inclination. Clearly, the proportion of high-velocity collisions increases for objects in higher-inclination orbits. If the calculations incorporate the population of objects detected by the Haystack radar in addition to the cataloged population, the plotted variation of collision velocity with altitude looks similar to Figure 4-9, but with slightly lower average collision velocities at all inclinations. In a 51.6-degree-inclination orbit, for example, the predicted average collision velocity with cataloged objects is 10.8 km/s, but the predicted average collision velocity with objects detected by Haystack is 9.2 km/s.

In semisynchronous orbits, orbital velocity is only about 3.9 km/s, so the maximum collision velocity is 7.8 km/s. In practice, however, because most spacecraft in these orbits operate in constellations with inclinations near 60 degrees, the average collision velocity is closer to 4 km/s. In GEO, collision velocities are lower still, both because of the low orbital velocities and because the spacecraft and rocket bodies in GEO are traveling in the same direction and have only minor inclination differences (as discussed in Chapter 3). The long-term average GEO collision velocity due to the various differences in inclination is about 0.5 km/s, much less than the average LEO collision velocity (but still about the speed of a rifle bullet).

The angle at which debris is likely to strike a spacecraft is important for spacecraft designers interested in protecting sensitive components. Figure 4-10 predicts the directions from which debris would impact the

FIGURE 4-10 Direction of orbital debris impact predicted for the LDEF.

SOURCE: M&D SIG NASA Model (Chobotov, 1991).

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