the fraction of objects that are not cataloged generally increases with altitude. It is possible that the total uncataloged population of large orbital debris could be as numerous as, or more numerous than, the cataloged population.

MEDIUM-SIZED DERBIS

The population of medium-sized (approximately 1 mm to 10 cm in diameter) debris is not nearly as well known as the population of large debris. As described in Chapter 2, the only measurements of the medium-sized debris population come from sampling of lower-altitude, higher-inclination LEO orbital regions with ground-based sensors. All other estimates of the size and characteristics of the medium-sized debris population are based entirely on extrapolations.

To a first approximation, it might be expected that medium-sized debris would be found in about the same orbits as large debris, since most medium-sized debris originates from large objects. However, all large objects may not contribute equally to the medium-sized debris population; some types of large object (such as rocket bodies that have been a source of explosive fragmentation) may produce much more debris than others. In addition, as described in Chapter 1, perturbing forces affect different sizes of debris differently. Medium-sized debris, which often has a higher ratio of cross-sectional-area to mass than large debris, will often be more strongly affected by atmospheric drag and thus will experience more rapid orbital decay.

Although there are no measurement data proving the origins of medium-sized debris, most likely the population is composed of fragmentation debris and mission-related objects (since nonfunctional spacecraft and rocket bodies are obviously large debris). The number of medium-sized debris objects detected is large compared to the number of large objects. Since it is generally believed that the majority of this population cannot be mission-related objects, they are most likely fragmentation debris. Consequently, breakup models can be useful tools in estimating some characteristics of the medium-sized debris population. Although there are large uncertainties in predictions of both the number and the initial velocities—and thus orbital parameters—of medium-sized objects ejected in a breakup (as described in Chapter 2), it is known that medium-sized fragments will generally be ejected from a catastrophic breakup with a greater range of initial relative velocities than large breakup fragments; this will place them into orbits with a wider range of altitudes, inclinations, and eccentricities (Johnson, 1985)

Ground-based sensors, particularly the Haystack radar, have provided the most detailed information to date on the population of me-



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