hazard posed by debris is to space operations. Although the current hazard to most space activities from debris is low, growth in the amount of debris threatens to make some valuable orbital regions increasingly inhospitable to space operations over the next few decades. A responsible approach to orbital debris will require continuing efforts to increase our knowledge of the current and future debris population, the development of tools to aid spacecraft designers in protecting spacecraft against the debris hazard, and international implementation of appropriate measures to minimize the creation of additional debris.

CHARACTERISING THE DEBRIS ENVIRONMENT

The debris environment is difficult to characterize accurately. First, the debris population changes continually as new debris is created and existing debris reenters the Earth's atmosphere. Detection of such changes requires that measurements of the debris environment be updated periodically. Second, only the largest objects can be repeatedly tracked by ground-based sensors; tracking of the numerous smaller pieces of debris is much more difficult. The U.S. and Russian space surveillance systems are able to track and catalog virtually all objects larger than 20 cm diameter in LEO. However, as altitude increases, the minimum-sized object that these systems are capable of tracking increases, until at GEO only objects larger than about 1 meter in diameter are presently cataloged.

Characterization of the debris population that cannot be cataloged must thus be accomplished by sampling the orbital debris flux at particular locations and times and using these data as a basis for estimating the characteristics of the general population. The flux can be sampled either directly (with spacecraft surfaces that are struck by debris) or remotely (by using ground- or space-based radars or optical telescopes that record debris passing through their fields of view). Presently, ground-based remote sensing is the most effective method for sampling the medium-sized (approximate diameter 1 mm–10 cm) debris population, and in situ impact sampling is the most effective method for measuring the small (approximate diameter <1 mm) debris population.

Current measurements of the debris environment contain gaps, such as a lack of information on objects smaller than 1 meter in diameter in GEO, on the small debris population at altitudes above 600 km, and on the medium-sized debris population above LEO. There are, however, several promising means for better characterizing the debris population. For example, large-aperture optical telescopes or telescopes equipped with charge-coupled devices could be employed to improve cataloging of large (approximate diameter >10 cm) debris in orbits above LEO,



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