A radio-thermal generator or RTG (also known as radioisotope thermoelectric generator) is a device that uses the heat derived from the decay of a radioactive isotope [V] to generate electricity. The first demonstrated RTG was produced in 1958 and displayed by President Dwight D. Eisenhower in January 1959. The Martin Company of Baltimore accomplished the research and design work, and Minnesota Mining and Manufacturing Company (now the 3M Company) developed the conversion system for transforming the heat into electricity. The RTG used the concept developed by Thomas Seebeck (1770–1831) in the thermocouple [IV] in which an electrical current is produced by keeping two metals at different temperatures in contact. In most RTGs the heat from the decay of 238Pu, an isotope of plutonium, was used to heat a thermocouple.
Larger and larger RTGs with greater power production were utilized in the space program of the National Aeronautics and Space Administration (NASA) from 1961 to about 1986. They were employed in the Pioneer, Viking, and Voyager space exploration vehicles. However, with the larger
power demands of the space vehicles that transported humans, the RTG was insufficient, and NASA adopted the use of fuel cells as a source of electric power. As a measure of the efficiency of RTGs, the ratio of watts produced per kilogram (or power density) reflected incremental improvement to the RTGs. Some of the first RTGs employed in Transit satellites in 1961 had a watt/ kilogram ratio about 1.29, while those on the Voyager space vehicle in 1980 produced 158 watts with a 4.2 watt/kilogram ratio.
In the mid- and late 1980s NASA phased out the use of RTGs, replacing them with improved fuel cells. In addition to the higher power requirements, RTGs caused concern that a satellite or space vehicle that accidentally crashed to Earth and that had an RTG for the production of electricity could create a radiological hazard on the ground. It was recognized that the adverse publicity from such an event could harm the space program more generally, so NASA discontinued their use.