The US space agency NASA has conceded defeat in its battle to free its Spirit rover trapped in the Sand on Mars.

The vehicle became stuck in soft soil back in May last year and all the efforts to extricate it have failed.

NASA says Spirit, which landed on the Red Planet ver six years ago, will no longer be a fully mobile robot, and has instead designated the once-roving scientific explorer a stationary science platform.

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.

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 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, Calif., plus analysis, modeling and reviews. In November, another wheel quit working, making a difficult situation even worse.

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.

It is one of two rovers Nasa landed on the planet in the January of 2004. The second vehicle, Opportunity, continues to roll freely on the surface.

Even in a stationary state, Spirit continues scientific research.

One stationary experiment Spirit has begun studies tiny wobbles in the rotation of Mars to gain insight about the planet's core. This requires months of radio-tracking the motion of a point on the surface of Mars to calculate long-term motion with an accuracy of a few inches.

There's a class of science we can do only with a stationary vehicle that we had put off during the years of driving, said Steve Squyres, a researcher at Cornell University and principal investigator for Spirit and Opportunity.