FIGURE 2.1 Measurements of orbital debris below 600 km are compared to the meteoroid flux at altitudes below 600 km. The orbital debris measurements (in some cases, averaged over several altitudes below 600 km) reflect use of the cataloged objects (99165 two-line elements), Haystack Auxiliary (HAX) radar, Haystack Long Range Imaging Radar, Goldstone radar, and returned spacecraft surfaces. At higher altitudes, the orbital debris flux increases with altitude, up to 900 km, where HAX has measured the flux of 1-cm objects as being about 10 times larger. The meteoroid flux remains nearly constant over this region. SOURCE: Courtesy of NASA-JSC, from Orbital Debris Program Office, “APPEL Orbital Debris Mitigation & Reentry Risk Management Course,” CD, NASA Johnson Space Center, Houston, Tex., 2010, Part 1B, pp. 18 and 27.

sizes (between 2 mm and 10 cm in LEO), an exposed surface larger than that of a typical satellite is required to obtain a meaningful sample of “impacts,” so ground telescopes and short-wavelength radars are used. However, neither telescopes nor radars actually track the objects that are detected; rather, they stay in a “staring mode” that essentially counts the number of objects passing through their relatively small field of view. While the debris is in the field of view, its direction of motion, signal strength, and range (for radar) or angular velocity (for telescopes) are measured. The largest source of uncertainty with these sensors exists in interpreting the signal strength in terms of the size or mass of the object passing through the field of view.

Signal strength is reported by telescopes in units of stellar magnitude, and by radar as radar cross section (RCS). Stellar magnitude is related to RCS statistically by using a sample of small fragments in which both RCS and magnitude are measured for each object.3 An advantage of detecting uncataloged debris with both radars and telescopes is finding debris that may not be seen by one or the other alone. In 1995, NASA began operations of the NASA-built and NASA-designed 3-meter Liquid Mirror Telescope (LMT). However, as a result of budget cuts, the LMT was shut down in 2001. Although the LMT was providing useful data on uncataloged debris, the amount of debris found was less than predicted as a result of its lower than expected albedo. The decision to discontinue opera-


3 D.J. Kessler and K.S. Jarvis, Obtaining the properly weighted average albedo of orbital debris from optical and radar data, Advances in Space Research 34(5):1006-1012, 2004.

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