This view of Victoria crater on Mars was taken by the rover Opportunity as it was looking southeast from Duck Bay towards the dramatic promontory called Cabo Frio. ((NASA/JPL/Cornell))

The NASA rover Opportunity sent stunning pictures of Mars back to researchers on Earth Friday as it begins to explore cliffs ringing the massive Victoria crater.

"It's probably the biggest crater we're ever going to get to with Opportunity," Doug McCuistion, director of the NASA Mars exploration program, said in a news conference Friday.

"Why is that important? The bottom line is it gives us a window on the past of the planet."

Victoria crater is about 800 metres in diameter and more than 60 metres deep — about five times the size of a large sports stadium. Opportunity had to cross roughly 10 kilometres of terrain from its landing site to reach the crater, a trip that took 21 months.

"It's not an easy engineering feat at all," McCuistion said. "We've been stuck in a few dunes and we had some mechanical problems, but thanks to the quality of the team, we've overcome all of those issues."

The crater is a "tremendously important scientific target," saidSteve Squyres, principal investigator with the Mars rover team.

"The rocks that we have been driving around for 21 months at Victoria crater are layered, like a stack of pancakes," he told reporters.

'Into the surface of Mars'

"We have been driving along the top of that stack, effectively seeing more or less the same rocks over and over and over again. But now, with this enormous hole in the ground in front of us, we have the capability to see what lies beneath — to see down into the surface of Mars."

He added that his team is looking for a safe place to enter the crater so that Opportunity can capture close-up images and use some of the sensors mounted on its robot arm to analyze the layers of exposed rock.

"What we want to do is find a safe path in and a safe path out," he said.

"Assuming that we do, and I think our chances of that are excellent, then we will definitely go down into the crater … but we're going to do it carefully, we're going to do it safely and we're going to do it when we're ready."

Although he couldn't give a specific timeline for a descent into Victoria crater, Squyres said the clock is ticking.

"The thing we have to keep in mind is that these vehicles are 10 times past their warranty," he said.

"This vehicle could die at any moment. We have no guarantee that it's going to last, so we have to use this vehicle aggressively. I can't tell you how long it's going to take, but we are not going to take a leisurely tour along the rim of this crater because our days are numbered."

The mission was expected to last until late April 2004, but the rover has proven surprisingly long-lived.

Mission extended

Launched on two separate rockets in the summer of 2003, Opportunity and its identical counterpart, the Spirit rover, were designed to cover about 100 metres each Martian day.

Spirit landed on Mars on Jan. 3, 2004, in the Gusev Crater, a wide basin that may once have held a lake. Opportunity arrived on Jan. 24 the same year, and explored in the Meridiani Planum. Both have found evidence of liquid water on Mars.

In April 2005, NASA extended the mission for another 18 months, although it says the rovers have developed some mechanical problems, including a right-front steering actuator that jammed on Spirit, which makes turning the vehicle more difficult.

In related news, NASA released highly detailed images from the Mars Reconnaissance Orbiter on Friday, including a bird's-eye shotshowing Opportunity as a tiny speck on the edge of the Victoriacrater.

Alfred McEwen, principal investigator for the Mars Reconnaissance Orbiter HiRISE camera, said the craft is orbiting 275 kilometres above the planet and is delivering shots with a resolution of 27.5 centimetres per pixel.

He pointed out that the orbiter's main mission does not start until early November, and that the images being transmitted at the moment are part of preliminary tests. But, he added, "Things are working fabulously well."