin air, digging at its rusty, red soil -- all from the safety of Earth.
"It's a breathtakingly beautiful place in a very stark way," he said. "There are mountains. There are deep canyons, spectacular craters. We've seen some of these things. Now, we've claimed the mountains. We've been down into the craters. We know what they look like."
Squyres is the principle investigator for the unmanned Spirit and Opportunity rovers that left for Mars in 2003. He says they have found "spectacular evidence" that Mars was once had pools of reddish water on its surface.
These days, Squyres works from his office at Cornell University, in upstate New York, where he is a professor of astronomy. He holds daily teleconferences with his team, some from other universities, most at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
That's where Squyres was in January 2004, when Spirit went tearing into the Martian atmosphere, slowed by its parachute and retro-rockets, landing safely, despite the odds. Opportunity followed three weeks later.
The rovers were sent to see if Mars could once have had liquid water and, therefore, perhaps been friendly to life. They found evidence within three months -- which was good, because in the Martian cold, the rovers were not expected to last much longer.
At some point, of course, a circuit will blow or a computer will crash, and one rover, then eventually the other, will die. But for now, both remain functioning more than a year beyond their expected lifespan.
"We're on day 550 of our 90-day mission," Squyres will often joke when he gives talks. He keeps having to raise the number.
Moving slowly and deliberately, automatically scanning the ground ahead of them to avoid hazards, the rovers have each gone miles from their landing sites, stopping to examine rocks, scrape away their outer layers and analyze their chemistry.
Many of their pictures are black and white, shot so that the engineers and scientists back on Earth can pick out potential targets of interest. Other pictures are in color, showing the rusty ground, the salmon-hued sky, and a ghostly blue shade that seems to settle over the Martian landscape at sunset.
"I feel when you look at the pictures that you get sort of a lonely feeling," Squyres said. "I mean, these rovers are really far from home. When you're out on the plains, man, there's nothing. And you look at those wheel tracks. And we've driven so far now that you look at those wheel tracks, and they just recede into the infinite distance. And, man, that little thing is just way on its own out there."
Originally, each rover was only designed to go about 600 yards. But Spirit's landing site, in particular, was less interesting than the science team had expected. It was covered with the kind of rock that forms around dormant volcanoes or meteor craters, not the kind that, on Earth, forms in the presence of water.
In search of better targets, Squyres and his team decided to go for broke.
"Over a period of about 60 days, two months, we sprinted, rover speed, across the plains, going as fast as we could," Squyres said. "We did 125 yards in one day. OK? And we sprinted across the plains. And on day 156 of the mission, we reached the base of the Columbia Hills. We crossed into the hills and everything changed."
The rocks became much more interesting. Spirit's instruments showed many of the minerals one finds in stream beds on Earth.
There was an added benefit to driving through mountain passes -- they're windy. Spirit's solar panels had became covered with fine Martian dust, blocking the sunlight they needed to produce electricity and keep the rover going. By sheer luck, gusts of wind cleaned them off.
On the other side of Mars, Opportunity got into a bigger fix. It accidentally drove into fine sand, where it got stuck for weeks.
"We were all pretty confident that we'd be able to get out," Squyres said. "But the first rule in a situation like that is, 'Do no harm.' Don't do anything dumb until you've analyzed the situation. We have rovers on the ground, here on Earth, that are duplicates of what we have up on Mars. And we use them all the time.
"We tried everything that you could imagine, different ways of operating the wheels and so forth. Turns out that, after lots and lots of research, the optimum way is basically put it in reverse and gun it."
Squyres, who has written a book called "Roving Mars," said the planet interests him because it could help humans learn about our place in the universe.
"Life might have originated on Mars. Did it? We don't know," he said. "But if you can show that life arose independently on two different worlds, just in this one solar system, it takes no great leap of imagination or faith, or anything else, to begin to believe that life might be common throughout the universe."
"And the reverse might also be important?" a visitor asked.
"The reverse might also be true," he said. "You might get to Mars and find that the conditions were once just right for life -- it was warm, it was wet, there were pools of water -- and you could search for years and find no evidence of life.
"That would be important too," Squyres said. "It means that life is pretty special."
ABC News' Ned Potter originally reported this story for "Nightline" on July 25, 2005.