For several decades, robots have been produced to carry out an assortment of tasks, including assembly-line manufacturing jobs (e.g., spot-welding), aerial surveillance, and domestic chores (e.g., vacuuming the living room carpet). The programming for specific and fairly repetitive-action tasks such as these is relatively simple. But in more recent years, robot design has become far more sophisticated. Today, many robots are being designed to act autonomously — that is, to react on their own to what they encounter in their environment.

In 2003, the Mars Exploration Rover mission was launched with the goal of learning more about the geological history of that planet, particularly the role that liquid water may have played. To carry out this task, twin robots — each the size of a golf cart — were sent to Mars. These "rovers," named Spirit and Opportunity, would be the first vehicles to roll across the surface of another planet.

To help fulfill the mission's research goals, Spirit and Opportunity were equipped with identical sets of custom-made scientific instruments, which included a panoramic camera to create high-resolution images and a spectrometer to determine mineralogy. Each rover also has a robotic arm to hold and manipulate miniature versions of instruments used by geologists on Earth, additional spectrometers to identify the elemental composition of rocks and soil, and a microscopic imager to enable close-up views of Martian features.

The rovers were also designed to exhibit great mobility over largely unknown terrain. Robotic engineers were challenged to develop wheels that were light enough to help keep down the weight of the spacecraft, compact enough to fit in the landing vehicle, and adaptable enough to allow the rover to maneuver off the lander and navigate around surface obstacles. Their solution? A flexible mobility system of six cleated, aluminum wheels, each 26 cm (10 in) in diameter, which enable the rover to traverse both sandy soil and rocks that are larger than the wheels' diameter.

Each rover was also designed with a jointed suspension system so that if one side of it climbed up over a rock, the other side went down to redistribute weight evenly over the all wheels and thereby prevent the possibility of tipping over. Despite — and perhaps also because of — their thoroughly considered design, the rovers could move at a maximum speed of only 5 cm/sec (2 in/sec).

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