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Glitch worsens on Mars rover's robotic arm

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A motor glitch has resurfaced aboard NASA's Mars rover Opportunity and may hinder some of the long-lived robot's ability to wield its mechanical arm.

The troublesome motor powers the sideways movement of the shoulder joint on Opportunity's instrument-tipped robotic arm and has suffered from intermittent stalling problems since 2005.

But the glitch appeared to have worsened last week, when the motor stalled much earlier than in the past. Opportunity's robotic arm is currently deployed and NASA engineers are determining whether an earlier fix can still be used to move the motor as desired.

"Motion of the stall-prone shoulder motor is necessary to unstow the arm," mission managers said in a statement. "So if the motor were to become unusable with the arm in the stowed position, the arm could not be deployed again."

The motor stalled on April 14 just after it was unberthed from its stowage hook, mission managers added.

If the motor failed outright while the arm was extended, Opportunity could still conduct science by compensating with other arm motors and some fancy wheelwork, NASA officials said.

"Even under the worst-case scenario for this motor, Opportunity still has the capability to do some contact science with the arm," said John Callas, project manager for Opportunity and its robotic twin Spirit at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The vehicle has quite a bit of versatility to continue the high-priority investigations in Victoria Crater and back out on the Meridiani plains after exiting the crater."

Like Spirit, Opportunity wields a single, 3-foot (1-meter) long robotic arm tipped with a turret containing four instruments to analyze the martian surface. The 13-pound (6-kg) arm contains five motors that provide sideways and vertical shoulder motion, as well as elbow and wrist movement.

The balky shoulder motor on Opportunity's arm began acting up in 2005, when it first stalled due to suspected degraded wiring.

By applying higher voltages than normal to the motor, rover engineers were able to overcome the stall and move the shoulder sideways. Since then, rover handlers have left Opportunity's robotic arm deployed overnight — rather than folded up at the front of the rover — after each day's drive to ensure it can be used for science even if the motor fails permanently.

Engineers are studying whether the same fix can be used for the renewed stall issue, and are holding off on moving Opportunity out of a sandy patch of ground inside Victoria Crater — a 2,625-foot (800-meter) wide depression on the martian plains of Meridiani Planum.

"We'll hold off backing out of the sand until after we've completed the diagnostic tests on the motor," Callas said. "The rover is stable and safe in its current situation, and not under any urgency. So we will take the time to act cautiously."

The Spirit and Opportunity rovers have been exploring Mars since their January 2004 landing. The hardy rovers are more than four years into what was initially planned to be a 90-day mission.