them into those orbits may remain in orbit for long periods of time. Many of these rocket bodies are in orbits that intersect those used by functional spacecraft.

Mission-Related Debris

Other space objects, referred to as mission-related debris, may be released as a result of a spacecraft's deployment, activation, and operation. Parts of explosive bolts, spring release mechanisms, or spin-up devices may be ejected during the staging and spacecraft separation process. Shortly after entering orbit, the spacecraft may release cords securing solar panels and other appendages or eject protective coverings from payload and attitude control sensors. The amount of debris released can be quite large; a detailed study of the debris released by one Russian launch mission revealed that 76 separate objects were released into orbit from either the launch vehicle or the spacecraft. Numerous debris may also be created during a spacecraft's active life. For example, during the first eight years of its operation, more than 200 pieces of mission-related debris linked with the Mir space station were cataloged. Although the occasional item accidentally dropped by a cosmonaut or astronaut may be newsworthy, the majority of this type of debris is intentionally dumped refuse. Since mission-related debris are often relatively small, only the larger items can be detected and cataloged by present-day ground-based surveillance networks.

Another type of mission-related debris comes from the operation of solid rocket motors normally used as final transfer stages, particularly on GEO missions. Current solid rocket fuel usually employs significant quantities of aluminum mixed with the propellant to dampen burn rate instabilities. However, during the burning process, large numbers of aluminum oxide (Al2O3) particles are formed and ejected through a wide range of flight path angles at velocities up to 4 km/s. These particles are generally believed to be no larger than 10 microns in diameter, but as many as 1020 may be created during the firing of a single solid rocket motor, depending on the distribution of sizes produced. While the orbital lifetimes of individual particles are relatively short, a considerable average population is suggested by examinations of impacts on exposed spacecraft surfaces. More than 25 solid rocket motor firings were conducted in orbit during 1993.

More recently, attention has been drawn to another side effect of solid rocket motors. Ground tests indicate that in addition to the large number of small particles, a smaller number of 1-cm or larger lumps of Al2O3 are also ejected during nominal burns. The only indication of the existence of such objects are data from ground tests carried out at

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