APPENDIX A SPACE RADIATION ENVIRONMENT The preliminary SPS Reference System (DOE/NASA, 1978) calls for workers to move from Earth to low Earth orbit (LEO) (altitude 500 km) for stays of varying lengths of time. Workers then travel to geosynchronous Earth orbit (GEO) (altitude about 36,000 km), following an elliptical trajectory (transfer ellipse, TE). These three SPS environments—LEO, TE, and GEO—have ionizing radiations of different quality, time dependence, and predictabi1ity of dose levels. The various components of the radiation environment are described in this section, and those important to each SPS stage are identified. TRAPPED ELECTRONS Large flux densities of electrons, trapped in the earth's magnetic field, are contained in an inner and outer zone separated roughly by the magnetic shell parameter,* L = 2.8 earth radii. The low energies of electrons in the inner zone are important only if persons are protected by very thin shielding (<0.5 g/cm^ aluminum) (Stassinopoulos, 1979). The outer zone contains flux densities of electrons which are greater in magnitude than those of the inner zone and have a larger fraction of high-energy particles. Maximum flux densities occur in the region of about 3.5 earth radii (approximately 21,300 km); the trapping region extends out to about 12 earth radii (76,540 km). Space vehicles transporting workers and materials from LEO must travel through the heart of the outer electron zone to reach GEO which, at 6.6 earth radii, is well within the outer zone of trapped electrons. Figure A-l illustrates the spatial variation of trapped electrons as a function of altitude and geographic latitude, the positions of LEO and GEO, and a representative pass for the transfer trajectory (TE). Two large temporal variations of the outer zone electron flux densities at the position of a GEO satellite have been identified (Stassinopoulos, 1980): 1. Diurnal variations—At GEO, electron flux densities vary over factors between 6 and 16 (depending on L value) between day and night. The maxima occur at about 1000 to 1100 hours, and the minima at about 2200 to 2300 hours, local time. The extent and times of these extremes also depend on electron energy and, to a small degree, on position in the eleven-year solar cycle. *The magnet shell parameter, L, denotes roughly a geomagnetic field line. The value gives the approximate geocentric distance, in earth radii, of the intersection of the field line with the geomagnetic equatorial plane.
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