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Last Updated: Saturday, 18 March 2006, 19:59 GMT
Front wheel on Mars rover stops
Mars Exploration Rover, artist's impression, Nasa
The rovers have gone on working far longer than anyone expected
One of the wheels on Nasa's Martian rover Spirit has stopped working.

The robotic vehicle is now dragging the wheel as it moves to a slope where it can get maximum sunshine on its solar cells to sustain it through the winter.

Spirit's right-front wheel has played up before because of a lubrication problem, but engineers on Earth were able to return it to normal operation.

This time, however, it appears to be the motor that rotates the wheel that has ceased to function.

"It is not drawing any current at all," said Jacob Matijevic, rover engineering team chief at the US space agency's Jet Propulsion Laboratory in Pasadena, California.

One possibility engineers are considering is that the motor's brushes - that deliver power to the rotating part of the motor - have lost contact.

Mission team work on an engineering model (Nasa/JPL)
Engineers can simulate solutions on an engineering model back on Earth
The motors that rotate Spirit's wheels have revolved more than 13 million times, far more than called for in the rovers' design.

The vehicle entered its third year on the planet on 3 January.

It has been exploring the geology of high ground in the 95km-wide Gusev Crater, which was created by an asteroid or comet impact early in Mars' history.

Spirit has already lost the use of its rock abrasion tool because the teeth have become too worn to grind the surface off any more rocks. Its wire-bristle brush, though, can still remove loose coatings to give imaging instruments a clearer view of rock specimens.

Spirit is trying to get to the north-facing side of elevated terrain known as "McCool Hill". The plan is for the rover to spend the southern-hemisphere winter on Mars tilted toward the Sun, to maximise the light falling on the vehicle's solar panels.

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