NASA's Opportunity rover is just days away from reaching the rim of Victoria Crater, the largest crater any rover has ever encountered on Mars. As of Tuesday, the rover was 20 metres away from its edge.

Rover managers estimated that they could reach their stopping point on Thursday or Friday.

"It's perhaps the greatest opportunity for exploration with these rovers since we've landed," says Mars Exploration Rover project manager John Callas.

Scientists want to study the horizontal layers of rock along the walls of the crater to learn more about the planet's geological history.

The Eagle Crater, in which Opportunity landed in 2004, gave geologists about 0.5 metres of layered rock to study. Endurance Crater, where Opportunity spent about six months, provided 7 metres of layers.

"I think we can get potentially several times that here," says Steve Squyres, the rovers' principal investigator and a planetary scientist at Cornell University in New York, US.

Victoria stretches between 750 and 800 metres across and appears to be at least 60 metres deep, which should allow scientists to study further back in time than ever before by looking at older rock layers at the bottom of the crater. (Watch an AVI movie of the crater, as seen from orbit.)

Opportunity's first stop along the rim will be a region named Duck Bay. From there it will survey two promontories on either side of it to determine which to drive to. From one of these outlooks, Opportunity will make a panoramic image of the crater that will guide further exploration.

"Our orbital reconnaissance of the crater suggests that there might be ways to drive into it," Callas told New Scientist.

Conceivably, Opportunity could spend its last days in Victoria. It has already defied expectations about its lifetime.

"Every day could be our final resting place," Squyres says. "We've exceeded the warranty. The rover could die at any moment."...