Pandora, the outer shepherd of the F ring

Updated October 19, 2026 · 4 min read

Pandora is a small, irregular moon of Saturn, about 81 km in mean diameter, that orbits just outside the narrow F ring, roughly 141,700 km from the planet's center. With Prometheus on the inside, it was identified by Voyager scientists as one of the two "shepherds" of the F ring. Cassini showed that Pandora's effect on the ring is much subtler than that of its partner, and that the two moons are locked in a chaotic gravitational relationship.

Discovery and name

Pandora was discovered in Voyager 1 images taken in October 1980, at the same time as Prometheus, by members of the Voyager imaging team including Stewart Collins and Don Carlson. The narrow F ring that the two moons flank had been found by Pioneer 11 a year earlier, in 1979, and the discovery of a moon on each side seemed to confirm the theory that small moons confine narrow rings.

In Greek myth, Pandora is the first woman, fashioned by the gods. Hesiod's Works and Days tells how she opened a jar (later popularized as a "box") and released evils into the world, with only hope remaining inside. In the story she becomes the wife of Epimetheus, the brother of Prometheus, so the names of Saturn's moons Prometheus, Pandora and Epimetheus echo one another.

A shepherd, but a discreet one

The shepherding theory, developed by Peter Goldreich and Scott Tremaine, holds that a moon on each side of a narrow ring transfers angular momentum to ring particles, pushing them away from its orbit, so that the ring is kept narrow between the two. Pandora's position outside the F ring fits that picture, and it does contribute to confining the ring.

Cassini's observations, however, showed that Prometheus, which is more massive and passes closer to the ring, produces most of the visible structure: streamers, channels and kinks. Pandora's influence is weaker and harder to isolate, and the F ring's narrowness is now thought to involve several factors, including Prometheus's resonances and collisions with small embedded moonlets. Pandora is therefore still described as a shepherd, but a secondary one. More on these interactions is on the shepherd moons page.

Resonances and chaos

Pandora's orbit is shaped by two relationships. It is close to a 3:2 resonance with Mimas, completing three orbits for every two of Mimas, and this resonance perturbs its motion. It also interacts strongly with Prometheus, the two moons being near a 121:118 resonance. Roughly every 6.2 years, when the orbits of Prometheus and Pandora come into closest alignment, they exchange small gravitational kicks.

These repeated kicks make the motion of both moons chaotic over the long term, meaning their positions cannot be predicted precisely far into the future. The phenomenon was spotted in 1995, during a ring-plane crossing, when Hubble observations showed Pandora ahead of its predicted position and Prometheus about 20 degrees behind.

The surface of Pandora

Pandora is more heavily cratered than Prometheus. Cassini's closest images, taken on 18 December 2016 from about 40,000 km, show that its craters are softened by a blanket of fine material, giving the surface a smooth, almost blurred look. Grooves and ridges also cross the surface.

Pandora's density of about 0.5 g/cm³ indicates a highly porous body, probably a loose aggregate of ice. It shares this low density with other ring moons such as Pan and Atlas.

Pandora in the crowded inner system

Between the A ring's outer edge and the co-orbital pair Janus and Epimetheus, a span of about 15,000 km, lie Atlas, Prometheus, the F ring and Pandora. This crowded zone makes Pandora a useful marker for testing models of orbital dynamics, ring confinement and the gravity of small bodies.

Sources

• NASA Science: Saturn's moons
• NASA: Cassini mission
• Goldreich, P. and Rappaport, N., "Chaotic motions of Prometheus and Pandora", Icarus 162 (2003)
• French, R. G. et al., "Saturn's wayward shepherds: the peregrinations of Prometheus and Pandora", Icarus 162 (2003)
• Hesiod, Works and Days, lines 54–105