Satellite observation - satobs for short - though not strictly astronomy, can be a very rewarding pastime that can be carried out with nothing more than a computer connected to the internet (and if you're reading this then you at least have access to one!) and your own eyeballs.

The Earth has many satellites, and you can guarantee you've seen the largest, most well known and brightest of them - the Moon! But of course here we are referring to artificial satellites, satellites that have been launched from the earth into orbit around the earth, starting back in 1957 with Sputnik, a little satellite launched by Russia that went around the earth for a while transmitting a radio signal to all and sundry.

Since then many thousands of satellites have been launched, and without them life today would probably be quite different. Some of them whiz around our planet at an extremely quick rate of knots, orbiting the earth once every 90 minutes or so - that's right, all the way around the planet in an hour and a half - that's Perth to Mackay and the Whitsundays in around 15 minutes! Some satellites go around much slower - once a day, which is very convenient because if they are launched correctly, they go around the earth at the same speed as the earth rotates, and they appear to be stationary in relation to a point on the earth's surface. Austar take great advantage of this fact to bring you satellite TV.

Okay, enough with the theory already! The fact of the matter is that you can see many of these satellites going around the earth! How then, when they're the size of a Volkswagen Beetle (the big ones) and they're 350Km away (at their closest). Well, in the early evening, and if you can get up in the early morning, the Sun may well be below the horizon, but a satellite, orbiting 400km above the earth, will be able to see you and the Sun, and sunlight may reflect off the satellite down to you. The satellite is acting like a big mirror. The great thing about this is that the satellite is whizzing along at some 27,000 Km/h, or 7.5 Km per second, and against the night sky appears as a star that is going in some hurry accross the sky - so fast in fact that it appears to be going even faster than a plane.

So how do we see them? NORAD plots the orbit of loads of stuff in space - from big satellites, to discarded rocket bodies, to bits and pieces astronauts drop when they're on a big job in space - don't tell me you've never dropped a shifter! All this stuff needs to be kept track of - you don't want to launch a multi-million dollar satellite only to have it wiped out by an errant bit of space junk - you need to know where it all is. In fact, starting with mission STS-114, NASA dock the space shuttle at the rear of the International Space Station to reduce the risk of it being damaged by space junk.

NORAD, and others, keep a track of everything in earth orbit that's big enough to track, regularly and publish data that's freely available that software and websites can use to predict whether a particular satellite will be visible on any given evening or morning - in fact, most will tell you, after you've plugged in your co-ordinates, what will be worth looking for over an entire evening. Tropical Stargazers even have a daily compilation of the more interesting and brighter satellites' element sets to download at our satellite elesets page.

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