Both Voyagers 1 and 2, launched during the summer of 1977 were originally slated to fly by two planets and operate for four years. But they accomplished much more.
Due to the fortuitous alignment of the outer four gas giants that occurs only once every 176 years, Voyager 2 was able to continue on to Uranus and Neptune after flying by Jupiter and Saturn. Utilizing the gravity assist technique, the normal travel time from Earth to Neptune was cut from 30 years down to 12.
The main key to Voyager's success was the extensive back-up systems that were installed on board the craft. Early in the mission, Voyager 2's prime receiver failed. For the rest of the flight, telecommunications were handled by a flawed but functional secondary receiver.
As the Voyagers traveled further and further from Earth, radio communications to and from the craft, travelling at the speed of light, took longer and longer to complete. At their first stop, Jupiter, the Voyagers were less than one hour's light time away. But when Voyager 2 was at Neptune, it took more than four hours to receive the radio messages sent to Earth.
