"One major reason is that NASA picks its propulsion systems based on its targets — and true exploration of the solar system and beyond hasn’t really been a serious goal, the Constellation plans for a return to the moon aside.<p><i>“The destinations dictate the power system,” said Rao Surampudi, a Jet Propulsion Laboratory engineer who works on the development of power systems.<p>By and large, it’s cheaper and easier to go with solar power or very low-power radioisotope generators[....]</i><p>Well, yes. But he's handwaving the fact that the choice is really a multi-argument function: where do we want to go, how fast do we want to get there, what do we want to do on the way, how much must we pay, and does its volume / mass fit into the launch vehicle's envelope?<p>Solar and such are good for long, slow, cheap trips where you aren't going to do a whole lot, and you're sure not going to make significant course corrections. With nuclear power, you've got energy to burn--you could run more instruments, get there faster, and even change destinations or come back if you want.