Hi Valarie,
The key here is not how far away from Earth a spacecraft is, but how far away from the Sun a spacecraft will have to travel. Since the intensity of sunlight falls of as the square of the distance away from the Sun, the further away a spacecraft will go from the Sun, the more difficult it is to use solar panels to provide electrical power. So far, solar panels have been used to power missions to the inner, terrestrial planets (Mercury, Earth, Venus, and Mars), as well as on missions to study the Moon and the Sun. However, past missions to the outer planets (Jupiter, Saturn, Uranus, Neptune) like the Pioneers, Voyagers, Ulysses, Galileo
and Cassini missions have all needed to use Radioisotope Thermal Generators
(RTGs) to generate electrical power. The Juno mission to Jupiter, which is currently being built, will be the first mission to one of the outer planets to use solar panels instead of RTGs. Some of my co-workers here at the University of Iowa are currently finishing construction of the Radio and Plasma Waves instrument for Juno, which will be launched in August 2011 on a five-year journey to Jupiter. You can read more about Juno here: http://juno.wisc.edu/index.html
Kris