http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Space http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_gen... Those are just RTGs.

http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Space http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_gen... Those are just RTGs. You can also put a proper fission reactor into space as well. This has been done in the past several times to power active radar surveillance satellites. They've been around since the early 70s, the TOPAZ-II from the early 90s could put out around 10kW. TOPAZ-I[aka TOPAZ] could do 5kW. As you would expect, they put out even more power as heat. Those 5kW-10kW numbers are just what you get in electrical power, through the thermionic converters, which are not particularly efficient. TOPAZ reactors are about half the mass of Voyager I. The upper bound on mass that we've already put into interplanetary space is probably the five S-IVB stages in heliocentric orbit right now, each with a dry mass of about 10,000kg. That's about 10 TOPAZ-II's or 30 TOPAZ's. There was a proposed manned Venus flyby mission using a wet-workshop S-IVB, which would have been much heavier.