The US Patent Office has inducted John Birden (1918-2011) and Ken Jordan (1929-2008) into the National Inventors Hall of Fame for developing the Radioisotope Thermoelectric Generator in the 1950s, sayiing in their announcement:
Radioisotope Thermoelectric Generator (RTG) – Birden and Jordan were working at Monsanto’s Mound Laboratory when they developed the RTG, a self-contained power source that obtains its power from radioactive decay. RTGs have powered most of the exploration vehicles the United States has launched into deep space, where the sun’s intensity is not sufficient to generate electricity with solar cells and steady, reliable power is needed in unmanned situations.
In honor of the occasion, Gary L. Bennett has provided the attached original 1954 report documenting their work combining a Polonium 210 isotope heat source with a Chromel-Constantan thermopile. We note the original effeciency of 0.1 – 0.2% was quite low, but still much better overall than then available alternatives.