The two rovers are identical, but will land at different regions of Mars. Spirit landed near the center of Gusev Crater, which may have once held a lake. Opportunity is expected to reach the Meridiani Planum, a region containing exposed deposits of a mineral that usually forms under watery conditions. The two landing sites were selected on the basis of intensive study of orbital data collected by the Mars Global Surveyor and Mars Pathfinder missions. Both sites show evidence of ancient water.

Spirit and Opportunity are larger, more mobile and better equipped than the1997 Mars Pathfinder rover. The mission for both rovers is the same - to seek evidence about whether the environment in two regions of Mars might once have been capable of supporting life. Each carries a sophisticated set of instruments to search for evidence about whether past environments at selected sites were wet enough to be hospitable to life.

"Think of Spirit and Opportunity as robotic field geologists," said Dr. Steve Squyres of Cornell University, Ithaca, N.Y., principal investigator for the Rovers' identical sets of science instruments. "They look around with a stereo, color camera and with an infrared instrument that can classify rock types from a distance. They go to the rocks that seem most interesting. When they get to one, they reach out with a robotic arm that has a handful of tools, a microscope, two instruments for identifying what the rock is made of, and a grinder for getting to a fresh, unweathered surface inside the rock."

Using images and measurements received daily from the rovers, scientists will command the vehicle to go to rock and soil targets of interest and evaluate their composition and their texture at microscopic scales. Initial targets will be near the landing sites, but later targets could be far afield. Sojourner traveled about the length of about one football field during its 12 weeks of activity on Mars. Each Mars Exploration Rover is expected to travel six to 10 times that distance during its three month prime mission.