NASA’s current missions to the International Space Station (ISS) and potential future exploration missions involving extended stays by astronauts on the lunar surface, as well as the possibility of Near-Earth Objects (NEO) or Mars missions, present challenges in protecting astronauts from radiation risks. These risks arise from a number of sources, including solar particle events (SPE), galactic cosmic rays (GCR), secondary radiation from surface impacts, and even nuclear isotope power sources transported with the astronauts. The serious early and late radiation health effects potentially posed by these exposures are equally varied, ranging from prodromal radiation sickness to cancer induction. Other possible effects include central nervous system damage, cataracts, heritable effects, impaired wound healing and infertility. Since opening in October 2003, the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Lab has enabled NASA to sponsor research focused on understanding and quantifying the radiation health risks posed by space radiation environments. While most aspects of the space radiation environments are now relatively well characterized, important uncertainties still exist regarding biological effects and thus the level and types of risks faced by astronauts.
In response to reviews and recommendations, NASA has developed an integrated research program that consists of openly solicited peer reviewed research to investigate the biomedical effects of simulated space radiation and provide the needed knowledge base on GCR and SPE radiobiology. While the main focus is on understanding cancer risks, the program also includes studies on potential degenerative risks to the central nervous system, the cardiovascular system and other tissues. In cooperation with the U.S. Department of Energy (DoE), NASA operates the NSRL, the only place in the U.S. where sophisticated biomedical experiments can be conducted using the heavy ions encountered in the space radiation environment.
For projecting cancer risk for ISS crews and to support trade study assessments of potential lunar, NEO, and Mars missions, NASA uses the model recommended by the National Council of Radiation Protection and Measurements (NCRP) Report No. 132. NASA also makes an uncertainty assessment in the NCRP model coefficients that describes errors in low LET human radio-epidemiology data, dose- and dose-rate effectiveness factors (DDREFs), radiation quality factors, and space physics. For astronaut occupational exposures, the 95 percent confidence level is used as a supplementary requirement as part of the Permissible Exposure Limit (PEL) of a no greater than
3 percent increase in the Risk of Exposure Induced Death (REID). The PEL standards are approved by the NASA Chief Health and Medical Officer.
NASA has updated its Space Radiation Cancer Risk Model based on recent developments including the following:
• BEIR-VII, UNSCEAR-2006, and other reports in the scientific literature have made new assessments of human radio-epidemiology data and DDREFs;
• New research results from NSRL have begun to modify the understanding of radiation quality and dose-rate effects; and
• NASA’s revised evaluation of uncertainty factors.
Because it is used to project the cancer risk for current ISS crews and future explorations missions, this NASA update to its Space Radiation Cancer Risk Model requires independent review and validation.
1. The committee will evaluate proposed updates to the NASA cancer projection model taking into consideration the following:
• Current knowledge of low-LET radiation cancer epidemiology,
• Effects of tissue weighting factors, radiation weighting factors, and DDREFs used in projecting risks, and
• Current uncertainties in Quality Factors, DDREFs and organ dose assessment.
This will be done taking into consideration possible qualitative differences between low LET and heavy ion biological effects to determine if the use of quality factors are appropriate or inappropriate for GCR risk assessments.
2. The committee will identify gaps in NASA’s current research strategy for reducing the uncertainties in cancer induction risks.