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A radioisotope thermoelectric generator (RTG, RITEG) is an electrical
generator that uses an array of thermocouples to convert the heat released
by the decay of a suitable radioactive material into electricity by the
Seebeck effect.

RTGs have been used as power sources in satellites, space probes and
unmanned remote facilities such as a series of lighthouses built by the
former Soviet Union inside the Arctic Circle. RTGs are usually the most
desirable power source for robotic or unmaintained situations that need a
few hundred watts (or less) of power for durations too long for fuel cells,
batteries, or generators to provide economically and in places where solar
cells are not practical. Safe use of RTGs requires containment of the
radioisotopes long after the productive life of the unit.