The ease with which a particular space object can be tracked depends on its optical or radar cross section (RCS) as well as its orbital parameters. In general, objects with larger optical or RCSs are more easily detectable than objects with smaller cross sections. Both the optical and the radar cross sections of particular space objects can vary greatly—which is not surprising for a collection of irregular-shaped objects. Uncertainty in the relation of RCS to actual size means that the smallest objects that these systems are able to catalog is uncertain, but since few objects in the SSS or SSN catalogs have an RCS of less than about 0.01 square meter, the commonly reported minimum trackable size has been 10 cm in diameter.

Recent radar range calibration of fragments produced in the laboratory, combined with measurements by short-wavelength radar and by ground telescopes (Henize and Stanley, 1990), have provided additional insight into the limiting size of the objects maintained in the catalog. These data indicate that for LEO orbital inclinations above about 30 degrees, the U.S. catalog contains some objects as small as 10 cm but is not complete at this size range. The catalog for LEO objects with inclinations greater than 30 degrees, however, is estimated to be 90 to 99 percent complete for objects larger than 20 cm.

As orbital altitude increases, the minimum size of debris that can be detected by ground-based sensors increases. However, this does not mean that the minimum-sized object that can be cataloged increases steadily with altitude. The opportunity for repeated observations and the predictability of an object's position in orbit also increase with altitude, making the maintenance of the orbital elements of a high-altitude detected object easier. Consequently, for altitudes below about 2,000 km, there is no simple statement of the limiting size of the catalog, other than that it is in the 10- to 30-cm range. However, radar detection sensitivity rapidly decreases with increasing altitude, and by 5,000 km the smallest

BOX 2-2Comparison of the SSN and SSS Catalogs

The U.S. and Russian space object catalogs are in general agreement for LEO objects greater than 50 cm in diameter. For space objects with diameters between 10 and 50 cm, the U.S. catalog is more complete. Above LEO, both catalogs generally maintain the orbital elements only of spacecraft and rocket bodies greater than 1 meter in diameter. Due to the lack of a worldwide network of sensors, the Russian space object catalog does not include objects in a significant portion of GEO and can only periodically track objects in highly eccentric, low-inclination orbits.

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