Radiation Shielding of Lunar Spacecraft
A description of the radiation environment to be encountered in space is presented. Based on available experimental data, a model of the spatial, spectral, and temporal variation of charged particles from the Van Allen belts and cosmic radiation is established.
The attenuation of charged particles has been calculated using data on the range and relative energy loss as function of energy for protons and electrons in various materials. Calculations have been performed to determine the relative effectiveness of varying thicknesses of different shielding materials as a function of geometry of the shield. Shield weight penalties as a function of material and threshold energy for a representative hazard chamber are presented. Tissue dose rate variation with time for a typical three-dimensional lunar trajectory is presented and compared to flight radially in the plane of the magnetic equator. Total integrated biological dose as a function of carbon shield thickness was calculated for four solar-flare events, and for flight radially out through the inner belt. Considering the amounts of ablation material, structure, and equipment surrounding the crew in a reentry vehicle, it was found that the additional shielding required for traversal of the belts is small. It is shown that approximately 10 g/cm2 of carbon shielding is sufficient to shield not only the inner-belt protons, but would also provide adequate protection against solar flares of the magnitude that occurred in March and August of 1958. However, it was also found to be impossible to shield against solar flares of the highest known energy within the reasonable weight limits of a typical lunar mission. Since adequate shielding cannot be provided against these very intense solar proton events, the probability of encountering these and and various lesser intensity flares is extremely important. Accordingly, the probability of encountering solar protons as a function of mission time is presented. These encounter probabilities are considerably reduced if flares can be predicted. Continuing effort in this area indicates that some degree of solar-flare prediction is feasible.
KeywordsSolar Flare Radiation Belt Solar Proton Radiation Shielding Solar Proton Event
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