capable of intense energy deposition over a very small volume of tissue; killing or damaging any cell that they transverse along a cylinder of tissue of very small diameter (microbeam). The quantification of the effects of exposure to the HZE particles is the subject of extensive debate among the radiobiologists involved in NASA's research programs. Conventional dosimetric concepts (as represented by the units of dose in the preceding table) are difficult to apply. The current trend of thinking holds that new concepts of radiation dose expression will be necessary for "bookkeeping" on this component of the total space radiation exposure problem. Recommendations are currently being formulated as to objectives for NASA's radiobiology research and they are expected to focus upon the HZE particles as uniquely NASA's problem and as a potentially serious source of radiation exposure. The key to assessing radiation hazards to personnel involved in such programs as the SPS is, of course, the accuracy of the models of the ambient space radiation environment. Currently the environment is well known qualitatively and the mathematical models describing it are adequate for determining the relative level of hazard. The model describing the geosynchronous electron environment, designated AE3, was stated to be reliable to about a factor of 2, at the time of its publication. Recent data from ATS-6 differ from the model AE3 by an order of magnitude in terms of calculated tissue absorbed dose. Thus, significant improvement in the environmental models in terms of absolute numbers of particles and their energy distribution, magnitude of cyclic variations and the periodicity of the variations and the potential for transient shifts must be accomplished before major design efforts are started, particularly for facilities in geosynchronous orbit. The space radiation environment will be a major driver on design of manned facilities and vehicles. Design to current radiation exposure standards will require increase in shielding effectiveness for long-term orbital facilities by factors of 2 to 5 over current design practices. LEO to GEO transfer for crews and radiation sensitive cargo will require high specific impulse vehicles with shielding effectiveness comparable to the Apollo CM. The radiation environment presents no impediment to further design and development; however, two areas require attention: (1) better quantitation of the environment and resultant radiation exposure liability, and (2) quantitation of the level of risk and the nature of the injury from long-term exposures to the galactic cosmic rays (HZE particles). Research on the latter is currently underway. In the area of microwave radiation and the hazards of exposures, the research has been "shot gunned" and it is difficult to collate the results into a coherent evaluation. Another shortcoming of all the research is the dosimetry technique since it is extremely difficult to make meaningful measurement of an RF energy field without distorting that field, particularly measurements of energy deposition into biological tissue.
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