Nasa extends Mars rover lifespan

The rovers are in good shape despite showing some signs of wear

The US space agency (Nasa) has approved up to 18 months of further operations for its twin Mars Exploration Rovers.

Solar-powered robot geologists Spirit and Opportunity have both found signs of a watery past on Mars since landing on the Red Planet in January 2004.

The rovers are showing signs of wear - for example, the teeth on Spirit's rock grinding tool seem to have worn down - but otherwise remain in good shape.

They were originally meant to carry out missions lasting just three months.

But Nasa scientists and engineers knew that if the robots did not suffer unexpected failures, they could go on for much longer.

The original mission cost $820m and the two previous mission extensions cost $15m. Nasa officials have declined to say how much the third will cost.

"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 Nasa's Jet Propulsion Laboratory (JPL) in Pasadena, California.

But he cautioned: "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."

Despite these concerns, both rovers are in good shape, mission scientists agreed.

"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," said Dr Erickson.

It could be a few weeks before Spirit's next rock drilling attempt, which will inform researchers about the extent of wear and tear on its grinding tool.

Opportunity has also been experiencing problems with its miniature thermal emission spectrometer (Mini-Tes) instrument. A troubleshooting process is underway to determine whether the instrument is still useable.