1979

Pioneer 11 at Saturn

The Pioneer 10 and 11 missions, launched in 1972 and 1973 respectively, were designed to provide the first reconnaissance of the outer solar system and beyond. Pioneer 10 made the first encounter with Jupiter in 1973, setting the stage for Pioneer 11’s flyby of that giant planet in 1974–1975. Unlike Pioneer 10, however, Pioneer 11 was on a trajectory that could be programmed to use Jupiter’s gravity to slingshot it on to a first-ever encounter with Saturn in 1979.

The Pioneer 11 encounter with Saturn was a great success, with the probe passing within about 13,000 miles (21,000 kilometers) of Saturn’s cloud tops on September 1, 1979. The mission carried cameras, magnetic-field and charged-particle instruments, cosmic-dust-particle and radiation counters, and other scientific instruments that provided planetary scientists with their first views of the environment at and around the ringed planet.

In a way, the Pioneer missions were pathfinders for the more ambitious Voyager probes to the outer solar system. For example, during the Saturn flyby, Pioneer 11 was directed to travel through Saturn’s ring plane to determine if small dusty or icy ring particles might pose a threat to spacecraft. They did not, thus enabling mission planners to direct Voyager 2 through the same region of the rings in order to get it on a trajectory that would allow subsequent encounters with Uranus and Neptune. Pioneer 11 data yielded other discoveries at Saturn as well, like the observation of the large, atmosphere-bearing moon Titan’s extremely low temperature (90 kelvins; perhaps too cold for life), the discovery (and near collision with) a new small moon and an additional ring; and detailed mapping of Saturn’s magnetic field, a large charged-particle structure similar in some ways to Jupiter’s magnetic field.

Like Pioneer 10, Pioneer 11 is on an escape trajectory from our solar system, now more than 83 astronomical units from the Sun and heading toward the center of the Milky Way galaxy. Contact was lost with the probe in late 1995, but the spacecraft carries a plaque, like Pioneer 10’s, that will, we hope, serve as an informative greeting for any galactic neighbors who discover the probe in the far future.

Pioneer 11 false-color image of part of Saturn and its rings, taken on September 1, 1979, when the probe was about 250,000 miles (400,000 kilometers) from the planet. Saturn can be seen through the rings’ Cassini Division, and the shadow of the rings can be seen silhouetted against the planet.