National Aeronautics and Space Administration mission scientists at the Jet Propulsion Laboratory in Pasadena, Calif., said Opportunity moved about 13 feet into the crater Tuesday -- just enough to position all of its six wheels inside the crater rim. It then backed uphill for about 10 feet.

Rover Project Manager John Callas said the driving commands included a precaution for the rover to stop moving if its wheels were slipping more than 40 percent. Slippage exceeded that amount on the last step of the drive, so Opportunity stopped with its front pair of wheels still inside the crater.

"We will do a full assessment of what we learned from the drive … and use that information to plan Opportunity's descent into the crater," said Callas.

The exploration is designed to investigate layered rocks exposed on the crater's interior slope that might reveal evidence of interaction between the Martian atmosphere and surface from millions of years ago, when the planet's atmosphere might have been different from present conditions.