Two successful spacecraft, Pioneer 10 and 11, provided the first detailed scientific information about the outer planets Jupiter and Saturn. The well-instrumented spacecraft explored Jupiter during flybys by Pioneer 10 in December 1973 and by Pioneer 11 in December 1974. After flyby, Pioneer 10 altered its path to explore Jupiter’s massive moon, Ganymede, and it continued travel out beyond the orbits of the outer planets and into interstellar space. Pioneer 11 used gravity assist from Jupiter to travel out to Saturn for exploration of that planet during a flyby in September 1979. It then journeyed out beyond the orbits of the outer planets and into interstellar space. Both spacecraft maintained contact with Earth for over 20 years and at distances well past the orbits of the outer planets.