In March 1972 NASA launched the Pioneer 10 probe to fly through the asteroid belt and explore the giant planet Jupiter. A year later it launched an identical sister probe, Pioneer 11, with the option of sending the probe on to Saturn afterwards. Pioneer 10 was accelerated to 51,670 km/h at launch, the highest speed any spacecraft had reached at the time. It emerged safely from the asteroid belt, suffering only a few minor impacts from dust particles and arrived at Jupiter in December 1973 in good shape after a billion km journey. It returned the best pictures ever of Jupiter, though its instruments suffered damage from the planet's powerful radiation belts. Jupiter's gravity then accelerated the probe into the depths of interplanetary space. In June 1983 Pioneer 10 had nosed beyond the orbit of Neptune, currently the outermost planet, and begun its eternal journey in interstellar space. Pioneer 11 followed its sister craft to Jupiter, arriving in December 1974. After sending back more images, it looped round the planet and was directed by gravity 'sling-shot' into a looping trajectory that five years later took it to Saturn. Its name was changed to Pioneer-Saturn. It flew past the ringed planet in September 1979, taking the best pictures yet of Saturn and discovering a new ring and a new moon. Afterwards, Pioneer- Saturn began its journey out of the solar system. In theory the Pioneer probes might one day be found by intelligent extraterrestrials somewhere else in space. With this in mind, they carry a plaque that shows where they came from and who sent them.
