bris is more frequently found in low inclination and eccentric orbits than cataloged large debris and that the large number of objects detected between 900 and 1,000 km are in near-circular orbits with inclinations around 65 degrees (Stansbery et al., 1994). The reported detection of objects with inclinations greater than 110 degrees may be a result of the high uncertainty in determining inclination for objects that are barely detectable (as described in Chapter 2).

As mentioned previously, the Haystack data suggest that there may be major sources of centimeter-sized orbital debris other than previously recorded breakups. The large number of objects in orbits between 900 and 1,000 km with orbital inclinations between 60 and 70 degrees suggests that there is a significant source of debris in this area. If this source were breakups, however, the debris would have been spread over a much wider area than is evidenced by the data. It thus seems possible that some of this debris may be the result of a previously unmodeled source. This possibility is supported by the polarization data from Haystack, which suggests that the objects have relatively smooth and spherical shapes, rather than the irregular shapes that would typically be created in a breakup. A combination of orbital and physical characteristics can be interpreted to suggest that these objects may be tens of thousands of 0.6-2.0 cm diameter liquid droplets of a sodium/potassium coolant leaking from the nonfunctional cores of Russian Radar Ocean Reconnaissance satellites (Stansbery et al., 1995; Kessler et al., 1995). Less evidence exists to suggest the sources of other concentrations of debris not predicted by models (such as the concentration of medium-sized objects detected by Haystack with inclinations between 25 and 30 degrees—another region in which few breakups have been observed [Kessler, 1993]).

SMALL DEBRIS

There is an extremely numerous population of small (<1 mm in diameter) debris particles in Earth orbit. Knowledge of the distribution of these particles comes, as described in Chapter 2, primarily from the examination of returned spacecraft material from such spacecraft as Solar Max and the LDEF and a few active measurements made on the LDEF, the Salyut and Mir space stations, EURECA, and the U.S. Space Shuttle. Since the returned materials and active measurements are all from spacecraft in orbits of 600 km altitude or less, uncertainty remains on how to extrapolate these data to higher altitudes. Some models predict that because of the lessening influence of atmospheric drag, the spatial density of debris smaller than 1 mm should increase with altitude up to at least 1,000 km.

Like medium-sized debris, small debris is all either mission-related



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