This image shows a screenshot from the software used by engineers to drive the Mars Exploration Rover Spirit. The software simulates the rover's movements across the martian terrain, helping to plot a safe course for the rover. The virtual 3-D world around the rover is built from images taken by Spirit's stereo navigation cameras. Regions for which the rover has not yet acquired 3-D data are represented in beige. The red dart to the left shows a target destination for the rover. Red lines indicate the path the rover's wheels will follow to reach the target, and the blue line denotes the path of the rover's "belly button," as engineers like to call it.