Radiation 



National Council of Radiation Protection, Naval Research Laboratory, National 

 Oceanic and Atmospheric Administration, Oak Ridge National Laboratory, 

 University of California, University of San Francisco, and U.S. Air Force School of 

 Aerospace Medicine. 



Radiation Environments and Biological Effects 



The first important problem in determining the radiation hazards to humans in 

 space is defining the radiation environments. A variety of measuring devices have 

 been carried on satellites and manned spacecraft, so that today much is known 

 about the radiation fields encountered in space. Since the fields are dynamic and 

 spatially varying, it is difficult to characterize them completely by measurements. 

 Parallel efforts in modeling are being carried out to provide more complete 

 estimates of these fields. 



The space radiation environment is divided into several different categories, 

 depending on the type of radiation and its location. The radiation in LEO is 

 primarily protons trapped in the Earth's magnetic field. In geosynchronous Earth 

 orbit (GEO), trapped electrons and bremsstrahlung radiation produced by space- 

 craft shielding are the predominant sources. The radiation in both of these sets of 

 orbits varies as a function of position and time. Outside the Earth's magnetic field, 

 radiation comes from large solar particle events and galactic cosmic rays (GCR). 

 Radiation from SPE's occurs sporadically and can be life threatening in intensity. 

 GCR radiation is a low-level, constant background radiation source. The interaction 

 of all of these radiation sources with the material of the spacecraft and its contents 

 alters intensity, spectral characteristics, and quality of the radiation. Table 1 

 summarizes the sources of radiation. 



The effects of radiation on humans are commonly grouped into two categories: 

 acute and long-term. Acute effects include radiation sickness and death; long-term 

 effects are carcinogenesis, teratogenesis, formation of cataracts, and damage to 

 nondividing cells. Figure 1A indicates the types of physiological responses 

 grouped under acute effects and the radiation doses that typically cause their 

 onset (3). Figure IB describes the temporal pattern of radiation effects following 

 exposure to radiation (3). For long-term missions and colonization, radiation injury 

 to embryos must also be considered. The extent and kind of biological effects 

 depends on the type of radiation, the dose, and the dose rate. In particular, the 

 presence of low and high LET radiation in the space environment has a great 

 impact on the biological effects. The deposition of energy within the cells of the 

 organism is different for low and high LET radiation, resulting in different 

 biological effects. For a given absorbed dose, the relative biological effectiveness 

 (RBE) is a function of radiation type (e.g., photons, particle species) and energy. 

 The RBE also depends on the particular tissue absorbing the radiation. 



Mission Scenarios 



In the next several decades, a number of different mission scenarios are plausible. 

 As discussed in the preceding sections, the radiation environment unique to each 

 scenario determines the type and magnitude of biological effects to be expected. 



55 



