JUST a few days from now, one of the Solar System’s most spectacular sights is set to vanish from the heavens. On Monday, 22 May, Saturn’s vast, swirling ring system will disappear from sight for an instant as the positions of Earth and Saturn conspire to give an edge-on view. Paradoxically a “ring plane crossing” such as this gives astronomers a chance to unlock some of the secrets of the ring system and the moons that chaperone these lanes of ice and dust. And because such an event takes place just once every 14 to 16 years, it will be eagerly watched by astronomers around the world.
Nearly four centuries have passed since Galileo peered through the earliest of telescopes to glimpse faint smudges next to Saturn – the first clues that the planet had rings. With modern telescopes, astronomers observing Saturn’s rings from Earth can pick out four distinct broad bands, which have been labelled A, B, C and D in order of decreasing radius. There is an obvious gap, known as Cassini’s division, separating the A and B rings, about two Saturn radii away from the centre of the planet(see Diagram).
In November 1980 and August 1981, NASA’s two Voyager spacecraft flew past Saturn to give us the most detailed look at the ring system to date. The best Voyager images delineate structure within the rings as fine as 10 kilometres across. And it is clear that the main rings are very thin – less than 1 kilometre from one side to the other – and consist mainly of chunks of water ice typically 1 centimetre to 5 metres across, along with some…
