result of the shield being struck by a given type and level of incident radiation. Equally important is knowledge of the effects of a given dose on relevant biological systems for different radiation types. Each of these components involves significant uncertainty that must be reduced to permit effective design of shielding, given that the level of uncertainty governs the amount of shielding. It is only prudent to design shielding that will protect space crew members from the predicted, but uncertain, high levels of biological effects from their exposure to radiation. At the same time, excess shielding, based on current cost estimates, would impose an excess expenditure at the level of tens of billions of dollars.2
An understanding of the scope of needed biological and physical data requires an explanation of certain aspects of radiation behavior and the biological impact. HZE particles impinging on shielding, or on human tissue, result in very dense ionization tracks (high LET) with numerous fragments that produce a spectrum of other energetic nuclei, protons, neutrons, and heavy fragments. The numbers of these other nuclei depend on the nature of the shielding and its mass per unit area. The energy loss of the individual particles depends on their types and energies. Thus each particle contributes to the radiation dose and biological response, which are dependent on the number of particles, their types, and their energies. The theoretical calculations of doses per particle type obtained thus far for relevant shielding materials must be verified by ground-based experiments, because the radiation field rate in space is too complex for sufficient experimental analysis. At the present time, the uncertainties in these measurements amount to a factor of ~2 or more (see “Estimates of Uncertainty in Radiation Risk Factors,” Chapter 2).
Ionizing radiation either directly affects cellular macromolecules or reacts with water to produce free radicals that affect these macromolecules by so-called indirect effects. These effects are mitigated somewhat by the presence of free radical scavengers in the surrounding medium. The scavengers are useful in reducing the effects of low-LET radiation but do not seem to result in any significant decrease in the damage caused by high-LET radiation.
The biological effects of fast charged particles depend on the nature of the particle (its charge and velocity) and on the specific biological end point under observation (e.g., cell killing, mutation at a specific genetic locus, chromosomal alterations, cell transformation in vitro, and tumor induction). The relative biological effectiveness (RBE) is taken as the ratio of the dose of gamma rays required to produce a specific effect to the dose of particle radiation required to produce the same level of effect. The RBE depends on the type of particle and the biological effect under consideration and may vary with the magnitude of the biological effect. More importantly, RBE varies greatly with the LET of the particle. For example, high-energy protons may have an RBE value approaching 1.0, whereas high-energy iron nuclei may have an RBE value approaching 40. For tumor induction in animals exposed at lower doses, the relationship between RBE and LET is known for only one tumor site—the Harderian gland in mice. As there are no equivalent data for tumor induction in humans for different LET values, it is necessary to extrapolate from cell and scanty mouse data to evaluate human risk.
Human radiation risk data, still being collected, are available from the analysis of cancer induction in the Japanese individuals exposed to acute doses of radiation resulting from the atomic bombs.3 These doses are not known precisely. As this radiation was primarily low LET, in order to estimate risks to humans in spaceflight conditions one must extrapolate from the RBE vs. LET data for cells in culture and small mammals to humans. In addition, one must extrapolate from the risks from acute exposures of humans to the low-dose-rate chronic exposures involved in space missions (except for the relatively acute exposures from solar particle events). As a general rule, as the dose rate decreases, the biological effect from a given dose also decreases. This dose reduction, in going from acute to chronic exposure, also depends on the biological system and may range from a factor of 2 to 10.4 The dose rate reduction factor for HZE particles is not well known but is probably closer to 1.5 Two other factors that must be considered, but whose impacts are currently unknown, are the effects of biochemical or cellular repair reactions following exposure to HZE particles and the effects of microgravity on such reactions. Thus, in estimating the risks to humans exposed to radiation in space, the uncertain factors are the radiation fields behind the shielding and the extrapolation, via cell culture and animal experiments, from the uncertain risks posed by acute low-LET exposure to risks posed by chronic high-LET exposure.