What Causes Space Junk?

Debris in orbit can come from many sources:

• Exploding rockets - This leaves behind the most debris in space.
• The slip of an astronaut's hand - Suppose an astronaut doing repair in space drops a wrench -- it's gone forever. The wrench then goes into orbit, probably at a speed of something like 6 miles per second. If the wrench hits any vehicle carrying a human crew, the results could be disastrous. Larger objects like a space station make a larger target for space junk, and so are at greater risk.
• Jettisoned items - Parts of launch canisters, camera lens caps, etc. 

Items initially placed into high orbits stay in space the longest.

The European Space Agency tracks more than 7,500 orbiting items with a width of 4 inches (10 cm) or more. Space debris may also be a reason why space shuttles typically orbit with their windows to the rear. This protects the astronauts onboard, at least to some degree. 

A special NASA satellite called Long Duration Exposure Facility (LDEF) was put in orbit to study the long-term effects of collisions with space junk. The LDEF was later brought back to Earth via a space shuttle for analysis.