Engineers spent this week simulating the Martian terrain at Nasa's Jet Propulsion Laboratory in Pasadena to figure out why the robot got bogged down and how to get it moving again.

 

Engineers performed several tests driving a dummy rover over a man-made sand dune.

 

Scientists sent the first new driving directions to Opportunity on Wednesday, commanding it to start inching down from the dune in a series of "mini-drives".

 

The six-wheeled Opportunity had driven about 39m of a planned 88.5m trip when its wheels started to slip on 26 April.

 

Going backwards

 

The rover, going backwards at the time, eventually stopped moving - its wheels stuck hub-deep in fine soil while trying to drive more than a 30cm high sand dune.

 

"Mars gives us surprises on a regular basis - some major, some minor," said project manager Jim Erickson. "This is something we consider a major one."

 

In the meantime, the rover has been taking pictures of its surroundings at the edge of an area known as the "etched terrain", where scientists believe they will uncover rocks exposed by the gentle erosion of wind.

 

Opportunity and its twin, Spirit, have been exploring opposite sides of Mars since landing in January 2004, and have uncovered geologic evidence of past water activity on the planet. Both rovers outlasted their primary, three-month missions long ago.

 

This is not the first time a rover has encountered a wheel problem.

 

In June 2004, Spirit's right front wheel became faulty. Spirit overcame the problem by driving in reverse, allowing it to drag the faulty wheel while driving itself with its five other wheels.