NASA: One more try to free plucky Mars rover

Washington - The Spirit Mars rover may have met its match in a patch of sandy soil that has ensnared it on Mars, but NASA scientists said Thursday they will make a last effort to free the little "rover that could."Spirit is one of two rovers that have far exceeded expectations and are now in their sixth year after an expected 90-day mission, exploring the Martian surface and making important discoveries about water on the Red Planet.

It has overcome obstacles no scientist thought it would, earning it kudos as "the little rover that could" after a popular children's picture book, "The little engine that could."

But Spirit ran aground in April when it broke through a hard shell of soil and its wheels got stuck in soft soil the consistency of talcum powder. Since then scientists have been at work on the ground running models in a sandbox at the Jet Propulsion Laboratory in California to work out a solution.

"This lonely spot may be where Spirit ends its adventures on Mars," said Doug McCuistion, NASA's director of the Mars exploration programme.

Beginning Monday, NASA will send signals to the rover in an effort to back it out of the ground where it is stuck. Moving just centimeters at a time, NASA ground controllers will attempt to steer Spirit backwards in the direction from which it came. The task could last until at least February, when an annual review of the programme is scheduled.

"If you have ever walked in a sandbox, it's very difficult to get traction and that's the situation that Spirit is in," said Ray Arvidson, a Washington University in St Louis scientist on the Mars exploration team.

The situation is complicated by a pointy rock located underneath the rover's belly that could become snagged.

If NASA is unable to free the rover, scientists may instead chose to let it stay put and focus on conducting science at that location until it dies. The site is likely an impact crater with coarse soil containing the highest sulfate content yet found on Mars, said Arvidson.

Spirit landed on January 3, 2004 and was followed by sister rover Opportunity three weeks later. Since then, the golf cart-sized crafts have provided scientists with valuable information about the Red Planet's wet history, while sending back 250,000 images and driving more than 13 miles.

Opportunity is still in good condition and moving toward the Endeavor crater for further exploration.

Perhaps the most important discovery of the mission to date was silica in Mars' soil that was uncovered by a dragging wheel on the Spirit rover. The mineral was seen as a likely product of a damp environment produced by hot springs or steam vents.

In 2007 NASA almost lost contact with Opportunity as it battled to survive a Martian dust storm that cut off its solar power source. Spirit has also faced some difficulties from dust, and it barely survived its third Martian winter that ended last December as its solar panels became coated with dirt.

The crafts' survival is even more remarkable because nearly two- thirds of Mars missions in the past have failed, with a European craft Beagle 2 launched just before Spirit and Opportunity failing to make any contact with scientists back home.