Watch your heads!

Since 1957, when the Russians launched Sputnik, the first man-made satellite, space has steadily filled with junk. The technical name is "orbital debris" - but call it what you want, it's still junk.

Orbital debris includes things like dead satellites as big as a large car, booster rockets from old space flights, or small pieces of junk - a door, a piece of solar panel.

Between 700 and 1,000 kilometres up, there are thousands of objects all whizzing around our planet at a speed of 28,000 kilometres per hour. America's space agency, NASA, keeps an eye on around 19,000 objects that are more than 10 centimetres wide. Occasionally, the Space Shuttle has to dodge one of these objects on its way up.

What goes up will come back down eventually. But the higher the altitude, the longer the object stays in orbit. After a satellite runs out of fuel, if it's about 600 kilometres up, it will stay up there for several years. If it's more than 1,000 kilometres up, it'll stay up there for centuries.

But get this: every day, a piece of space junk falls back to earth! Usually, it either burns up before reaching earth, or it lands in the ocean or in a desert, well away from where people live.

I remember when I was growing up, an old US space laboratory came down in several pieces over Australia. No one knew exactly where the thing would land, so I spent a couple of nights in our backyard in Sydney looking up at the sky - at the same time, feeling a little bit scared of being whacked on the head by a piece of space junk.

Eventually, the space lab landed thousands of kilometres away from me, in Western Australia. Here, you can read a news article about it, or watch this old BBC report.