How do Radioisotope Thermoelectric Generators (RTGs) work for spacecraft power?

RTGs convert heat from radioactive decay to electricity using thermoelectric effect. Components: Heat source - Plutonium-238 (87-year half-life, alpha emitter), Thermoelectric couples (SiGe or PbTe), Cold side radiator, and Radiation shielding. Operation: Radioactive decay produces heat (~250W thermal per kg of Pu-238), Temperature difference across thermocouples generates voltage, No moving parts, extremely reliable. Efficiency: ~6-7% thermal to electric (improving with new materials). Applications: Deep space missions (beyond Jupiter where solar power inadequate), Planetary surface operations (Mars nights, lunar poles), and Long-duration missions (Voyager still operating after 45+ years). Disadvantages: High cost, Limited Pu-238 supply, Safety concerns (launch accidents), and Export restrictions. Newer designs (Multi-Mission RTG) improve efficiency and reduce plutonium requirements.