The images stock a treasury of scientific information on scales from microscopic detail to features on the horizon scores of kilometers or miles away, and even include glimpses of Mars' moons, Earth and the Sun.

They also provide an always-current understanding of the surrounding terrain for use by the team of rover wranglers planning each day's activities on Mars.

There are now more than twice as many images from the two rovers as from NASA's three previous Mars surface missions combined: Viking Lander 1, Viking Lander 2 and Mars Pathfinder.

"The cameras on Spirit and Opportunity have been reliable, sharp eyes for our adventure of exploring some amazing places on Mars," said Dr. Justin Maki of NASA's Jet Propulsion Laboratory, Pasadena, Calif., an imaging scientist on the rover team.

"The pictures continue to be stunning. One big difference from earlier Mars surface missions is that the rovers continue to show us new places and new sights."

All raw images that reach Earth from the rovers are posted online at http://marsrovers.jpl.nasa.gov/gallery/all Captioned pictures, including the 50,000th image and panoramas assembled from many individual raw images, are posted at http://marsrovers.jpl.nasa.gov/gallery/press/.

Both rovers have successfully completed their three-month primary missions and their first mission extensions. They began second extensions of their missions on Oct. 1.

Counting stereo instruments as separate right and left cameras, each rover carries nine cameras.

The stereo panoramic cameras have taken most of the images. Spirit's accounts for 35 percent of the all images from the rovers so far; Opportunity's, 32 percent. Color pictures from these cameras combine individual frames taken through different filters.

Mosaic image products stitch together many contiguous frames for a larger view. A single 360-degree color panorama uses more than 100 individual images.

Usually when a panoramic camera is used, it takes a series of shots of the calibration target through different filters to aid in accurate interpretation of the other shots it takes. It is no surprise that Spirit's calibration target happened to be the subject in the 50,000th image, since it has become the single most photographed subject on Mars.

Spirit's front hazard-avoidance camera (also two cameras for stereo views) has the next highest fraction of the rovers' image catalog at 9 percent. That signifies the importance of this low-slung camera in Spirit racking up 3.6 kilometers (2.3 miles) of driving so far.

Opportunity has driven 1.6 kilometers (1 mile) and its front hazard-avoidance camera has taken 3 percent of all rover images. Totals for the rear hazard- avoidance cameras are about one-fifth of the number from the front cameras on each rover.

Each rover's stereo navigation camera sits up on the mast with the panoramic camera but takes wider-angle images without filters. Spirit's navigation camera has taken 7 percent, and Opportunity's 6 percent, of all rover images.

Some days when Spirit was driving long distances, Opportunity was busy examining bedrock exposures and soil patches with its microscopic imager. That camera on Opportunity has taken 4 percent of all rover images; the one on Spirit, 2 percent.

Each spacecraft had a 10th camera on the bottom of its lander, which contained the rover during the descent through Mars' atmosphere. Those descent cameras each took three images, as planned, during the final minute before impact.

NASA's Viking Lander 1 returned 3,542 images while it operated for 79 months beginning in 1976. Viking Lander 2 returned 3,043 images while it operated for 43 months, also beginning in 1976. Mars Pathfinder returned 16,635 images from its lander and 628 from its Sojourner rover during 12 weeks of operation in 1997.

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