Alex writes:
Actually, conventional RTGs have a comparable specific power performance to the current state-of-the-art Stirling Radioisotope Power Sources (SRPS). However, the advanced SRPS being contemplated might, when combined with a putative sub-kilowatt electric propulsion system, enable this method to fly on outer planetary spacecraft without radical mass reduction. One will note that the SPRSs carry about a factor of four less fuel (by mass) than typical RTGs in the first place, so SPRS-powered electric propulsion for, say, Pluto exploration is a definite possibility.

Alex R. Blackwell
University of Hawaii

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After I posted my question, I saw SPRSs being mentioned in a site I visited yesterday, but it was very briefly and it eluded that it was something that NASA has had in mind as a power source for at least a couple of years now.

I'm sure someone can come up with a proposal or two.

Good to see you, Alex.

Marcia