grow in time unless deliberate actions are taken to minimize the creation of new debris. This predicted increased hazard will force spacecraft designers and operators to take countermeasures against the threat of debris or to face a heightened risk of losing spacecraft capability due to impacts. Projected future increases in the debris hazard already have had an effect on the design of LEO spacecraft (such as the International Space Station) that are large and have long projected functional lifetimes and, thus, a significant probability of colliding with damaging debris.

Concern about the orbital debris hazard has grown in the last decade. A number of events, including the breakup of several rocket upper stages and the replacement of Shuttle windows after impacts by small particles, helped to increase awareness of the problem, as did the need to factor space debris considerations into the design of Space Station Freedom. Reports by the American Institute of Aeronautics and Astronautics (AIAA), the European Space Agency (ESA), the U.S. Interagency Group on Space, the International Academy of Astronautics (IAA), and the Japan Society for Aeronautical and Space Sciences also served to define the problem better and to offer some suggestions on its mitigation.

Knowledge about orbital debris also has grown over the last few years. New data on the debris population have been gathered from a multitude of sources, ranging from LDEF, the European Retrievable Carrier (EURECA), and Mir—all of which collected data from the impacts of small debris in space—to the Haystack radar, which collected data on previously undetectable medium-sized debris. These new data have served to improve the models used to estimate the current characteristics and predicted growth of the overall debris population.

Despite these efforts, there remains much that we do not know about orbital debris. The primary reason is the fundamental difficulty of studying small, fast-moving, often dark objects orbiting hundreds or thousands of kilometers above the Earth. Our knowledge also is limited because

BOX 1 Other Effects of Orbital Debris

In addition to presenting a collision hazard to space operations, orbital debris can also have other detrimental effects. For example, debris can affect astronomical observations by leaving light trails on long-exposure photographs with wide fields of view. In addition, debris reentering the atmosphere can potentially harm people and property on the ground. In the past, this has been a minor hazard, since most reentering debris objects burn up completely in the atmosphere. However, there have been some exceptions (e.g., Kosmos 954, Skylab, and Salyut-7/Kosmos 1686), and the exact number of objects surviving reentry is unknown.

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