BOX 4-1 The Meteoroid Environment

Meteoroids revolve about the Sun and steadily rain upon the Earth and on objects in Earth orbit. About 40,000 (±20,000) metric tons of meteoroids enter the Earth's atmosphere each year (Love and Brownlee, 1993). The intensity of this natural environment is often used to establish a threshold of concern for the orbital debris environment.

The onset of space exploration, particularly human space flight, prompted efforts to assess the potential hazard posed to spacecraft by the natural meteoroid environment. Numerous measurements conducted during the 1960s, including the large-area meteoroid detectors deployed by three Pegasus spacecraft in 1965, revealed that the threat of colliding with a meteoroid capable of inflicting significant damage to a spacecraft was remote. (The probability that a square meter of exposed surface in LEO will be struck by a 1-cm-diameter meteoroid during a year in space is about one in a million.) Simple design features are capable of protecting spacecraft against the predominately small and light (average meteoroid density is about 0.5 g/cm3) particles.

Figure 4-5 shows the estimated meteoroid flux that will be experienced at 500-km altitude. The meteoroid flux varies slowly with altitude due both to shielding by the Earth (which can decrease the flux by as much as a factor of two at low altitudes) and to the Earth's gravity field (which can increase the flux near the Earth by as much as a factor of two) (Kessler, 1972). Just above the Earth's atmosphere, the average velocity of a meteoroid is about 17 km/s; at lunar distances, the average velocity is about 15 km/s. Average meteoroid collision velocities with orbiting spacecraft would be higher by a few kilometers per second, depending on the orbit of the spacecraft (Kessler, 1969).

probability with medium-sized objects at high inclinations may not be as great as the estimated increase shown (for cataloged objects) in Figure 4-4.

High Earth Orbits

Estimates of collision probabilities in high Earth orbits are less accurate than LEO collision probability estimates due to the sparse information available on the HEO debris population. (As described in Chapter 2, there are no measurements above LEO of the small debris population, the medium-sized debris population, or even the smaller objects in the large debris population.) It is certain, however, that the chance of collision with cataloged objects is generally much lower in HEO than it is in LEO. As shown in Figure 3-3, the average spatial density of cataloged objects in even the relatively densely populated semisynchronous and geosynchronous orbits is about 100 times lower than the average spatial density of cataloged objects in LEO. In less densely populated high Earth orbits, the spatial density of cataloged objects is often 1,000 times lower

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