The Mars probes Spirit and Opportunity were designed to be robotic geologists. The size of small golf carts, the twin probes are about as heavy as, and move around with the speed of, a Galapagos tortoise—about 130 feet (40 meters) a day. They were equipped with many of the instruments that a geologist on Earth might carry on a scientific expedition. Spirit and Opportunity were designed to be remotely controlled by scientists on Earth, but were also programmed with a small amount of autonomy to adapt to immediate conditions that they encountered on Mars.