I'm very pro-nuclear and strongly support the use of RTGs in interplanetary spacecraft. However RTGs are heavy, expensive, require environmental impact reports and can't be turned off (they're always producing heat). Not being able to turn them off is a big nuisance if they're under a heat shield. During interplanetary transfer, coolant must flow through plumbing from the carrier bus to the RTG and then back to a radiator that dumps the unwanted heat to space. If you look at the pictures of the Viking spacecraft on Mars, you'll see lots of chopped off pipes sticking straight up from the spacecraft. These are old coolant pipes that got cut away by explosive cutters. If you upgrade MER to use RTGs then you have to throw the old proven MER design in the trash and start from a clean piece of paper. In essence you're reinventing MSL with all the baloney associated with in.

Gary