The aging but intrepid Mars rover Opportunity is set to embark on a two-year mission it may never complete -- a 7-mile (12-km) journey to a crater far bigger than one it has called home for two years, NASA said.

The golf-cart-sized robot with a wobbly front wheel climbed out of Victoria crater earlier this month and scientists at NASA's Jet Propulsion Laboratory in California are steering the probe toward a crater more than 20 times larger, dubbed Endeavor.

But with the rover able to travel only 110 yards (100 metres) per day, the mission control team at JPL said it could take two years for Opportunity to reach its destination. There is no guarantee the vehicle will survive the trip.

Opportunity, like its twin rover Spirit, semi-idle for the moment on the opposite side of Mars, is well past its original three-month life expectancy.

The 7-mile stretch between Victoria and Endeavor craters matches the total distance the rover already has covered in the 4 1/2 years since landing on the planet.

"We may not get there but it is scientifically the right direction to go anyway," said Steve Squyres of Cornell University, the principal science investigator for the project. "This crater (Endeavor) is staggeringly large compared to anything we've seen before."

The two rovers arrived on Mars in January 2004 to begin an ambitious geologic expedition aimed at finding signs of water, and by extension determining whether the planet was ever sufficiently moist to support life.

The probes are equipped with a range of sophisticated laboratory equipment and cameras to explore rocks and soil on the martian surface.

Scientists are eager to get a glimpse into Endeavor, a bowl measuring 13.7 miles (22 km) across, where they expect to find a much deeper stack of rock layers than those at Victoria.

"But even if we never get there, as we move southward we expect to be getting to younger and younger layers of rock on the surface," Squyres said.

Also along the way are small rocks strewn about the surface that appear to have been dug up by meteor impacts farther away, giving scientists the chance to examine material that otherwise would be too deep to reach, he said.

Opportunity still has the use of all six of its wheels, though the right front wheel can no longer be steered. Self-navigation software for both rovers was updated in 2006.

The two probes also benefit from high-resolution pictures of the planet's surface from NASA's Mars Reconnaissance Orbiter, which can be used to help JPL teams steering the rovers avoid potential obstacles.