Pioneer 11 was the second craft (after Pioneer 10) launched by NASA to study Jupiter and the outer reaches of our Solar System. It became the first man made craft to pass by Saturn and together with Pioneer 10, became the first objects from Earth to leave the Solar System. Both craft carried a plaque, bearing a message from mankind, in addition to drawings of a man and woman, and our Solar System's place in the galaxy.
Pioneer 11 was launched on 5 April 1973, atop an Atlas/Centaur/TE364-4 launch vehicle. It successfully negotiated the asteroid belt on 19 April 1974, whereupon the thrusters were fired to increase the spacecraft's velocity by another 63.7 m/sec. At the same time, the aiming point at Jupiter was adjusted to 43,000 kilometers above the cloud tops. The close approach also allowed the spacecraft to be accelerated by Jupiter's massive gravity to a velocity 55 times that of the muzzle velocity of a high speed rifle bullet-173,000 kph, allowing it to be catapulted across the Solar System to cover the 2.4 billion kilometers to Saturn.
During its flyby of Jupiter on 2 December 1974, Pioneer 11 obtained dramatic images of the Great Red Spot, made the first observation of the immense polar regions, and determined the mass of Jupiter's moon, Callisto.
Pioneer 11 reached Saturn on 1 September 1979, flying within 21,000 kilometers of Saturn and taking the first close-up pictures of the planet. Onboard instruments located two previously undiscovered moons (Epimetheus and Janus) and an additional ring, charted Saturn's magnetosphere and magnetic field and found its planet-size moon, Titan, to be too cold for life. Hurtling underneath the ring plane, Pioneer 11 sent back pictures of Saturn's rings. Strangely, the rings, which normally seem bright when observed from Earth, appeared dark in the Pioneer pictures, and the dark gaps in the rings seen from Earth appeared as bright rings.
Following its encounter with Saturn, Pioneer 11 explored the outer regions of our Solar System, studying the Solar Wind and cosmic rays entering our portion of the Milky Way. In September 1995, Pioneer 11 was at a distance of 6.5 billion kilometers from Earth. At that distance, it takes over 6 hours for a radio signal (traveling at the speed of light) to reach Earth.
In September 1995, the available power fell below that needed for continued scientific observations, and the spacecraft was officially retired.