On September 1, 1979, NASA’s Pioneer 11 spacecraft performed the first Saturn flyby in the history of space exploration, at a distance of 21,000 km (13,000 miles) from Saturn’s cloud tops.

Today’s (September 1) story of what happened this day in Science, Technology, Astronomy, and Space Exploration history.

Pioneer 11’s historic first Saturn flyby

Pioneer 11 was launched from Cape Canaveral on April 5, 1973 on top of an Atlas-Centaur rocket. It was actually intended as a backup to its twin, Pioneer 10. By the time Pioneer 11 launched, Pioneer 10 (launched on March 2, 1972) had already safely traversed the asteroid belt and was on its way to the very first encounter with Jupiter in December 1973.

Pioneer 11
Artist’s concept of Pioneer spacecraft (either 10 or 11, since these spacecraft are twins) and the Milky Way Galaxy. Image: NASA

After Pioneer 10 successfully completed its observations of Jupiter, in May 1974 mission planners retargeted Pioneer 11 to use the giant planet’s gravity to slingshot the spacecraft to encounter Saturn.

Pioneer 11 completed its crossing of the asteroid belt on April 14, 1974, and encountered Jupiter on December 2, taking the most detailed images of the Great Red Spot and mapping Jupiter’s polar regions. The spacecraft’s closest approach to Jupiter was at 42,828 km (26,612 miles).

Course corrections in May 1976 and July 1978 refined the spacecraft’s trajectory toward Saturn.

In late July 1979, Pioneer 11 began its observations of Saturn. By that time, Voyager 1 and Voyager 2 had been launched, had completed their fly-bys of Jupiter, and were on their way to Saturn as well.

Mission managers at NASA had to choose where to target Pioneer 11’s flyby of Saturn to provide maximum support to the twin Voyager spacecraft.

Options included a more scientifically interesting but riskier passage through Saturn’s inner rings or a less interesting pass through the outer rings but which would be a pathfinder for Voyager 2 that would have to take that same path to ensure the proper gravity assist to send it on to Uranus and ultimately to Neptune.

After much deliberation, managers decided it was more important to ensure that Voyager 2 complete the planned grand tour of the outer planets of our solar system.

Pioneer 11 Image of Saturn and Its Moon Titan - the first Saturn flyby
The first Saturn flyby: This image from Pioneer 11 shows Saturn and its moon Titan. The irregularities in ring silhouette and shadow are due to technical anomalies in the preliminary data later corrected. At the time this image was taken, Pioneer was 2,846,000 km (1,768,422 miles) from Saturn. Source: NASA

On September 1, 1979, Pioneer 11 performed the first Saturn flyby in the history of space exploration: it passed within 21,000 kilometers (13,000) miles of Saturn’s cloud tops at a velocity of 114,263 km/h (71,000 mph).

During the encounter, Pioneer 11 sent back data on the planet, its rings, and its satellites, including 440 images. Based on the data, scientists learned that Saturn’s atmosphere consists mostly of liquid hydrogen and confirmed that the planet had a magnetic field.

Images of Saturn’s moon Titan revealed an orange cloud-shrouded globe, with a global temperature of minus 315o F. Pioneer 11 discovered two new moons and one new ring orbiting Saturn. The spacecraft completed its study of the ringed planet on October 5, 1979.

Pioneer 11 performs the first Saturn flyby on September 1, 1979
On September 1, 1979, Pioneer 11 performed the first Saturn flyby. Image: Illustration of Pioneer 11 near Saturn, by Rick Guidice. Source: NASA

NASA ends operations

On February 25, 1990, Pioneer 11 became the 4th human-made object to pass beyond the orbit of the planets, after Pioneer 10 (June 13, 1983), Voyager 1, and Voyager 2 (August 5, 1979).

By 1995, Pioneer 11 could no longer power any of its detectors, so the decision was made to shut it down.

On September 29, 1995, NASA’s Ames Research Center, responsible for managing the project, issued a press release that began, “After nearly 22 years of exploration out to the farthest reaches of the Solar System, one of the most durable and productive space missions in history will come to a close.”

It indicated NASA would use its Deep Space Network antennas to listen “once or twice a month” for the spacecraft’s signal, until “sometime in late 1996”.

One day after that press release, NASA engineers terminated routine contact with Pioneer 11 on September 30, 1995, when the spacecraft was 6.5 billion km (approximately 43.4 AU, see notes 1) from Earth, but continued to make contact for about 2 hours every 2 to 4 weeks.

On November 24, 1995, scientists received a few minutes of good engineering data, but then lost final contact once Earth moved out of view of the spacecraft’s antenna. Pioneer 11 remains silent since then.

Current status of Pioneer 11

As of August 2022, Pioneer 11 is estimated to be 109.153 AU (1.63291×1010 km; 1.01464×1010 mi) from the Earth and 109.81 AU (1.6427×1010 km; 1.0207×1010 mi) from the Sun; and traveling at 11.182 kilometers per second (40,260 km/h; 25,010 mph, relative to the Sun) and traveling outward at about 2.36 AU per year.

Pioneer 11 is now headed toward the constellation of Aquila, northwest of the constellation of Sagittarius. Barring an incident, it will pass near one of the stars in the constellation in about 4 million years.


  1. An Astronomical Unit (AU) is the average distance between Earth and the Sun, which is about 93 million miles or 150 million kilometers. Astronomical units are usually used to measure distances within our Solar System.


M. Özgür Nevres

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