Life Sciences in the Space Program 



Geosipichronous Orbit 



A mission in geosynchronous orbit around the Earth faces radiation from several 

 sources: 1) electrons in the outer radiation belt, 2) bremsstrahlung from electron- 

 shielding interactions, 3) GCR, and 4) SPE's. The electrons are high energy, and 

 doses rise very rapidly as shielding decreases to values of less than 2 g/cm**2 (at 1 

 g/cm**2, the dose is approximately 5 rads/day). With greater than 2 g/cm**2 of 

 shielding, bremsstrahlung dominates and not much is gained with additional 

 shielding; doses range from tens to hundreds of millirads per day, depending on 

 the parking longitude (4). Compared to the first two sources, GCR contributes a 

 smaller but, nevertheless, significant dose. Because of the contribution of fast 

 particles, shielding does not have a profound effect on dose rate. A rough estimate 

 of the dose rate is on the order of 100 millirem per day with no shielding and 50 

 millirem per day behind 4 g/cm**2 Al (5). (The corresponding physical doses are 

 approximately 30 millirads per day and 20 millirads per day, respectively.) Doses 

 from SPE's vary considerably, corresponding to the wide range of magnitudes of 

 the events. 



Curtis tabulates a number of doses for solar particle events occurring in Solar 

 Cycle 19 (1958 - 1961). for shielding of 2 g/cm**2 Al, skin doses were typically 

 100-200 rads, and doses 4 cm deep in tissues were in the range of 20-50 rads. 

 Behind 5 g/cm**2 Al, the corresponding doses were 20-80 rads and 15-30 rads, 

 respectively. These doses are of a magnitude sufficient to produce acute effects. 



Finally, doses from all sources received in transit from LEO to GEO are somewhat 

 less than 1 rem (6). 



Lunar Colony 



A colony on the surface of the Moon would have no natural magnetic or 

 atmospheric shielding from galactic cosmic rays or SPE's. At solar minimum, the 

 annual dose-equivalent rate due to GCR is approximately 30 rem per year (7). As 

 discussed above, doses from SPE's can be substantially greater. Because of the 

 penetrating nature of GCR, substantial amounts of shielding are needed to stop 

 the riZE (high atomic number, Z, and high energy, E) component. Nuclear 

 interactions between the GCR and the shielding result in production ol neutrons, 

 complicating the dosimetry and the calculation of biological effects. Figure 2 

 illustrates the dose-depth relationships lor GCR in lunar material, indicating the 

 complexity of calculating shielding (7). The cosmic rays are significantly attenuated 

 after tens of grams per square centimeter of shielding. However, nuclear 

 interactions result in the buildup of a significant quantity of neutrons, which have 

 a high biological effectiveness. 



( liven lifetime exposure limits, it becomes clear that if individuals are to spend 



on the Moon, substantial shielding would be necessary. Since some amount 



time would presumably be necessary to perform the colony tasks, it 

 t be advisable to build sleeping quarters deep below the surface. Very well 

 :afe havens would also be needed for the occasional giant solar particle 





