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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop
The large uncertainties in space radiation and biological effects that exist at present increase the cost of missions owing to the large safety margins required as a consequence. These uncertainties also limit the ability to judge the effectiveness of risk mitigation methods, such as improvements in shielding or biological countermeasures. Operational measures and radiation shielding are currently the main means of reducing radiation risk; improved biological markers have the potential to enable improved early diagnostics; discovery of means of biological prevention and intervention may lead to significantly more powerful methods, including better radioprotectants, to overcome the biological consequences of exposure to radiation. Continued basic research has the potential to address all of these key issues effectively.1
The challenges described here can be overcome, and NASA is making progress on many of them. But the hazards of space radiation to future space explorers can only be reduced with the assistance of the solar and space physics science community and effective collaboration between the scientists and the space operations community.
Cucinotta, F.A., M.-H.Y. Kim, and L. Ren. 2005. Managing Lunar and Mars Mission Radiation Risks. Part I: Cancer Risks,Uncertainties, and Shielding Effectiveness. NASA/TP-2005-213164. NASA, Washington, D.C.
Townsend, L.W., F.A. Cucinotta, and J.W. Wilson. 1992. Interplanetary crew exposure estimates for galactic cosmic rays. Radiat.Res. 129:48-52.
Extraordinary shielding (~300 to 500 g/cm2) would be necessary to protect astronauts if their radiation limits were set at levels comparable to those of occupationally exposed individuals on Earth (e.g., workers at nuclear power plants) or at the even lower exposure limits established for the general public. However, astronaut limits for operations in low Earth orbit (LEO) are approximately an order of magnitude higher than limits for earthbound radiation workers (at present 50 centi-sievert [cSv] per year for astronauts, with a lifetime limit that depends on age and sex; however, no limits have been established as yet for Mars missions). This difference is due to the shorter career exposure times for astronauts (generally assumed to be no more than 10 years) versus possible 40+ year career exposure times for radiation workers on Earth. The LEO limits for astronauts, although higher than limits for earthbound radiation workers, are based on a 3 percent excess cancer mortality risk. Shielding needed to attain this elevated level of permitted exposure is much less than the heroic value of 300 to 500 g/cm2, generally being somewhere in the range of 20 g/cm2 or somewhat above. For example, for 20 g/cm2 aluminum shielding, Townsend et al. (1992) calculate 50 cSv per year at solar minimum, but Cucinotta et al. (2005) now estimate closer to 75 cSv/year using the newer transport codes and environmental models. The effect of these levels on astronaut risk is not sufficiently well known at this time and is a subject of active research. Future human spaceflight depends on the outcome of this research. Note that the most relevant radiation protection quantity is the radiation risk, as represented by the dose equivalent, which represents risk of developing a fatal cancer. Most of the dose equivalent is contributed by the heavy ion component of the GCR spectrum and not the protons. Dose is important, but only for possible acute radiation syndrome effects (radiation sickness) resulting from very large SEP radiation exposures. Dose is relatively small from GCR particles, being only around 20 centi-gray (20 rads) annually during solar minimum, of which only about 7 rads come from protons of all energies (Townsend et al., 1992). “Gray (Gy)” is the name for the unit joule per kilogram when that unit is applied to the absorbed dose. “Absorbed dose” is defined as the energy imparted by ionizing radiation per unit of mass.