The Mars exploration rover Opportunity cautiously ventured hardly any way down a slope leading to the floor of the Martian Victoria Crater, then backed out to check wheel slippage.

NASA officials had feared the rover, if it descended down the slope, might have been unable to climb out of the crater formed eons ago by a huge object such as an asteroid slamming into the Martian surface.

Then, as soon as Opportunity successfully proved it could climb up the steep grade and exit the crater, the rover reversed itself and descended down the slope in earnest. It ended the day about 20 feet down the slope.

This begins a multi-week investigation of crater geology that may yield clues to Martian conditions long ago.

The first destination inside the crater is a light-toned layer of exposed rock that may preserve evidence of interaction between the Martian atmosphere and surface from millions of years ago.

Victoria exposes a taller stack of ancient rock layers than any crater Opportunity has visited previously during the nearly 44 months the rover has prowled the surface on Mars, a mission that originally was planned for three months.

"We want to maintain a safe egress route out of the crater for Opportunity, and by completing the back-up drive over the sand ripple at the rim, we have confirmed that we have one," said John Callas, Mars rover project manager at the NASA Jet Propulsion Laboratory in Pasadena, Calif. "Opportunity is now exploring the interior of Victoria Crater."

Opportunity and its sister rover Spirit have shown themselves to be made of tough stuff. Aside from operating multiple times longer than expected, they recently survived blistering dust storms that shut off sunlight required for adequate electrical power generation, a storm that for a time imperiled the two craft.