NASA has approved another 18-months of crawling around on the Red planet for the two Mars rovers, Spirit and Opportunity. The rovers, which were originally designed to explore the surface of Mars for just three months, are holding up so well after their fourteen month stay on the red planet, that mission scientists say they are having to make long term plans for them.

Spirit got an unscheduled spring-clean in March this year, when a dust devil swept past, clearing Martian sand from its solar panels, considerably improving its power output. Scientists had been pushing the solar-powered rover quite hard to get it out of the shadow of a rocky slope dubbed "Husband's hill". The power boost means it is making good progress.

Dr Steve Squyres of Cornell University, principal investigator for the rover's science instruments, was concerned that the rover would be critically short of power as the Southern Hemisphere's summer drew to a close (around June). He says he still wants to get the rover over the hill and into the next basin, but that there is now more flexibility about the amount of science they can do.

Meanwhile, Opportunity is nearing a region called "Etched Terrain". Scientists think they will find rocks exposed by wind erosion, rather than by impact events, and rocks from a different period in Mars' history. Squyres described Opportunity's new target as "a journey into the unknown, something completely new". The rover has much dustier solar panels than its twin, but can still roll for as much as three hours in a day. It has now covered a total distance of three miles - eight times the distance originally planned for.

Jim Erickson, rover project manager at NASA's Jet Propulsion Laboratory, warned that although plans are being made, either mission could easily be totally derailed by 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," he said.

Both rovers are showing their age, a little. Spirit's drill's teeth seem to have worn away after grinding through five times as much rock as expected, and Opportunity's miniature thermal emission spectrometer is playing up.

"But right now, both rovers are in amazingly good shape," Erikson continued. "We're 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." ®

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