That isn’t rare: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge... The Pionee

That isn’t rare: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge... The Pioneer probes used them to power their radios decades after launch. There aren’t real alternatives.