radioisotope thermoelectric generator

A radioisotope thermoelectric generator (RTG, aka radioisotope power system, RPS) is a device that uses radioactivity to produce electricity, functioning as a kind of (electric) battery, i.e., a self-contained source of electricity. It generates power using the thermoelectric effect (a means of inducing a voltage from a difference in heat) from radioactive heating in its radioisotope heat unit (RHU), the heat resulting from radioactive decay of an included radioactive isotope. Small units can offer a small amount of power for years, but providing substantial power results in considerable weight, and a radioisotope with a high energy density is useful. Unfortunately, the most energy-dense isotopes are subject to fission or are related to such isotopes, which has resulted in limiting their use. RTGs have been used by space probes and satellites from the first, and are still used for space missions requiring more electric power than practically capturable from the Sun, typically for planet exploration beyond Earth's orbit. An example RTG is the MMRTG.