September 1, 1979. On this date, NASA’s Pioneer 11 came within 13,000 miles (21,000 kilometers) of Saturn, making it the first spacecraft ever to sweep closely past that world. The spacecraft found a new ring for Saturn – now called the “F” ring – and also a new moon, Epimetheus. There were two Pioneer spacecraft. They were used to investigate Saturn’s rings and determine if a trajectory through the rings was safe for the upcoming Voyager visits. They paved the way for the even-more-sophisticated Voyager spacecraft, which were launched in 1977 … and ultimately for the wonderful Cassini mission to Saturn, which has been orbiting the planet since 2004 and which has provided unprecedented and spectacular views of Saturn and its rings and moons.
Scientists said that Pioneer 11 also enabled them to get a sense of Saturn’s internal composition. It had long been realized that Saturn is not very dense; if you could find an ocean large enough to hold it, it would float on water. Pioneer 11 showed Saturn likely has a relatively small core for an outer gas giant world – only 10 times Earth’s mass – and that the planet is mostly liquid hydrogen.
Pioneer 11 is still sailing away from Earth, even though its transmissions died several years ago. As far as scientists know, it’s off towards the center of our Milky Way galaxy, that is, generally in the direction of our constellation Sagittarius.
Botton line: On September 1, 1979, Pioneer 11 came closest to Saturn.