JIMO mission by NASA which was designed to observe the various moons of Jupiter (Europa, Ganymede and Callisto specifically) but lost funding in 2005. It was to have an ion propulsion engine called HiPEP (powered by a small fission reactor) and a Brayton conversion system which would convert reactor heat into electricity (which would have been a 1000 times more productive than a solar panels). Source- solarsystem.nasa.gov More info- en.wikipedia.org