The risks to personnel in space from the naturally occurring radiations are generally considered to be one of the three or four most serious limitations to human space missions, as noted in two reports of the National Research Council/National Academy of Sciences (1996, 1998) and the NASA Critical Path Roadmap. The main objective of the Core Project of the Radiation Effects Team for the National Space Biomedical Research Institute is to study the consequences of radiations in space in order to develop countermeasures, both physical and pharmaceutical, to reduce the risks of cancer and other diseases associated with such exposures.
During interplanetary missions, personnel in space will be exposed to galactic cosmic rays, including high-energy protons and energetic ions with atomic masses of iron or higher. In addition, solar events will produce radiation fields of high intensity for short but irregular durations. The level of intensity of these radiations is considerably higher than that on Earth's surface, and the biological risks to astronauts is consequently increased, including increased risks of carcinogenesis and other diseases. Carcinogenesis from space radiation is one of the major health concerns for long term space travel. Protons are the most abundant component of the space radiation in both solar particle events and in the galactic cosmic rays. The corresponding dose-rates for proton exposure range from 10-3 to 0.1 Gy/hr, with possible accumulated doses of roughly 0.5-2 Gy in a large solar particle event or for interplanetary travel. These dose-rates are lower than those used in therapeutic treatment of cancers or encountered in epidemiological studies of A-bomb survivors.
This group is examining the risk of cancers resulting from low-dose, low-dose rate exposures of model systems to photons, protons, and iron by using ground-based accelerators which are capable of producing beams of protons, iron, and other heavy ions at energies comparable to those encountered in space. The specific aims of this work include in-vivo studies of carcinogenesis resulting from exposures to low doses of energetic heavy- ions, protons, and photons. We have successfully conducted a series of experiments using a 1-GeV iron beam at the Brookhaven National Laboratory and 250-MeV protons at Loma Linda University Medical Center's proton synchrotron facility. As part of these studies, this group is investigating the potential for the pharmaceutical, Tamoxifen, to reduce the risk of breast cancer in astronauts exposed to the level of doses and particle types expected in space. These data are essential for an improved evaluation of the cancer risks from radiation in space. Nevertheless, this is only the second large-scale study of this type and only the first including a study of a chemopreventive agent. Although the experiments are only in the preliminary stages, extensive data have been forthcoming and are reviewed in this report.
Theoretical studies are being carried out as part of this project in a collaboration between scientists at NASA's Johnson Space Center and Johns Hopkins University. The theoretical studies, in coordination with the experimental program, have provided methods and predictions which are being used to improve our evaluation of radiation risks to be encountered and to evaluate appropriate strategies for countermeasures. Continued collection and analysis of data from this project over the next three years will enhance the precision of our estimates of biologic response and reduce the unacceptable uncertainties associated with present risk assessments of activities in space which increase vulnerability and costs.
Although the work in this project is primarily directed toward problems associated with space travel, the problem of protracted exposures to low-levels of radiation is one of national interest in our energy and defense programs, and the results may suggest new paradigms for addressing such risks.