Callas has worked on the Mars rovers' mission since 2000 and five other Mars missions since joining JPL in 1987. He succeeds Jim Erickson, who has switched to a leadership role on NASA's Mars Reconnaissance Orbiter team. Callas grew up near Boston and graduated from Tufts University in Medford, Mass. He earned his doctorate in physics from Brown University in Providence, R.I.

"Even though the rovers are well past their original design life, they still have plenty of capability to conduct outstanding science on Mars," Callas said in a statement. "The JPL operations team and the remote science team working on the project are the best in the solar system at what they do."

Callas takes over as one of Spirit's six wheels has stopped working. Dragging that wheel, the solar-powered rover must reach a slope where it can catch enough sunshine to continue operating during the Martian winter. The period of minimum sunshine is more than 100 days away, but Spirit gets only enough power for about one hour per day of driving on flat ground � and mission controllers said that power supply is dropping fast.

Spirit's right-front wheel became a concern once before, when it began drawing unusually high current five months after the January 2004 landing on Mars. Driving Spirit backwards redistributed lubricant and returned the wheel to normal operation. Last week, during the 779th Martian day of what was originally planned as a 90-Martian-day mission, the motor that rotates the wheel stopped working.

"It is not drawing any current at all," said JPL's Jacob Matijevic, rover engineering team chief. One possibility engineers are considering is that the motor's brushes, which deliver power to the rotating part of the motor, have lost contact. The motors that rotate Spirit's wheels have rotated more than 13 million times, far more than called for in the rovers' design.

Spirit's solar panels have been generating about 350 watt-hours of electricity daily for the past week, or down about 15 percent since February and less than one-half their output during the Martian summer.

Because the Martian winter is approaching, mission controllers are driving Spirit toward the north-facing side of a hill called McCool, which is part of the seven Columbia Hills, named after the crew members lost in the shuttle Discovery disaster on Feb. 1, 2003.

Controllers intend for Spirit to spend the southern-hemisphere winter with its solar panels tilted as far as possible toward the Sun to maximize its power output.

Opportunity is closer to the equator, so does not need to winter on a slope like Spirit. Opportunity spent has most of the past four months at Erebus Crater, part of the Meridiani Planum. It examined layered outcrops, while the rover team determined and tested a strategy for dealing with degraded performance by a motor in the shoulder of its robotic arm.

Opportunity left Erebus this week and is on a 2 kilometer (1.2 mile) journey to a large crater called Victoria.

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