Check out this computer-generated image of all the space junk floating around the Earth. Some of the bits are pieces of spacecraft, dead satellites and even flecks of loose paint!
They range in size from as big as a car, to as small as a speck of dust. But even the tiniest bits can damage a space shuttle, because they're travelling around the Earth so fast.
Some of the more unusual litter has been deliberately chucked out by astronauts. Last year, crew on board the International Space Station had no room on board for an old fridge, so they chucked it out into space!
The problem is there's not much scientists can do to clean up the junk. If the rubbish is small, it can be sent back to Earth in a space shuttle. But if it's too heavy, it's just left up there.
Sometimes it gets burnt up in the Earth's atmosphere, but that doesn't happen often, and blowing it up just makes more debris. So cleaning up space isn't going to be easy.