updated 12/10/2009 5:47:55 PM ET 2009-12-10T22:47:55

After months stuck fast in deep Martian sand, NASA's embattled rover Spirit is facing a debilitating wheel problem that has slowed its escape efforts and could paralyze the long-lived robot.

Spirit has been trapped wheel-deep in sand for nearly eight months and engineers on Earth have been spinning the Mars rover's wheels in recent weeks to try and force it free. But the rover's right-rear wheel has been stalling, and now may be broken for good.

"These wheel stalls have been such a complication, that we really haven't gotten started," said rover project manager John Callas of NASA's Jet Propulsion Laboratory in Pasadena, Calif., in a recent interview on Spirit's escape attempts.

Recent tests "continue to indicate a troubled wheel, which may leave the rover with only four operable wheels," mission managers said Thursday in a status update.

Spirit is about the size of a golf cart and has six wheels in all. One of those wheels - the right-front wheel – has been broken since 2006. Spirit and its robotic twin Opportunity – which is working fine – have been exploring Mars since January 2004. Since then, they have outlived their initial 90-day mission several times over.

The new wheel trouble for Spirit began during recent escape attempts. The right-rear wheel began stalling when commanded to spin in concert with the four other working wheels in an attempt to back the rover out of its Martian quagmire.

NASA engineers are unsure if they can free Spirit with only four working wheels. The team is on the clock, too. The rover's current position and tilt aren't favorable for surviving the next southern Martian winter, which starts in May 2010.

Rover controllers have been testing a rock drill on Spirit's robotic arm that uses the same motor controller as the balky right-rear wheel to see if they can better understand the problem. The team has also not ruled out an external jam that may be blocking the wheel's motion, and is considering extra steering tests as well.

"We have to get going," Callas told "Winter is coming, and there's now the question of Spirit being able to survive the winter at its current location. So having a mobile rover gives us a much better chance at surviving the winter. That clock is ticking; that's only six months away."

A faulty wheel on the Mars rover Spirit is complicating efforts to free it from a sand trap where it has been stuck for the past eight months.

NASA said Thursday it will continue to run tests to see if there's any movement in the right rear wheel. Spirit, which landed on the red planet with six working wheels, lost the ability to move its right front wheel three years ago.

Engineers are studying whether it's possible to free Spirit with only four operable wheels.

Spirit became bogged down in soft soil in April during a routine drive. NASA last month outlined a rescue plan with little progress made.

Spirit and its twin Opportunity landed on Mars in 2004 and have outlasted expectations.

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