Pioneer 11, the first spacecraft at Saturn

Updated October 19, 2026 · 5 min read

Pioneer 11 was the first spacecraft to fly past Saturn. Launched by NASA in April 1973, it swung past Jupiter in December 1974 and reached Saturn on 1 September 1979, passing about 21,000 km above the cloud tops. It discovered the narrow F ring, made the first measurements of Saturn's magnetic field, and showed that a spacecraft could cross the ring plane just outside the main rings, which shaped the route of Voyager 2.

A Jupiter probe that became a Saturn pioneer

Pioneer 10 and Pioneer 11 were built as twin scouts for the outer Solar System. Their first job was to prove that a spacecraft could cross the asteroid belt and survive Jupiter's intense radiation belts. Pioneer 10 did both in 1973. Because it succeeded, mission planners at NASA's Ames Research Center could send its sibling on a bolder path.

Pioneer 11 flew past Jupiter on 3 December 1974, much closer than Pioneer 10 and over the planet's southern hemisphere. The encounter was designed so that Jupiter's gravity would fling the spacecraft back across the Solar System toward Saturn, a journey of almost five more years. After the Jupiter flyby, the spacecraft was informally called Pioneer Saturn.

The probe was simple by modern standards. It was spin-stabilized, carried no true camera, and built its pictures with an imaging photopolarimeter that scanned the scene line by line as the spacecraft rotated. Power came from four radioisotope thermoelectric generators, because sunlight at Saturn is about 90 times weaker than at Earth.

The encounter of 1 September 1979

Pioneer 11 approached from above the ring plane, dived through it about 38,000 km beyond the outer edge of the A ring, passed beneath the rings and made its closest approach roughly 21,000 km above the clouds. It then crossed the ring plane again on the way out. Each crossing carried a real risk: no one knew how many small particles lay outside the visible rings, and an impact at encounter speeds could have destroyed the spacecraft.

The images, transmitted at a very low data rate across 1.5 billion km, were coarse compared with what Voyager would send a year later. They still delivered the first close-up views of the rings, including their unlit side, and of Titan as a small orange disk.

What Pioneer 11 discovered

• The F ring. A thin, isolated ring just outside the A ring, about 3,400 km beyond its edge. Voyager later revealed its knotted, braided structure.
• A small moon on a near-collision course. Pioneer 11 passed a few thousand kilometers from a small moon that was later identified, most likely, with Epimetheus, one of the two co-orbital moons Janus and Epimetheus.
• Saturn's magnetic field. The spacecraft measured a magnetic field and a magnetosphere for the first time. Surprisingly, the field's axis was aligned with the rotation axis to within about one degree, a puzzle that Cassini later sharpened.
• Internal heat. Infrared measurements confirmed that Saturn radiates more energy than it receives from the Sun, a clue to its internal processes (see Saturn's interior).
• Titan is cold. Measurements indicated a frigid moon, a first data point before Voyager 1's close pass in 1980.

The charged-particle instruments also showed that the rings sweep up particles in the radiation belts, creating sharp drops in radiation where the ring material sits. Gaps in those profiles hinted at moons and ring material that were not visible in the images.

A pathfinder for Voyager 2

The most consequential result was not a discovery about Saturn but a test of navigation. Voyager 2 could only continue to Uranus and Neptune if it crossed Saturn's ring plane close to the planet, roughly where Pioneer 11 had passed. Pioneer 11 survived its crossings with no significant damage, which gave NASA the confidence to keep Voyager 2 on the trajectory that led to the only visits ever made to Uranus (1986) and Neptune (1989). The two Voyager encounters are described on the Voyager 1 and 2 page.

After Saturn

Pioneer 11 left Saturn on an escape trajectory and kept returning data on the solar wind and cosmic rays. Its power output declined as its plutonium decayed, and NASA ended routine operations in September 1995; the last signals were received later that year. The spacecraft is heading toward the constellation Aquila (the Eagle) and, in about four million years, will pass near one of its stars.

Like Pioneer 10, it carries a gold-anodized aluminum plaque designed by Carl Sagan and Frank Drake, with artwork by Linda Salzman Sagan, showing a man and a woman and the location of the Sun relative to 14 pulsars. It is the oldest message of its kind to leave the Solar System, predating the Voyager Golden Records by four years.

The full sequence of visits continues with Cassini-Huygens and is summarized in the Saturn timeline.

Sources

• NASA Science: Pioneer 11
• NASA Ames Research Center
• Fimmel, R. O., Van Allen, J. A., Burgess, E., Pioneer: First to Jupiter, Saturn, and Beyond, NASA SP-446, 1980