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Astronomy

The Dynamics of Saturn's F Ring and Its Shepherd Moons

Quick fact

The shepherd moons Prometheus and Pandora are only about 86 and 81 kilometers across, yet their gravitational tugs are enough to confine the F ring to a width of just a few hundred kilometers—astonishingly narrow compared to Saturn's main rings.

Why this is interesting

Saturn's F ring is a thin, kinked ribbon of ice—but what keeps it so narrow and why does it look braided? The answer lies in two tiny moons that exert surprising gravitational control from either side.