The Mars Exploration Rovers act as robot geologists while they are on the surface of Mars. The Mars rover is powered by radioactive thermoelectric generator (dark cylinder on rear). Radioisotope thermoelectric generators work by converting the heat generated by a radioactive isotope to electricity using an array of thermocouples. In particular, Pu-238 fuel, which generates about half a kilowatt of heat per kilogram of isotope, has been used to power space missions and remote military installations since the early 1960s. It has Autonomous Planetary Mobility for enabling the rovers to make decisions and avoid hazards on their own - for instance its new path-planning software has helped the rovers avoid mission barriers. When rocks are unavoidable, the inherited suspension system allows for easier maneuverability. It has technologies for severe environments including heaters for batteries to cope with the extreme cold.