The word "robot" was coined in 1934 by the Czech playwright Karel C apek from the Czech word robota, meaning "compulsory labor." While this original meaning still applies to most Earth-bound robots, robots in space have broken through the tedium to become great explorers. They work in environments that may be harmful to humans or in situations where sending a human crew would be too costly. They have been sent as advanced guards to measure the temperature, evaluate the atmosphere, and analyze the soil of other worlds to determine what human explorers can expect to find.

What, exactly, is a robot? A broad definition considers any mechanism guided by automatic controls to be a robot; a very narrow definition requires a robot to be a humanoid mechanical device capable of performing complex human tasks automatically. Robots in space have fallen somewhere in between these extremes. They generally...