A quick tour of the solar system
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Next we move out to Uranus. If it were at its closest to Saturn it would take 37 days to get there at a million miles and hour – but if it were the far side of its orbit it could take three months or more.
The first thing that strikes you about Uranus is that it is a blue planet – it’s like a plain blue ball in space. Underneath the atmosphere of methane it’s mostly water, which gets denser and denser as you go in. There probably isn’t an ocean surface – it just gets foggier and denser.
There are plenty of moons to explore, though. One of the oddest is Miranda, a small moon about 300 miles (480 km) across. This looks a bit like a walnut, with strange grooves across it. Scientists think it was split apart in a collision millions of years ago, and managed to reform itself. There are also ice cliffs about 12 miles (20 km) high. Imagine climbing those – you could do it quite easily, because you’d only weigh a fraction of what you do on Earth.
Onwards and outwards to another blue planet, Neptune, taking at least 42 days to get there. This is almost a twin of Uranus, and again we can’t land on the planet itself. As well as several small moons it has one large moon, Triton, which, guess what, is another ice world, with a surface temperature of -235ºC. A frozen moon like this is the last place you might expect to find activity, but for some reason there are 5-mile (8 km) high fountains of nitrogen gas in the pink ice that covers much of the moon . Scientists think that the feeble heat from the Sun is just enough to warm up pockets of gas below the ice sheet which then burst out and escape.
Beyond Neptune lie many other worlds, of which the best known is Pluto, a strange world. Most books call it a planet, but in reality it is tiny, only half the size of Mercury. It also has a very strange orbit, which sometimes brings it closer to the Sun than Neptune, though the two can never collide. At the moment it is as far from Neptune as Neptune is from the Earth, so it would take about 75 days – 2½ months – to get there and the same length of time to get back to Earth.
What would we find on Pluto? No spacecraft has ever visited the planet, but it’s probably very similar to the ice moons that circle Uranus and Neptune. It has a moon of its own, about half its size, called Charon. From out there, the Sun would appear much dimmer than from Earth – it would be about 2000 times dimmer than the Sun does from Earth, but that still makes it by far the brightest object in the black sky and it would drown out all the other stars.
Are there any more planets out there? It depends on what you call planets. In 2003, a much more distant world was discovered which has been named Sedna . This is about half the size of Pluto, and its orbit around the Sun is even more extreme.
Then in 2005 came the discovery of a body even larger than Pluto, and with a very strange orbit that occasionally brings it closer than Neptune. This planet is often referred to a Xena, but it has not yet been given an official name. Read the report by one of the discoverers here.
There are many other smaller objects out there as well, and it’s quite possible that some may be discovered that are larger still. Astronomers now call these Kuiper Belt Objects, and many say that Pluto and ‘Xena’ are just the largest known Kuiper Belt Objects rather than being planets.
If you were to take one of these icy objects and fling it closer to the Sun, its ice would start to turn to gas when it got close enough to be warmed up. This is just what we see when a comet comes along. The gases stream from it to create a tail, which may separate out into the gas and the dust. The most famous comet of recent years came along in 1997 and was called Hale-Bopp after the two amateur astronomers who first spotted it. Most comets turn up unexpectedly, because they spend thousands of years so far from the Sun that we can’t detect them. Only if their orbits bring them close to the Sun do we ever get to see them. Usually they are faint, but occasionally one becomes bright enough to be seen without a telescope. There was one in the sky in early 2005, called Comet Machholz.
What if we travel even farther from the Sun? At a million miles an hour it takes us 2½ months to get to Pluto, but to get to the nearest star would take much longer – 3000 years! What if we could travel at the fastest speed possible, which is the speed of light – six million million miles an hour? It would still take over four years to get to the nearest star, and to travel round our galaxy of stars it would take thousands of years. Even in Star Trek the journeys they take between stars are supposed to take many days or even weeks. We have no idea how we would ever travel even close to the speed of light, so these journeys will have to remain just in stories.
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Markings on Miranda
A gas fountain or geyser on Triton
Pluto (left) and Charon
An artist’s view of Sedna