I think that's exactly the point - Gigantor arrays (or any Solar panels AFAIK) shouldn't be able to give you nearly enough power to run a vessel at that distance. After all, the energy you can get falls off with the inverse-square of distance - and I'm not sure whether it's implemented for solar panels in the game. So, there does appear to be a gap into which such a system could fit without unbalancing the game. In my opinion, the nuclear reactors do not have to have a large weight penalty (Russia built orbiting reactors at under 400kg each, although they lacked shielding), but they would have to require a large science input. There should, by that logic, be two reactor types - one for probes (light, unshielded) and one for manned vessels (heavy, shielded).