Flight over the outer part of Saturn's rings. The planet Saturn is encircled by an extensive system of rings, composed of small particles of water ice up to a few metres across at most. This is the outer part of the main ring system, with the thin F Ring seen first, followed by the 2600-kilometre-wide Roche Division. After this, the edge of the bright, dense A Ring is seen, which lies 136,775 kilometres from the centre of Saturn. The two prominent gaps in the A Ring are called the Keeler Gap (35 kilometres wide) and the Encke Gap (325 kilometres wide). Despite their great extent, the rings are extremely thin - the A Ring is the thickest ring and is still less than 30 metres deep, and many regions are thinner than that. One effect of this is that the rings are invisible when they are edge-on to Earth. The formation of the rings is not fully understood. It is thought that they are the remains of an icy moon that was either ripped apart by tidal forces, or destroyed in a collision with another body.