Current problems in astroradiobiology.

H U M A N FLIGHT in extra-atmospheric regions has created, for the radiobiologist, a number of new problems resulting from the fact that the radiation environment in space is peculiarly different from what natural and man-made terrestrial radiation sources can provide. Already before the advent of the satellite era, when the ceiling of manned aircraft progressed from the troposphere deeply into the stratosphere and finally all the way up to ~he top of the atmosphere, these new aspects were recognized. Especially one problem has found the early attention of those concerned with radiation hazards in high altitude flight. We mean the controversial issue of the heavy nuclei of the primary cosmic radiation. In 1948, soon after the discovery of the heavy nuclei, C. F. Gell 2 directed attention to the fact that components of the primary cosmic ray beam at extreme altitude and in free space would pose a special problem. Today, twelve years later, the question of the microbeam effectiveness of heavy nuclei is still unsolved. When radiation hazards in space flight are discussed nowadays, the newly discovered high intensity proton radiation fields of various origin are in the center of interest. In fact, attention seems so much concentrated on these new phenomena that the unsolved heavy nuclei problem is in a certain danger of no longer being seen in its true significance. This finds a certain justification in the circumstance that the exposure hazard in proton radiation fields is an acute one which might well reach or surpass the incapacitation threshold, whereas the heavy nuclei hazard is of the low-dosage long-term