Orbital Rads Copyright (c) 1993, 1996 by Sean Barrett Here are some random comments on radiation dose rates in Earth-orbit. The South Atlantic Anomaly is a region where the proton and electron belts (the "van Allen" belts) dip down to a low altitude, due to the offset and tilt of the Earth's magnetic field and the slight bulge of the earth. As its name implies, this anomaly is located in the South Atlantic, with its centre at about the same latitude at South Africa but on the South America side of the Atlantic. The proton belt extends out to about 3.5 Earth radii, and the electron belts to about 8.5 Earth radii. The particle fluxes in the belts are lower during periods of solar maximum activity (the extra heat from the sun expands the atmosphere causing higher absorption of particles from the belts) than solar minimum. Beyond the trapped belts, it is just solar flare particles (protons mostly) and cosmic rays that provide the dose. At Space Station Mir (52 degree inclination, 400 km circular orbit) an unshielded human would take a total dose in 1 year of 146 kilorads. This dosage is well over three hundred times the LD30/50. However, the vast majority of this dose is beta, and a person shielded by 4mm of aluminum (a thin hull) would only take 150 rads -- one-third the LD30/50. At Space Station Fred (28.5 deg inclination, 500 km circular orbit) an unshielded human would take a total dose in 1 year of 17 kilorads, and one inside a 4mm Al hull would take 170 rads. Conclusion 1: in low orbit, you don't need special shielding -- a hull does fine. In geosynchronous orbit, unshielded, 73 megarads a year. Death becomes certain in minutes, occurs in hours. That 4mm hull drops the dose to 27 kilorads in a year. Wear your lead skivvies. Conclusion 2: in Clarke orbit, you need shielding. You need eighteen inches (50cm) of lead to drop that annual dose to a level legal in the US for continuous habitation. If you want to read up on the subject: 1. Vette,J.I. et al, "AP8 Trapped Proton Environment for solar maximum and solar minimum", NSSDC 76-06, NASA-GSFC (1976) 2. Vette,J.I. et al, "AE8 model for inner and outer zone electrons at solar maximum and solar minimum...", see NSSDC reports 72-06, 72-13 (AE4), 72-10 (AE5), and 76-04 (AE6). 3. Feynman J., "A New Interplanetary Proton Fluence Model", J. Spacecraft and Rockets 27, pp403 (1990) 4. Seltzer,"SHIELDOSE: a computer code for space shielding radation dose calculations", TN-1116 (NBS), 1980. The Z(mm) column is the shielding thickness in mm of aluminium. It is assumed that the H20 target is at the center of a spherical shell of Z mm of Al. ------------ For MIR (52 degree inclination, 400 km circular orbit) total dose after 365.25 days: No solar proton events were included in the analysis. DOSES FOR A PERIOD OF 365.250 DAYS 4*PI DOSE AT CENTRE OF ALUMINIUM SPHERES with thickness Z(mm) RADS H2-O Z(MM) Z(G/CM2) TOTAL |( ELECTRON BREMS TRP PROT SOL PROT ( ============================================================================== 0.050 0.014 1.460E+05 |( 1.454E+05 1.050E+01 6.082E+02 0.000E+00 ( 0.100 0.027 7.725E+04 |( 7.703E+04 6.923E+00 2.172E+02 0.000E+00 ( 0.200 0.054 3.096E+04 |( 3.085E+04 4.198E+00 