TESS’s top five
ith Saturn at opposition this month, now is a great time to see it. Even a modest scope will reveal the distinctive ring system. Other planets may have rings, but none as spectacular as Saturn’s.
Galileo was first to observe the rings in
1610, but he was unsure if he was seeing two moons or whether Saturn had handles or ‘ears’. And why did they seem to come and go every year or so?
In 1656 Christiaan Huygens was first to suggest that Saturn had a ring that was not attached to the planet, and in 1675 Giovanni Domenico Cassini identified the formation as being composed of a series of rings and gaps, identifying the A and B rings and the largest gap, which was named the Cassini Division after him. It was thought the rings were solid or liquid, but in 1859
WJames Clerk Maxwell proved they were made of particles and were orbiting Saturn independently. The main rings have been named in the order of their discovery; so the C ring was discovered in 1850, the D ring in 1933 and the E ring in 1967. The diffuse Phoebe ring, discovered in 2009, extends an enormous 6–16.2 million km from Saturn.
Four missions have had close encounters with Saturn so far. Pioneer II was first, passing by in 1979 when it discovered the very narrow F ring. Normally collisions between particles would spread the ring both inward towards the planet and outward to create a wide, diffuse ring. But the F ring is kept narrowly confined by the influence of two shepherd moons, called Prometheus and Pandora.
In 1980, Voyager 1 identified the G ring, and the following year Voyager 2 discovered several smaller ringlets when it passed by. In July 2004, the Cassini