What is Pioneer 11?

Pioneer 11 is an unmanned space probe, the 2nd ever to cross the asteroid belt and visit Jupiter and its moons. Unlike its predecessor and sister craft, Pioneer 10, it also visited Saturn, using a gravitational assist from Jupiter. The probe then exited the solar system, heading in the direction of the constellation Aquila (The Eagle). It will pass near one of the stars there in about four million years.

Pioneer 11 was launched from Cape Canaveral, Florida, USA, on 6 April 1973, a month after its sister craft. Although originally intended only to visit Jupiter, Pioneer 11 was redirected in mid-mission to visit Saturn. Ironically, the plaque accompanying Pioneer 11, meant for possible extraterrestrials, indicates that it left the solar system through a Jupiter assist, though this is false.

On 4 December 1974, a year and a half after launch, Pioneer 11 passed to within 34,000 km of Jupiter's cloud tops, taking fantastic images of the Great Red Spot and measuring the mass of the moon Callisto. Using a gravity assist, it continued on to Saturn.

Pioneer 11 passed by Saturn on 1 September 1 1979, reaching to within 21,000 km of the cloud tops. The first probe ever to orbit Saturn, it took measurements of the density of ring particles, making sure that the planetary zone would be safe for the incoming Voyager craft, which had already passed Jupiter and were en route. If it was determined that the ring particles would be dangerous to the probes, they would have been routed away from the immediate vicinity of the planet to avoid the rings, but would have missed the gravity assist opportunity to visit the other outer planets, Uranus and Neptune. It determined that the dust was too thin to damage space probes there.

During its visit to the Saturnian system, Pioneer 11 imaged and almost collided with the small moon Epimetheus, the existence of which had been suspected but not confirmed by astronomers. Among the other measurements it made was that Saturn's moon, Titan, was likely too cold for life.