Each of the four giant planets in our Solar System is surrounded by flat, annular features known as planetary rings. Planetary rings are composed of vast numbers of small satellites, which are unable to accrete into large moons because of their proximity to the planet.

When Galileo Galilei first observed Saturn's rings in 1610, he believed them to be two giant moons in orbit about the planet. However, these ‘moons’ appeared fixed in position, unlike the four satellites of Jupiter which he had previously observed. Moreover, Saturn's ‘moons’ had disappeared completely by the time Galileo resumed his observations of the planet in 1612. Many explanations were put forth to explain Saturn's ‘strange appendages’, which grew, shrank and disappeared every 15 years (Fig. 11.1a). In 1656, Christiaan Huygens finally deduced the correct explanation, that Saturn's strange appendages are a flattened disk of material in Saturn's equatorial plane, which appear to vanish when the Earth passes through the plane of the disk (Fig. 11.1b).

For more than three centuries, Saturn was the only planet known to possess rings. Although Saturn's rings are quite broad, little structure within the ring system was detected from Earth (Fig. 11.2). Observational and theoretical progress towards understanding the physics of planetary rings was slow.