NASA's Opportunity rover (search) stretched out its rear wheels Thursday in preparation for rolling onto the surface of Mars, possibly a day earlier than expected, and the space agency said its twin Spirit "will be perfect again" after repairs of crippling software problems.

Except for its robotic arm, Opportunity finished unfolding and standing up from its landing position and was ready to travel the final 10 feet from its lander to the surface of Mars, possibly late Saturday, project manager Pete Theisinger told The Associated Press.

NASA earlier had said Opportunity could be ready to reach the surface by Sunday.

Engineers continued to make progress on the Spirit rover, which has been sidelined for more than a week on the other side of Mars.

They were preparing to wipe its flash memory clean of science and engineering files that have stymied its software, said Jennifer Trosper, a mission manager. The fix, likely to be made Friday, could restore Spirit to full health.

"I think it will be perfect again," Theisinger said.

Spirit resumed acquiring science data Thursday, using its panoramic camera to focus on two rocks scientists have called Cake and Blanco.

To smooth Opportunity's way to the martian surface, the National Aeronautics and Space Administration (search) adjusted the forward tilt of its lander, pushing the platform downward so the tips of the preferred exit ramp dug into the martian soil.

Engineers consider roll-off the riskiest maneuver of the surface portion of each mission.

Spirit has resumed using its high-gain antenna, which speeds transmission of the data needed to debug the rover's problems.

The first picture it took in more than a week was a simple black-and-white photograph showing the rover's robotic arm pressed to a football-sized rock, exactly as it was poised before the problems started.

"It's more indication of Spirit's slow progression back to health," said Cornell University astronomer Steve Squyres (search), the mission's main scientist.

NASA landed the $820 million pair of rovers on Mars to hunt for geologic evidence that would show whether the planet ever was wet enough to support life. Spirit landed Jan. 3, followed by Opportunity on Jan. 24.

Opportunity remained in excellent shape, save for a 15-watt power loss that NASA continued to monitor. Engineers believe a heater in the shoulder of its robotic arm has been turning on every night, even though it's needed only when the arm is in use.

Theisinger said the heater did not present an immediate concern but could become an issue later in the mission, when it could cut into the rover's ability to fully recharge its batteries.

Spirit landed in Gusev Crater (search), a 95-mile-wide depression that once may have contained a lake. Opportunity landed 6,600 miles away, on a broad plain called Meridiani Planum. It too ended up in a crater, this one just 20 yards across.

NASA remained unsure of Opportunity's precise location. The Mars Global Surveyor satellite, in orbit around Mars, is expected to take detailed pictures of the area during the weekend.