Radioisotope thermoelectric generators (RTGs) operate by converting heat from the natural decay of radioactive isotopes into electricity, rather than through nuclear fission like reactors. Plutonium-238, a key isotope used in RTGs for its long half-life and decay properties, can indeed be a byproduct of certain nuclear fuel cycles, including those associated with weapons programs, although dedicated production methods are employed. RTGs are critical for long-duration space missions where solar power is insufficient or unreliable, providing a continuous and stable power source for spacecraft.