GOLDEN, Colo. – After more than three and a half years of operation, NASA's twin Mars rovers continue to thrive, showing no signs of stopping as they relay a wealth of valuable data back to Earth about that ancient world's past, present and future.

NASA announced Oct. 15 that it was extending, for a fifth time, the activities of the trailblazing robots – Spirit and Opportunity – perhaps through 2009.

The six-wheeled, golf-cart sized dual rovers that make up the Mars Exploration Rover (MER) project, have been, quite literally, on a mechanical and scientific roll since they landed on opposite sides of Mars in January 2004. They continue to function well, NASA officials said, despite the fact that each one only had an engineering warranty of 90 days of life to carry out their respective missions.

Slow motion science

The solar-powered rovers can reach a top velocity of 5 centimeters per second on flat hard ground. "When you use these vehicles on a day-to-day basis, it just feels excruciatingly slow," said Steve Squyres, lead scientist for the MER mission at Cornell University in Ithaca, N.Y., adding: "It's this wonderful adventure that's unfolding in incredibly slow motion."

The longevity of the rovers has translated into unexpected scientific payoff, particularly in the case of Spirit, Squyres told Space News in an Oct. 18 phone interview.

"Spirit didn't make its biggest discovery until 1,200-plus sols [a sol is a Martian day] into the mission," Squyres said. When Spirit analyzed a patch of Martian soil that was very rich in silica, it provided some of the strongest evidence yet that ancient Mars was much wetter than it is now. The processes that could have produced such a concentrated deposit of silica require the presence of water.

Spirit is now busy at work exploring an intriguing site called "Home Plate," said William Bruce Banerdt, MER project scientist at the Jet Propulsion Laboratory in Pasadena, Calif. "I wouldn't say there's anything like a consensus, but there is opinion that this is some sort of volcanic feature," he told Space News in an Oct. 18 phone interview.

Banerdt said overhead imagery of the area collected by NASA's Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment showed that other features in that vicinity have a volcanic look to them. "If the rover grants us enough time for more observations of these other features, we hope to be able to begin putting together the story of this area and its evolution ... perhaps in a very active volcanic environment," he said.

"As the rovers continue to survive, the story has been getting more and more exciting," said Larry Crumpler, research curator for volcanology and space sciences at the New Mexico Museum of Natural History and Science in Albuquerque and a member of the MER science team.

Crumpler suggested that Spirit's current exploration zone within Gusev Crater – in a range of hills that were on the distant horizon from its landing site – is an area ripe for new revelations.

"There is abundant evidence for not only water, but water that may have been present for a considerable time, or water that was warm, maybe even hot. All of these factors bode well for some climactic discoveries," Crumpler explained in an Oct. 19 e-mail.

Crumpler said members of the MER science team are having the time of their lives. "Look for some of us to be riding along with the rovers until the bitter end, however far in the future that may be. I am sure there will be a major science question still hanging just around the next ridge," he said.

Mobility on Mars

"Before the MER mission, you would be surprised at how difficult it sometimes was to convince people of the value of mobility on the Martian surface," Squyres added. "But it took a very capable set of rovers and a couple of very interesting places on Mars to try and convince the world of that."

For instance, the Opportunity rover is now perched inside the rim of Victoria Crater, a feature 800 meters across and well over 70 meters deep that was excavated by a meteor impact millions of years ago. To get to that site within Meridiani Planum, the robot slogged roughly 6 kilometers from its landing site.

"The longevity of the mission has allowed us to examine Victoria Crater both from the rim and now in-place within the inner rim of the crater," said William Farrand, a research scientist at the Space Science Institute in neighboring Boulder and a member of the MER science team.

"We are just getting started with our examinations of Victoria's stratigraphy ... so it is too early yet to say what those investigations will yield," Farrand said in an Oct. 18 e-mail. "But we would never have had this opportunity if the rovers had not lasted as long as they have."

Farrand said that, potentially, Mars scientists will gain a better understanding of how the sedimentary rocks at Meridiani Planum formed. "We know that water played a huge role here," he said, but added that a lot of the details have yet to be worked out, with the investigations at Victoria Crater providing additional information to better work out that sedimentary history.

Squyres said a decision is forthcoming about just how far down into Victoria Crater Opportunity will venture. The decision will be driven both by science and safety considerations, he said. "If we lose a wheel down inside the crater, I'm not sure we'll ever get out," Squyres said. "There's a lot of motivation for us to wrap up the science quickly and get back out on the plains," he said.

The Spirit rover is struggling now due to a breakdown in its right-front wheel, Squyres pointed out.

Remote maintenance

Squyres says the longevity of the rovers is due to several factors.

"The first is that we built really good hardware. The components were selected extremely carefully and everything was tested very, very rigorously." Furthermore, the vehicles were designed for remote maintenance, imbued with the ability for ground controllers to troubleshoot and maintain the robots from a distance, he said, such as utilizing heaters on every single actuator to counter the temperature swings on Mars.

Mars rover operators also have received lucky breaks. In particular, little gusts of wind have repeatedly swept away sheens of dust that collect on rover solar panels, Squyres said. "That's been just good fortune."

Another reason for the long-term survivability of the rovers has been the ability to drive and park the vehicles on steep terrain, thereby tilting solar arrays significantly towards the Sun. "We've survived two winters on the planet that way ... and the third one is coming ... and we're going to try and survive that one too," Squyres said.

Squeezing out more science

Last July, Spirit and Opportunity were subjected to global martian dust storms that reduced sunlight reaching the solar-powered rovers. That situation elevated concern regarding survival of electrical and mechanical components on board the robots. Ground controllers put in place low-power procedures that enabled the machines to survive the severe dust storms. With clearing skies, the rovers were put back on line to continue their data gathering.

But, in the end, the rovers are not immortal, Squyres said. "If Mars doesn't kill them, we're going to wear them out. We have driven them so hard," he said.

The key now, Squyres added, is to continue each day and find that balance between safety and aggressiveness, as well as squeezing as much science out of the rovers as possible.

"When I sit down and look at the traverses that Opportunity and Spirit have followed, the data that we've collected, the discoveries that we've made and the papers that we've written ... I sleep pretty well at night," Squyres said. "I'm going to be the last guy to turn out the lights when the last rover dies. I've got to see this through the end."