FIGURE 1 Classes of space objects.

collision with objects as small as a centimeter in diameter could damage or prove fatal to most spacecraft, depending on where the impact occurs. Impacts with the much more numerous debris particles that are a millimeter or less in diameter can damage optics, degrade surface coatings, or even crack windows.

There have not yet been any confirmed incidents in which collision with orbital debris has severely damaged or destroyed a spacecraft, but there have been a number of spacecraft malfunctions and breakups that might have been caused by impacts with debris. Smaller debris particles have certainly pitted windows (of the U.S. Space Shuttle and the Salyut and Mir space stations) and marred the surfaces of spacecraft such as the Solar Maximum Mission spacecraft (Solar Max) and the Long Duration Exposure Facility (LDEF). This type of low-grade damage is probably very widespread in low Earth orbit (LEO), but much of it goes undetected, because most spacecraft are not returned to Earth for examination.

Since the late 1970s, studies of the debris population using modeling techniques have predicted that the hazard from orbital debris is likely to