Alcaray said:

"..because it has a small (by terrestrial standards) nuclear reactor. Not because it is a "space ship". Out in the Jupiter neighborhood, a spacecraft is usually more concerned with staying warm, than it is with staying cool."

JIMO carries a nuclear reactor to power a xenon fueled ion propulsion system. That ion propulsion system is what transports JIMO to the Jovian system without wasting time on gravity assist flybys. Also that same propulsion system will allow JIMO to cruise the Jovian system and loiter in low orbit about interesting moons, e.g. Europa. Once on-orbit, that same nuclear power system will allow high power data transmission back to Earth at unheard of data rates. JIMO represents a quantum leap in spacecraft design.

Gary