NASA Rover Labors Toward Hill, Winter Sun

The Mars rover Spirit has lost power in one of its six wheels, slowing its trek toward a slope where the solar-powered vehicle should catch enough sunlight to keep operating through the Martian winter.

It wasn’t clear why Spirit’s right front wheel stopped working. Because the wheel was drawing no power, engineers at NASA’s Jet Propulsion Laboratory in La Canada Flintridge speculated that the problem was a broken electrical wire or malfunctioning motor.

Spirit, driving backward and dragging its broken wheel, is inching toward McCool Hill, where it is to spend the winter facing the sun.

With days growing shorter as winter approaches in the southern hemisphere of Mars, Spirit gets only enough power for one hour of driving a day on flat ground. Spirit can cover 40 feet a day if its path is clear. It was 200 feet short of its goal on Friday.

The hobbling rover’s progress stalled Monday, about 300 feet from the slope, when the rover ran into a mound. The obstacle would not have posed a challenge if all six wheels had been working, said John Callas, project manager for the rover mission. After breaking free Tuesday, Spirit limped only 12 feet farther.

To continue functioning, Spirit must generate more than 250 watt-hours each day. The vehicle’s solar panels generated 350 watt-hours of energy daily last week, down 15% from February.

In addition to the shorter days, dust on its solar panels blocks sunlight, interfering with charging.

Plummeting temperatures are also a problem.

The deepest winter day in the southern hemisphere is Aug. 5.

Spirit’s right front wheel became balky in June 2004, five months after it landed on Mars. JPL mission controllers fixed the problem by driving Spirit backward to redistribute lubricant.

Spirit and its twin, Opportunity, landed on opposite sides of Mars and have significantly outlasted their planned 90-day mission.

Opportunity, which remains in good condition, is closer to Mars’ equator, so it does not need to winter on a slope.