Dear Urban Astronomer
How exactly do satellites work? Who invented them, and how many are up there now? I heard somewhere that there must be millions! Is this true?
All artificial satellites have one thing in common: they are in orbit around something (usually the Earth), and will in theory stay up there indefinitely all by themselves. If by “How do they work”, you meant “How do they stay up there”, then prepare for a brief lesson on how Orbits work. We all know that when something orbits the Earth, it moves in a big circle in space around the planet, and has no gravity on it. As you might expect with such a simple answer, this is wrong. An orbiting object is really just a falling object that manages to keep missing the ground. Confused? Okay, imagine standing on top of a very tall tower, with a cannonball. You heave it up, puny scientist muscles a-quiver, and drop it out the window. It falls straight down towards the centre of the Earth, in accordance with Newton’s Laws of Universal Gravitation. So far, so expected. You then take a second cannonball and load it into a cannon. You aim it out the window at the horizon, and fire. This second cannonball flies out horizontally at first, but gravity takes effect immediately, making the path of the ball curve downwards until it thuds into the dirt. Again, no big surprise there. But now we take a third cannonball, and load it into a really big cannon, with a huge charge of gunpowder. You aim it as before, and light the fuse. This time the ball travels really far – so far that it actually vanishes over the horizon before landing. Now you start to notice something interesting: The ball took a little longer to land than you expected (Because you used Newton’s laws to calculate exactly how long it would take for the ball to land), and after much thought you realise that it’s because the Earth is round – the ball traveled so far that the ground where it lands is actually lower down than where it started (relative to the ball, of course). This intrigues us, so we build bigger and bigger cannon, firing the ball further and further until suddenly it hits you in the back of the head – it managed to fly all the way around the world before it landed. It’s travelling so fast that the ground drops out beneath it at the same rate that it tries to fall! And once an object is moving at such a speed that it falls forever without ever reaching the ground, we say that it is in orbit. Because gravity weakens with distance, an orbit from very high up moves more slowly than one closer to the ground. There is a specific height (about 35000km) where the speed exactly matches the rate at which the Earth rotates. If you set up an orbit at that altitude and lined up with the Equator will match the motions of the ground below – same speed and direction – then anything in that orbit will appear to be motionless relative to the ground. We describe such orbits as “Geostationary”.
So who invented the satellite? Well some people say that it was the famous science fiction author Arthur C. Clarke, but he actually came up with the specific idea of having a communications satellite in a geostationary orbit. This is the sort of satellite that the dish for your satellite TV service points at. But the first ever artificial satellite was built and launched by soviet engineers in the 1950’s. They called it “Sputnik”, and I suppose that this means that the satellite was invented in the USSR at the height of the cold war.
So how many satellites are up there right now? According to the UCS Satellite Database, there are just under 1000 active satellites in Earth orbit that are still working. Which seems a lot, but there is a lot of stuff up there which isn’t in the database – defunct satellites, tools dropped by astronauts, discarded rocket stages left over from launches, even the scattered debris from collisions. Junk, in other words. Space junk is thankfully a self-correcting problem. The highest thinnest parts of the Earth’s atmosphere extend many hundreds of kilometres upwards so that everything will gradually slow down by tiny amounts each month and eventually fall back to Earth. Unfortunately it will take years, if not centuries, for the problem to completely go away, and as we increase our activity in space we’ll find ourselves putting more and more junk up there faster than it can clean itself.
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