WASHINGTON (AFP) – NASA admitted defeat Tuesday saying efforts to free the Spirit rover bogged down by Martian sand were over and instead the plucky robot was hunkering down to brave the harsh Mars winter.

"Spirit is not dead; it has just entered another phase of its long life," said Doug McCuistion, director of the Mars Exploration Program at NASA headquarters in Washington.

"It looks like Spirit's current location on Mars will be its final resting place."

Earlier this month NASA celebrated Spirit's bountiful, six-year stint on the Red Planet, way longer than the three months it was forecast to last.

But in April the tireless, 180-kilogram (400-pound), six-wheel robot broke through a crusty surface layer and hit sand at one edge of the Troy crater, west of the Home Plate plateau, in the Martian southern hemisphere.

All attempts to extricate it have failed. Dead in its tracks, Spirit cannot shake off the Martian dust that has been slowly accumulating on its solar panels, preventing its batteries from recharging.

Now with winter arriving in May, the Spirit will not get enough light from the sun to be able to recharge, NASA said.

"Solar energy is declining and expected to become insufficient to power further driving by mid-February. The rover team plans to use those remaining potential drives for improving the rover's tilt," NASA said in a statement.

NASA hopes to be able to tilt the Spirit enough to keep it going through the winter.

"Getting through the winter will all come down to temperature and how cold the rover electronics will get," said project manager John Callas.

"Every bit of energy produced by Spirit's solar arrays will go into keeping the rover's critical electronics warm, either by having the electronics on or by turning on essential heaters."

Even though it will be stationary, Spirit will still be able to carry out studies and has already begun to monitor tiny wobbles in the rotation of Mars to gain insight about the planet's core.