1.051E+02 0.000E+00 ( 0.300 0.081 1.591E+04 |( 1.582E+04 3.013E+00 7.929E+01 0.000E+00 ( 0.400 0.108 9.450E+03 |( 9.381E+03 2.362E+00 6.711E+01 0.000E+00 ( 0.500 0.135 6.246E+03 |( 6.185E+03 1.971E+00 5.915E+01 0.000E+00 ( 0.600 0.162 4.482E+03 |( 4.427E+03 1.719E+00 5.348E+01 0.000E+00 ( 0.800 0.216 2.745E+03 |( 2.698E+03 1.422E+00 4.635E+01 0.000E+00 ( 1.000 0.270 1.934E+03 |( 1.890E+03 1.252E+00 4.256E+01 0.000E+00 ( 1.500 0.405 1.064E+03 |( 1.026E+03 1.021E+00 3.696E+01 0.000E+00 ( 2.000 0.540 6.788E+02 |( 6.445E+02 8.891E-01 3.340E+01 0.000E+00 ( 2.500 0.675 4.529E+02 |( 4.211E+02 7.979E-01 3.105E+01 0.000E+00 ( 3.000 0.810 3.082E+02 |( 2.781E+02 7.296E-01 2.936E+01 0.000E+00 ( 4.000 1.080 1.514E+02 |( 1.239E+02 6.331E-01 2.687E+01 0.000E+00 ( -------------- FOR Shuttle (SSF), 28.5 deg inclination 500 km circular orbit: DOSES FOR A PERIOD OF 365.250 DAYS 4*PI DOSE AT CENTRE OF ALUMINIUM SPHERES 0RADS H2-O 0 Z(MM) Z(G/CM2) TOTAL |( ELECTRON BREMS TRP PROT SOL PROT ( ============================================================================== 0.050 0.014 1.740E+04 |( 1.702E+04 1.205E+00 3.818E+02 0.000E+00 ( 0.100 0.027 9.535E+03 |( 9.185E+03 7.996E-01 3.491E+02 0.000E+00 ( 0.200 0.054 3.953E+03 |( 3.635E+03 4.759E-01 3.179E+02 0.000E+00 ( 0.300 0.081 2.108E+03 |( 1.809E+03 3.328E-01 2.990E+02 0.000E+00 ( 0.400 0.108 1.311E+03 |( 1.028E+03 2.534E-01 2.832E+02 0.000E+00 ( 0.500 0.135 9.118E+02 |( 6.421E+02 2.054E-01 2.695E+02 0.000E+00 ( 0.600 0.162 6.897E+02 |( 4.320E+02 1.745E-01 2.575E+02 0.000E+00 ( 0.800 0.216 4.693E+02 |( 2.298E+02 1.379E-01 2.394E+02 0.000E+00 ( 1.000 0.270 3.699E+02 |( 1.417E+02 1.170E-01 2.281E+02 0.000E+00 ( 1.500 0.405 2.691E+02 |( 6.133E+01 8.944E-02 2.077E+02 0.000E+00 ( 2.000 0.540 2.267E+02 |( 3.384E+01 7.492E-02 1.928E+02 0.000E+00 ( 2.500 0.675 2.034E+02 |( 2.067E+01 6.569E-02 1.826E+02 0.000E+00 ( 3.000 0.810 1.888E+02 |( 1.342E+01 5.925E-02 1.754E+02 0.000E+00 ( 4.000 1.080 1.698E+02 |( 5.479E+00 5.049E-02 1.643E+02 0.000E+00 ( ------------------- Geosyncronous: DOSES FOR A PERIOD OF 365.250 DAYS 4*PI DOSE AT CENTRE OF ALUMINIUM SPHERES 0RADS H2-O 0 Z(MM) Z(G/CM2) TOTAL |( ELECTRON BREMS TRP PROT SOL PROT ( ============================================================================== 0.050 0.014 7.344E+07 |( 7.341E+07 5.348E+03 0.000E+00 3.331E+04 ( 0.100 0.027 4.556E+07 |( 4.552E+07 4.015E+03 0.000E+00 3.393E+04 ( 0.200 0.054 2.268E+07 |( 2.265E+07 2.746E+03 0.000E+00 3.533E+04 ( 0.300 0.081 1.337E+07 |( 1.333E+07 2.096E+03 0.000E+00 3.704E+04 ( 0.400 0.108 8.606E+06 |( 8.565E+06 1.691E+03 0.000E+00 3.954E+04 ( 0.500 0.135 5.900E+06 |( 5.856E+06 1.418E+03 0.000E+00 4.249E+04 ( 0.600 0.162 4.252E+06 |( 4.201E+06 1.226E+03 0.000E+00 5.012E+04 ( 0.800 0.216 2.445E+06 |( 2.395E+06 9.822E+02 0.000E+00 4.949E+04 ( 1.000 0.270 1.528E+06 |( 1.486E+06 8.336E+02 0.000E+00 4.099E+04 ( 1.500 0.405 5.846E+05 |( 5.551E+05 6.257E+02 0.000E+00 2.890E+04 ( 2.000 0.540 2.695E+05 |( 2.469E+05 5.145E+02 0.000E+00 2.203E+04 ( 2.500 0.675 1.389E+05 |( 1.216E+05 4.461E+02 0.000E+00 1.683E+04 ( 3.000 0.810 7.743E+04 |( 6.358E+04 4.008E+02 0.000E+00 1.345E+04 ( 4.000 1.080 2.714E+04 |( 1.776E+04 3.442E+02 0.000E+00 9.033E+03 (