No guts, no glory. Cassini is worth it. Sure we need to be ultra careful, but progress is never without some risk. Renewable resources are limited to earth and near earth environments and fossil fuels are polluting as well as non-renewable. Minn. I believe that the opposing side opposing of RTGs must examine what things are already affecting their personal well being and health and consider that a person might in fact receive more radiation from basking under the sun on the beach or smoking cigarettes.
Space exploration is about learning new things and exploring.
Risk is an inherent part of exploration. The opponents of nuclear fuel are attempting to stifle growth of all humankind's knowledge, for their own timid selfish reasons. While the idea of manifest destiny may be archaic in political terms, in my opinion it should be the rule in terms of space exploration. I would personally be willing to risk everything to bring mankind one step closer to the stars. We need to grow, efforts to stop this growth are counter to more than 5,000 years of human civilization.