A Stirling radioisotope generator (SRG) is a type of radioisotope generator based on a Stirling engine powered by a large radioisotope heater unit. The hot end of the Stirling converter reaches high temperature and heated helium drives the piston, with heat being rejected at the cold end of the engine. A generator or alternator converts the motion into electricity. Given the very constrained supply of plutonium, the Stirling converter is notable for producing about four times as much electric power from the plutonium fuel as compared to a radioisotope thermoelectric generator (RTG).
The Stirling generators were extensively tested on Earth by NASA, but their development was cancelled in 2013 before they could be deployed on actual spacecraft missions. A similar NASA project still under development, called Kilopower, also utilizes Stirling engines, but uses a small uranium fission reactor as the heat source.