Washington, Jan 30 (IANS) After six years of unprecedented exploration, NASA's Mars Exploration Rover Spirit will no longer be a fully mobile robot.

NASA has designated the once-roving scientific explorer a stationary science platform after efforts during the past several months to free it from a sand trap were unsuccessful.

The venerable robot's primary task in the next few weeks will be to position itself to combat the severe Martian winter.

If Spirit survives, it will continue conducting significant new scientific research from its final location. The rover's mission could continue for several months to years.

'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.

'We told the world last year that attempts to set the beloved robot free may not be successful. It looks like Spirit's current location on Mars will be its final resting place.'

Ten months ago, as Spirit was driving south beside the western edge of a low plateau called Home Plate, its wheels broke through the crusty surface and churned into soft sand hidden underneath.

After Spirit became embedded, the rover team crafted plans for trying to get the six-wheeled vehicle free using its five functioning wheels. The sixth wheel had quit working in 2006, limiting Spirit's mobility.

The planning included experiments with a test rover in a sandbox at NASA's Jet Propulsion Laboratory in Pasadena, California, plus analysis, modelling and reviews. In November, another wheel quit working, making a difficult situation even worse.

Recent drives have yielded the best results since Spirit became embedded. However, the coming winter mandates a change in strategy.

It is mid-autumn at the solar-powered robot's home on Mars. Winter will begin in May. Solar energy is declining and expected to become insufficient to power further driving by mid-February, said a NASA release.

The rover team plans to use those remaining potential drives for improving the rover's tilt. Spirit currently tilts slightly toward the south. The winter sun stays in the northern sky, so decreasing the southward tilt would boost the amount of sunshine on the rover's solar panels.