[petascale]

Probably an RTG [1], it's what NASA normally uses. It's thermoelectric [2]: Heat (in this case from radioactive decay) is converted directly into electricity without moving parts.

[1] https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

[2] https://en.wikipedia.org/wiki/Thermoelectric_effect#Seebeck_...