Nasa was also researching a stirling engine based RTG (as opposed to thermoelectric), but lack of funding has put that mostly on ice https://en.wikipedia.org/w

Nasa was also researching a stirling engine based RTG (as opposed to thermoelectric), but lack of funding has put that mostly on ice https://en.wikipedia.org/wiki/Advanced_Stirling_radioisotope_generator https://en.wikipedia.org/wiki/Advanced_Stirling_radioisotope...