Radiation Hazards to Crews of Interplanetary Missions Biological Issues and Research Strategies (1996) / Chapter Skim
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3: How to Reduce Risk and the Uncertainty in Risk Estimates
3: How to Reduce Risk and the Uncertainty in Risk Estimates
Pages 35-41
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From page 35... ...
Knowledge Base Development (current studies indicate that the particle distribution and energies that occur behind the various potential shield materials are critically dependent on the fragmentation and secondary particle production that occurs when spacecraft shielding is struck by radiation. Even cross sections for protons, which have been studied extensively both experimentally and theoretically in the most heavily supported computer modeling codes, show disagreements by a factor of 2i between the values calculated from models and measurements in energy regions for which there previously were no data.
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From page 36... ...
This amount of beam time compares with about 400 hours per year previously available for similar research studies at the now-closed Berkeley BEVALAC. From the predicted cross sections for the secondary particles and the maximum count rates of the most sensitive detectors to detect these particles in the apparatus, it is possible to estimate typical time periods necessary to accumulate a sufficient number of counts at a specified beam rate, so that the random error in total counts is minimized.
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From page 37... ...
At a recent space shielding workshop, liquid hydrogen was identified as the optimum shield material.3 Accepting this fact, metal hydrides i.e., lithium hydride would appear to be prime candidates for consideration as potential shield materials. Current calculations with the HZETRN/NUCFRG2, a transport code utilized by Langley Research Center investigators, indicate that the production of fragments and secondary particles is such that increasing the thickness of aluminum shielding has conflicting effects depending on the risk model in use.
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From page 38... ...
The costs of such a program, in time and money, have to be compared with the costs of using currently incomplete and inaccurate databases and radiation shielding models and then allowing for excess shielding in the spacecraft design to compensate for the maximum uncertainty associated with the poorly known parameters. Using the Apollo mission experience as a guide, Wilson has estimated that at an overall uncertainty factor of 3 for the risk posed by exposure to HZE particles, the increased costs of compensating for that uncertainty by using excess shielding in the spacecraft for the Mars mission are about $10 billion; at an overall uncertainty factor of 6, the value is close to $30 billion.6 These figures far exceed NASA's annual research budget for particle physics and biology research of approximately $4.5 million per year.7 Accepting increased costs as being associated with increased uncertainty, the question then arises: What are the sources of uncertainty in the current shielding design information?
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From page 39... ...
Based on previous studies, it is the general consensus of the task group that the optimal time for a planetary mission is probably during solar minimum, despite the increased total fluence and dose equivalent associated with the increased galactic fluence.~5 This consensus is in part based on the total estimated dose equivalents for each flight scenario, but also is based on the variability of the doses expected from solar events. Moreover, the potential for acute effects arising from exposure at high levels is greater from solar events, although such large exposures are unlikely even during solar maximum except for specific cases such as a large SPE exposing personnel engaged in extravehicular activities.
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From page 40... ...
Conclusions Assuming the goal of allowing time for crew members, in space or on another planetary body, to reach shielded locations, current monitoring systems using visual observations for predicting SPEs are not acceptable. The fact that a space vehicle during its travel may be connected to a flare on the sun by magnetic field lines that originate on the far side creates a blind spot for Earth-based monitoring stations.
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From page 41... ...
Chapter 1 in: Shielding Strategies for Human Space Exploration: A Workshop (John W Wilson, Jack Miller, and Andrei Konradi, eds.)
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