Any kind of small object in space that might provide a hazard to navigation. Especially refers to artificial objects. On bad days, the Hubble Space Telescope.

There was an interesting show on TLC about spacebourne debris. Typical space junk travels at thousands of miles per hour, and to be called "dangerous" only has to be a few millimeters in size. A satellite (or other spacecraft) can be destroyed or damaged by a piece of space junk. Either way, more space junk will probably be broken off the damaged craft and become trash, due to the collision.

This brings up the possiblity of a chain reaction, where space junk from damaged spacecraft flys about only to damage other spacecraft, generating more junk, etc.

It becomes more interesting when you think of how many satellites are already orbiting up there, and the fact that a project called Teledesic wants to add around 300 more. If a satellite "chain destruction" were to occur, the Earth would be surrounded by millions of little orbiting spacecraft bits whizzing about at hundreds of miles per second.

It could form a sturdy barrier making space travel from earth impossible, and sending out a strong "GO AWAY" message to any visitors of Earth. Astronomers are not thrilled about this.

To top it all off, Teledesic is a pet project of Bill Gates.
See also a preposterous flotilla of deadly orbiting space-junk

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