eclipses) over a 30-year period. The state of the art of solar cells is now at a level where lifetimes of 10 years are achievable and, in fact, have been projected for INTELSAT IV. The primary life-limiting factor is solar radiation damage which causes a logarithmic decay of solarcell effectiveness. It is estimated that exposure at this altitude for 30 years would result in degradation of 20-25 percent in power for present solar cells protected by 6 mils (150 microns) of glass. The additional 20-25 percent cost and mass penalties associated with such shielding are, of course, unacceptable so other solutions must be found. The problem of radiation damage is further complicated by the necessity of traversing the Van Allen radiation belts at a relatively slow rate (30 days or more) with essentially unknown but potentially very severe effects. Attempts to reduce solar cell costs may also be limited because the proposed processes may also increase radiation susceptibility. For example, one means of improving solar cell efficiency is to reduce the base resistivity; this, however, increases susceptibility to radiation. Another factor is the impact of micrometeroids; however, in synchronous orbit, the SSPS is expected to suffer only a one percent loss of solar cells from this cause during its 30 year lifetime. Improvements in environmental resistance will require: • Improved radiation resistant materials • Radiation spectral tailoring to minimize unusable solar radiation • Meteorite hardening • Improved annealing techniques • Material selection to prevent thermal cycle fatigue. (5) Optimized Concentration Factor The overall array performance can be improved by incorporating mirrors to concentrate the sun's radiant energy on the solar cells. This concentration can be implemented in many ways and at various levels and can result in significant SSPS cost reductions by reducing the nurriber of solar cells required.
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