Saturn

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If there were a prize for the most favourite planet, apart from Earth of course, it would probably go to Saturn. Even experienced astronomers get a thrill when they see that lovely planet with the amazing rings around it. You don't actually need a very big telescope to see them, as long as it magnifies at least about 30 or 40 times. Any less than that and you can still see that there is something rather elongated about the planet, but you can't see that they really are rings that don't touch the planet anywhere.

Our view of the rings changes all the time over the 29½ years that Saturn takes to go around the Sun, because of their tilt. Sometimes we see them wide open, like the picture on this board, but at other times we see them more edge on. When this happens, which is in 2025 and 2038, they can appear so thin that they are hardly visible even in large telescopes.

Your telescope might also show Titan, Saturn's largest moon, as a tiny speck of light close to the planet. This was the target of the European Space Agency's Huygens probe, which made the perilous journey down to the surface of Titan in 2005. No-one was really sure what it might land on, as Titan's thick and hazy atmosphere hid the surface details. Some scientists thought there might be methane seas, which we now know to be correct. So the lander had to be prepared for either solid ground or a liquid, as it descended on a parachute landing system made not far from here, in Denham.

In the event, it landed on solid ground, though it appeared to be icy rather than rocky. The lander's camera showed smooth pebbles, probably made of water ice, that looked as though they had been washed over by a stream. But at a temperature of -180º C, this wasn't water but liquid methane.

Saturn, and Titan, are ten times farther away from the Sun than we are on Earth, so the lighting is as dim as twilight on Earth. If you look across to the scale Sun, where the trail starts, you can see how big the Sun would appear from Titan, as this is a true scale model.

And if we made a model of Saturn, and matched not only its size, but its density, we'd be in for a surprise. The planet itself, for all its size, is mostly hydrogen and helium, which are the lightest elements. Its density is about 70% that of water, which is about the same as wood, so our scale model of Saturn would actually float on water! For comparison, the density of the Earth is 5½ times that of water, so you'd have to make the scale model out of heavy stone such as iron ore. And of course the model Earth would sink like, er, a stone!

Your walk to the next planet, Uranus, will take you seven or eight minutes even if you don't hang about. At a typical walking speed of 5 km/h, you will be whizzing along at about 20 times the scale speed of light! In Star Trek terms, that's between Warp 2 and Warp 3.

Photo: A closeup of Saturn from the Cassini spacecraft.
Credit: NASA/JPL/Space Science Institute