In the 1970s I was working at GE’s Space Division in Valley Forge as the energy engineering manager. One of my groups was responsible for the design of a radioisotope power supply that would supply DC power for the Voyager spacecraft. For details see http://voyager.jpl.nasa.gov/ We had won the contract to provide the Jet Propulsion Laboratory in Pasadena the power supplies for the two spacecraft that were launched in 1977. A key reason for our selection was that we had supplied radioisotope power supplies before for Apollo Missions 13 and 14 experiments.
The one for Apollo 14, SNAP 27, was set up to power a number of experiments that were left behind on the moon. It was operating four years after placement when monitoring of the experiments was stopped by NASA, http://en.wikipedia.org/wiki/Apollo_Lunar_Surface_Experiments_Package
These power units, commonly called RTGs, radioisotope thermoelectric generators, are the only good candidates for powering interplanetary missions like Voyager. The solar power is reduced as the square of the distance from the sun and so the size of solar arrays needed to power a mission even at Mars becomes quite unwieldy. The figure to the right shows the two RTGs mounted in a test fixture pre-flight.
The Voyager mission has shown that RTGs can provide power for years. They are in their 33rd year of operation and about to leave the solar system. And JPL estimates that the two Voyagers could continue to operate until 2025. Now it takes a long time to talk with them. They are about 10 billion miles away! My guesstimate is that it is 13 hours one way for a communication. So if JPL were to command one to “turn right” it would be 26 hours before they would know the command was executed correctly.
So many people had much more to do with the successful missions than we did but its nice to know we had something to contribute and that our RTGs are still going after 33 years!