The U.S. space agency says a motor in the robotic arm of its Mars rover Opportunity that began stalling more than two years ago has become more troublesome.

National Aeronautics and Space Administration engineers at the Jet Propulsion Laboratory in Pasadena, Calif., are diagnosing why the motor, one of five in the robotic arm, stalled April 14. NASA said the engineers are also examining whether the motor can be used and assessing the impact on Opportunity's work if the motor is deemed unusable.

The malfunctioning motor controls sideways motion at the shoulder joint of the rover's robotic arm. Other motors provide up-and-down motion at the shoulder and maneuverability at the elbow and wrist. A turret at the end of the robotic arm has four tools that are used to study rock and soil composition and texture.

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 the twin rovers Opportunity and Spirit.

Opportunity and Spirit landed on Mars in January 2004 to begin a three-month mission. They have continued operating for more than four years.