As of September, the rovers are in the second extension of their mission, which was initially expected to last three months.
On Oct. 29, Spirit's panoramic camera transmitted the 50,000th image from the pair of rovers. The photo showed the camera's calibration target with rocks and soil of "Columbia Hills" -- the site Spirit examined on its 260th Martian day, or sol -- in the background. The calibration target includes reference colors and a shadow-casting post.
A Martian day is about 40 minutes longer than one on Earth.
As of Tuesday at a point opposite the planet from Spirit, Opportunity had gone as far east as it could inside "Endurance Crater" and had begun a remote-sensing campaign to capture panoramic images of Burns Cliff and to make other scientific readings.
"Opportunity remains healthy and in an extremely advantageous solar array attitude," reported mission managers at NASA's Jet Propulsion Laboratory.
Scientists had hoped to steer Opportunity out of the crater without the need for the rover to backtrack over ground it has previously studied, but the terrain appears too treacherous for the six-wheeled robotic explorer.
Opportunity will try a southward route or retrace the path it used to enter the crater in June.
"We've done a careful analysis of the ground in front of Opportunity and decided to turn around," rover project manager Jim Erickson said in a statement, according to the Associated Press. "To the right, the slope is too steep -- more than 30 degrees. To the left, there are sandy areas we can't be sure we could get across."
The rover has already found in the stadium-size crater layers of rocks that have given scientists evidence that the area was once wet. Before starting its retreat, the rover will study a 33-foot-high cliff to determine whether some layers were deposited by wind, rather than water.
The solar-powered machines are reportedly in good health as they continue to explore the planet and have resumed full operations after about two weeks of not driving in mid-September while communications were unreliable because Mars was passing nearly behind the Sun, according to the Cornell University rover team.
The rovers' programming has become more efficient since their January landing. The planning and operations that must occur twice each day, between the time the previous day's data reach Earth until commands for the next day's activities are sent to the rovers, now takes an average six or seven hours rather than the 17 hours it once took, the university said.