London, Nov 17: Reports indicate that NASA's Opportunity rover on Mars has malfunctioned due to the failure of two of its most important instruments, thus halting research work.

The instruments that have broken down are the rover's rock grinding tool and its infrared spectrometer.

The rock grinding tool on Opportunity is having problems with one of its encoders which monitors the brush that clears away rock dust produced by the grinder. The encoders also tell the rover's computers whether the grind head is moving or not. The grinding tools help the rovers study the interior of rocks by cutting away the surface.

The rover team has also suspended the use of Opportunity's miniature infrared spectrometer, which it uses to study the chemical composition of rocks from afar, because it has become blinded by dust.

A major cause behind this malfunction is that both the rovers were originally designed to last just 90 days on Mars. But their presence on the planet for almost 4 years, has had drastic effects on the longevity and the wear and tear of their instruments.

The rovers' lifetimes were originally expected to be limited by dust accumulating on the panels. If dust reduced harvestable solar power too much, the rovers would have trouble keeping their electronic innards warm enough to survive the cold Martian nights, especially in the winter.

"But gusts of wind have cleaned off both rovers' solar panels from time to time, allowing them to weather the coldest nights," said project manager John Callas of NASA's Jet Propulsion Laboratory in Pasadena, California, US.

"Because of these helpful winds, the rovers will be limited more by how long their components can last against wear and tear," added Callas.

But the rover's team is working on fixing both the instruments.

For Opportunity's rock grinder, or rock abrasion tool (RAT), the rover team did work out a way, but accidentally rotated the brush the wrong way, bending the bristles.

"Some testing is needed to make sure it is safe to continue using the grinder with the bent brush," New Scientist magazine quotes Callas as saying. "Our expectation is that we will be able to resume grinding operations with the RAT within a couple of weeks," he added.

As for the infrared spectrometer, the team is hoping to be able to shake the dust off by jiggling the affected part of the instrument.

The breaking down of the two instruments is not surprising as both the rovers have already experienced several parts failures in the past.

Spirit's right front wheel seized up in March 2006, due to a broken motor. Since then, the rover team has been driving Spirit in reverse and dragging the dead wheel in order to compensate for the problem.

Opportunity 's right front wheel is also injured. It can still spin, but since April 2005, it has no longer been able to turn in different directions to help the rover make turns. Opportunity can still drive reasonably well, however, since the other front wheel and the two back wheels can still swivel.

An 'arthritic' instrument arm has also caused problems for Opportunity, a problem that cropped up in November 2005. The motor at the arm's shoulder joint has degraded and is not producing as much torque as it was designed to.

Despite the various age-related problems, Callas is optimistic about the future of the rovers. "I'm planning to keep these rovers going for years more," he said. "They're still very effective robotic geologists," he added. (ANI)