Fig. 4-39. Sporadic and Stream Average Total Meteroid Environment (Omnidirectional) The meteoroid flux-mass environment shown in Figure 4-39 was calculated on the assumption that the distribution of meteoroid orbital directions with respect to the Earth is uniform. Actually, the majority of meteoroid orbits are close to the ecliptic plane as shown in Figure 4-40. It is possible to preferentially orient the SPS radiators to take advantage of this anisotropic distribution of meteoroids in space. Figure 4-42 shows that as the SPS orbits the Earth and the Earth orbits the Sun, the SPS is always pointing towards the Sun. The smaller figure shows the radiator oriented to be in the plane of the ecliptic and edgewise to the main meteoroid flux. Fig. 4-42. SPS Radiators Can be Preferentially Oriented Figure 4-42 showed the radiator placed in the plane of the ecliptic. Figure 4-43 shows the flux concentrated at a low angle to the ecliptic plane. This angular concentration extends around the leading edge of the radiator from helion to antihelion, as was shown in Figure 4-41. Thus, the radiator sees the meteoroid flux impinging in a concentration at an angle of approximately 15° to its plane of motion. Fig. 4-43. Flux Seen by Radiator The radiator consists of thousands of small tubes spaced at 50 mm (2 inches) to 75 mm (3 inches) apart, depending upon design. These tubes are most vulnerable to meteoroid damage since penetration would allow escape of coolant. Protection of the tubes by some form of barrier, therefore, is extremely important. To facilitate the design of a minimum weight barrier, a refined flux-mass model was derived taking into account the orientation of the flux concentration. Fig. 4-41. Resultant Interaction With Object in Earth's Orbit Both these distributions are apparent flux densities as observed from Earth; however, they clearly indicate the anisotropic distribution of meteoroids in space. Fig. 4-40. Meteoroid Motion The graph on the left of Figure 4-41 shows the observed meteoroid flux with respect to the ecliptic, and that on the right presents the distribution with respect to solar longitude, in the plane of the ecliptic. These figures were obtained from Reference (2).
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