An important source of information about the debris environment is the study of impact pits on surfaces that have been exposed to space in Earth orbit. All spacecraft collide with very small orbital debris particles and meteoroids; consequently, spacecraft surfaces returned to Earth are found to have many small craters resulting from hypervelocity impacts. In most cases, these craters are too small to have any effect on the operation of the spacecraft. However, by examining them, important clues can be obtained on the sources of orbital debris, and the rate that it is changing. The Long Duration Exposure Facility (LDEF) was a bus-sized spacecraft that was returned after ~5.7 years in low Earth orbit. Over 20,000 impacts have been documented on LDEF, approximately 1,000 of which have been chemically analyzed in an attempt to determine the origin of the projectile. Critical surfaces, such as the windows, on the Space Shuttle are examined after every flight. Other surfaces include those from the Solar Max satellite and the Hubble Space Telescope.