NASA has approved up to 18 more months of operations for Spirit and Opportunity, the twin Mars rovers that have already surprised engineers and scientists by continuing active exploration for more than 14 months. The mission will be extended through September 2006.

The rover mission had previously been extended 11 months beyond the successful three-month prime mission.

“We now have to make long-term plans for the vehicles because they may be around for quite a while,” said Jim Erickson, rover project manager at NASAʼs Jet Propulsion Laboratory (JPL), Pasadena, Calif.

Erickson cautioned though, “Either mission could end tomorrow with a random part failure. With the rovers already performing well beyond their original design lifetimes, having a part wear out and disable a rover is a distinct possibility at any time. But right now, both rovers are in amazingly good shape. Weʼre going to work them hard to get as much benefit from them as we can, for as long as they are capable of producing worthwhile science results.”

The current science exploration has gone far beyond the roversʼ original mission, which was simply to determine whether or not water had ever been present on Mars.

“As we continue to explore, we continue to find new things,” said Steve Squyres, the Goldwin Smith Professor of Astronomy at Cornell and principal investigator for the roversʼ science instruments. “Mars keeps throwing stuff at us.”

Spirit is approaching the summit of the highest of the Columbia Hills, named Husband Hill for Rick Husband, commander of the Columbia space shuttle. (The seven hills making up Columbia Hills are named for the seven Columbia crew members.) “As we work higher, we are finding new materials,” Squyres reported. “We are finally getting evidence, for example, that there is some layering of the rocks.” What they are seeing looks like volcanic layering that has been altered by water, he added.

Meanwhile, Opportunity is heading south at record speed and is within a few football fieldsʼ length of a region called “Etched Terrain,” which looks like a pattern of parallel grooves and ridges in pictures taken from orbit. Squyres hopes this area might show rocks that have been exposed by wind erosion.

“All the other rocks weʼve looked at have been exposed by cratering, a violent process that made the geological history jumbled like it had been through a blender,” he said.

He said that if the rover can continue past that area it will come to Victoria Crater, which is 900 meters in diameter and has 40-meter-tall cliffs that could reveal much deeper layers of rock. The rover just may be able to drive down what appear to be eroded ramps. “It could be the biggest thing of the entire mission if we can get to it,” Squyres said.

To reach Etched Terrain, rover planners have been pushing the rover fast. Opportunity has overtaken Spirit in total distance driven. It has rolled more than three miles – eight times the original goal. On March 20, Opportunity also set a new Martian record of 722 feet in a single dayʼs drive. The long drives take advantage of crossing a plain so smooth itʼs “like an East Coast beach,” said JPLʼs Jeff Favretto, mission manager on the Opportunity shift in recent weeks.

Also, Opportunityʼs solar panels, though now dustier than Spiritʼs, still generate enough power to allow driving for more than three hours on some days. One reason the rovers are still rolling is a “miraculous” cleaning of dust off of their solar panels by strong Martian winds.

It had been expected that dust would accumulate on the panels until they were no longer able to absorb enough solar energy to recharge the batteries, and that would be the end of the mission.

But a sudden wind cleaned the dust off Opportunityʼs panels several weeks ago, and just a few days ago, several days of wind blowing through the Columbia Hills did the same for Spirit. When Spirit first landed, it was collecting about 900 watt-hours per day, Squyres said, and a month ago that was down to 350 watt-hours.

“We think death comes at around 280,” he noted. But two days ago, after a wind treatment, Spirit was back up to 830 watt-hours per day. “Itʼs like the vehicle just came off the showroom floor,” Squyres exulted.

Meanwhile, he said, Opportunity is racking up about 600 to 700 watt-hours per day. The jump in power output has taken some urgency out of Spiritʼs southward climb.

With Mars now beginning southern-hemisphere spring, the sun is farther south in the sky each day. If not for the panel cleaning, Spirit might be facing the prospect of becoming critically short of power if it is still on the north-facing slope in early June.

“We still want to get to the summit of Husband Hill and then head down into the ʻInner Basinʼ on the other side,” Squyres said. “But now we have more flexibility in how we carry out the plan. Before, it was climb or die.” Although cresting the hill is now not as crucial for solar energy, it still offers the allure of possibly finding exposed rock layers not yet examined, plus a vista of surrounding terrain. In orbital images, the Inner Basin farther south appears to have terracing that also hints of layered rock.

Both rovers do have some signs of wear and exposure. Spiritʼs rock abrasion tool shows indications that its grinding teeth might be worn away after exposing the interiors of five times more rock targets than its design goal of three rocks. Researchers probably wonʼt know the extent of wear until Spiritʼs next rock-grinding attempt, which may be weeks away. Also, troubleshooting continues for determining whether Opportunityʼs miniature thermal emission spectrometer is still usable after tests indicated a problem last month.

All other instruments on both rovers are still working normally.

“Despite the fact that the vehicles appear to be in excellent health, this could all be over tomorrow, and we live with that daily,” Squyres said. “They could last another year or two, or could die today. We voided the warranty on these things a long time ago.”