Even known pieces of space junk can pose an unexpected threat to satellites.
In July 1996 a piece of space junk tracked by the U.S. government bisected the antenna on France's Cerise military reconnaissance satellite (shown in an artist's conception)--a relatively minor collision. The satellite collision on January 10, 2009, by contrast, added hundreds of pieces of debris to the thousands tracked by the U.S. government.
Space agencies can track objects that are about four inches (ten centimeters) across or larger. But smaller pieces that are impossible to track can cause just as much damage if they hit a satellite, space shuttle, or the International Space Station.