Hi Valarie,

Until very recently, spacecraft visiting the outer reaches of our solar system had to rely upon RTGs (Radioisotope Thermoelectric Generators) which power the spacecraft by converting the heat generated by the decay of plutonium-238 (Pu-238) fuel into electricity using devices called thermocouples. Even some of the missions to Mars have used RTGs because of concerns that solar panels would not provide a reliable power source. You can read about RTGs and some of the spacecraft that used them for power here:

http://solarsystem.nasa.gov/rps/rtg.cfm

The Juno mission to Jupiter is the very first mission to the outer planets (Jupiter, Saturn, Neptune, Uranus) to use solar panels instead of RTGs, thanks to advances in technology. Even though the solar panels used on the Juno mission are much more efficient than the solar panels used years ago, the solar panels on Juno still have to be quite large because Jupiter only receives about 1/25th of the sunlight we receive here on Earth. The Juno solar panels are 66 feet (20 meters) across. You can read more about Juno here:

http://www.nasa.gov/missi...ex.html#.UydHmoU60uQ

Kris