Washington - The US space agency NASA began sending signals late Monday to the Spirit Mars rover in an effort to back the robotic device out of a patch of sandy soil where it has been stuck on the Red Planet.

NASA ground controllers were attempting to steer Spirit backward in the direction from which it came, moving it just centimetres at a time. The task could last until at least February, when an annual review of the programme is scheduled.

Spirit is one of two rovers that have far exceeded expectations and are now in their sixth year of what was planned as a 90-day mission, exploring the Martian surface and making important discoveries about water on Earth's neighbour.

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

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.

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.

Spirit landed on January 3, 2004, and was followed by sister rover Opportunity three weeks later. Since then, the golf cart-sized craft have provided scientists with valuable information about the Red Planet's wet history, while sending back 250,000 images and driving more than 21 kilometres.

Opportunity is still in good condition and moving toward the Endeavour 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 survival of the craft 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.  (dpa)