Sunday marks the sixth year of two rovers exploring terrain on Mars, NASA's Jet Propulsion Laboratory (JPL) said on Saturday.

Designed at JPL, Spirit landed on the Red Planet on Jan. 3, 2004 and its twin, Opportunity, arrived three weeks later. The rovers were built to last for three months, but have lasted six Earth years, or 3.2 Mars years, JPL said in a statement.

In 2004, Opportunity discovered the first mineralogical evidence that Mars had liquid water. The rover recently finished a two-year investigation of a half-mile wide crater called Victoria.

Since landing, Opportunity has traveled more than 11 miles and sent back more than 132,000 images.

JPL spokesperson Guy Webster said nine months ago, Spirit's wheels fell through a crusty surface layer into loose sand hidden underneath. Efforts to escape this sand trap barely have budged the rover.

"The rover's inability to use all six wheels for driving has worsened the predicament. Sprit's right-front wheel quit working in 2006, and its right-rear wheel stalled a month ago," Webster said.

"Surprisingly, the right-front wheel resumed working, though intermittently. Drives with four or five operating wheels have produced little progress toward escaping the sand trap. The latest attempts resulted in the rover sinking deeper in the soil."

If mobility is not possible, the next priority is to improve the rover's tilt, while Spirit is able to generate enough electricity to turn its wheels.

"Sprit is in the southern hemisphere of Mars, where it is autumn, and the amount of daily sunshine available for the solar- powered rover is declining," Webster said.

"This could result in ceasing extraction activities as early as January, depending on the amount of remaining power. Spirit's tilt, nearly five degrees toward the south, is unfavorable because the winter sun crosses low in the northern sky."

Webster said unless the tilt can be improved or luck with winds affects the gradual buildup of dust on the solar panels, the amount of sunshine available will continue to decline until May. Spirit may conk out.

"At the current rate of dust accumulation, solar arrays at zero tilt would provide barely enough energy to run the survival heaters through the Mars winter solstice," said Jennifer Herman, a rover power engineer at JPL.

In February, NASA will assess Mars missions, including Spirit, for potential science benefits versus costs to determine whether to keep them going. Meanwhile, the team is planning additional research about what a stationary Spirit could accomplish as power wanes.

"Spirit could continue significant research right where it is," said Ray Arvidson of Washington University in St. Louis, deputy principal investigator of the rovers. "We can study the interior of Mars, monitor the weather and continue examining the interesting deposits uncovered by Spirit's wheels."