For more than 50 years, NASA’s robotic deep space probes have carried nuclear batteries provided by the U.S. Department of Energy. Even the crewed Apollo moon landings carried nuclear powered equipment. However, the United States’ supply of plutonium-238, which fuels these batteries, called radioisotope thermoelectric generators (RTGs), is running low. Experts worry that ambitious planetary science missions in the future may have to be put on hold until more of the radioactive substance is available. Learn about nuclear power used on NASA space probes in this infographic. Source: All about our solar system, outer space and exploration