Hi Chris,
Until recently, the spacecraft sent to explore the outer planets (Jupiter, Saturn, Neptune, Uranus, Pluto) have relied upon radioisotope thermoelectric generators, or RTGs to provide power. An RTG is not a nuclear reactor, but it does use highly radioactive materials to produce power. The New Horizons mission to Pluto is powered by an RTG. The Voyager missions also used RTGs, as did other recent planetary science missions such as Cassini. You can read more about RTGs and how they work here:
http://en.wikipedia.org/w...moelectric_generator
The Juno mission, which is currently on its way to explore Jupiter, is the very first mission to the outer planets to use solar panels to power the spacecraft. This was accomplished by advances in solar cell technology and by making the solar panels very large. You can read more about the Juno mission here:
http://missionjuno.swri.edu/
Kris