Space debris

nik 1 by the Soviet Union on 4 October 1957. Since that time, some 4500 additional launches have taken place. Today 850 active satellites are in orbit, supporting a wide range of civil and military uses. The US owns and operates roughly half of those satellites, as shown in figure 1. As a result of this space activity, a tremendous amount of debris has been left orbiting in space. Orbital debris is any human-made object in orbit that no longer serves a useful purpose. It comes in the form of discarded equipment and rocket stages, defunct satellites, bolts and other hardware released during the deployment of satellites, and fragments from the breakup of satellites and rocket stages. Space debris is a growing concern. With their high speed in orbit, even relatively small pieces of debris can damage or destroy satellites in a collision. Since debris at high altitudes can stay in orbit for decades or longer, it accumulates as more is produced. As the amount grows, the risk of collisions with satellites also grows. If the amount of debris at some altitudes becomes sufficiently large, it could become difficult to use those regions for satellites. There is currently no effective way to remove large amounts of debris from orbit, so controlling the production of debris is essential for preserving the longterm use of space. The debris issue gained prominence in January 2007 when China tested an antisatellite (ASAT) weapon that destroyed one of its defunct weather satellites, the Feng Yun-1C (FY-1C), at an altitude of about 850 km. The test added significantly to the debris population near that altitude. (See PHYSICS TODAY, March 2007, pages 29 and 100.)