FIGURE 4-2 Flux of LEO cataloged objects, 1994 (assumed velocity 10 km/s).

SOURCE: Prepared by Kaman Sciences Corporation based in part on U.S. Space Command data.

acquired at a variety of altitude regimes at different times during a seven-year period. The uncertainty of debris population estimates is, however, reflected by the error bars in the figure.

Figure 4-1 predicts the average number of collisions with different sizes of debris that a spacecraft in a ''typical" low Earth orbit will experience in a 10-year orbital lifetime. For example, the probability that a spacecraft in LEO with a cross-sectional area of 10 square meters will collide with an object larger than about 1 cm in diameter over its 10-year functional lifetime can be seen to be somewhere between one in a hundred and one in a thousand. The figure also predicts that the same spacecraft will be struck by about one 1-mm- to 1-cm-diameter particle and somewhere between 100 and 1,000 particles with diameters between 0.1 mm and 1 mm during this time period.

The chances of a spacecraft in LEO being struck by debris can vary significantly from those estimates, depending on the spacecraft's particular orbit. Figure 4-2 shows the variation in flux for cataloged objects in LEO as a function of altitude. While this figure does not show the uncataloged debris flux, the Haystack data have shown that uncataloged debris as small as 0.7 cm in diameter follow a similar distribution to the cataloged flux throughout much of LEO (as discussed in Chapter 3). Figure 4-3 shows an estimate by NASA's EVOLVE model of the flux of large