The U.S. National Research Council's views of the radiation hazards in space

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Abstract

The author was the Chairman of a Task Group on the Biological Effects of Space Radiation formed as a result of discussions between NASA and the U.S. National Research Council's Committee on Space Biology and Medicine — a committee under the U.S. National Research Council's Space Studies Board. The Task Group was asked to review current knowledge on the effects of long-term exposure to radiation in space and to consider NASA radiation shielding requirements for orbital and interplanetary spacecraft. The group was charged with assessing the adequacy of NASA planning for the protection of humans from radiation in space and with making recommendations regarding needed research and/or new shielding requirements. This manuscript is a summary of the findings and recommendations of the Task Group. Beyond the protection of the Earth's atmosphere and its magnetosphere, the exposure to ionizing radiations far exceeds that on Earth. Of all the risks astronauts may face, this one is probably the most straightforward to control — by providing adequate shielding. However, because shielding adds weight, cost and complexity to space vehicles, it is important for designers to have a good quantitative understanding of the true risk and its degree of uncertainty so as not to under- or overshield spacecrafts. The extrapolations from our knowledge of ionizing radiation effects of low linear energy transfer (LET) to the risks from high-atomic-number high-energy energetic (HZE) cosmic rays are very uncertain because the necessary experiments on the effects of such particles have not been carried out and the extrapolation from low-LET to very high-LET has great uncertainties. These uncertainties were enumerated by the Task Group, and the types of experiments needed to minimize the uncertainties were described. The report found that, because of the small amounts of available time for biological research at HZE accelerators, it would take more than a decade of effort to obtain the answers to a narrow set of key questions that would facilitate reduction in risks and identification of the types of shielding needed.

Introduction

The Space Studies Board (originally the Space Science Board) of the U.S. National Research Council provides external and independent scientific and programmatic guidance to NASA. It is an advisory, consultative, correlating, evaluating body but not an operating agency in the field of space science. The Board has responsibility for strategic planning and oversight in the basic subdisciplines of space research. One of its committees, the Committee on Space Biology and Medicine, had undertaken to carry out a comprehensive review of the status of research in the various fields of space life sciences and develop a science strategy which could guide NASA in its long-term research and mission planning. One of its objectives was to develop the foundation of knowledge and understanding that will make long-term manned space habitation and/or exploration feasible.

I joined the Committee on Space Biology and Medicine in the summer of 1995. In the spring of that year, I attended a NASA Workshop, held at Brookhaven National Laboratory, devoted to space radiation effects. The workshop was held at Brookhaven because the laboratory had, and still has, the only accelerator capable of providing high-atomic-number high-energy energetic (HZE) particles, such as iron nuclei at an energy of 1 GeV/nucleon, suitable for irradiating biological samples. I picked up lots of useful information and I was asked to make a presentation to the Committee on Space Biology and Medicine of what was known in the general area of the health effects of the radiation environment in deep space. The cosmic ray flux above the Earth's atmosphere consists of a range of particles from protons to iron nuclei (atomic number 26) with a broad energy distribution going as high as 106 MeV/nucleon. The maximum in these distributions is between 102 and 103 MeV/nucleon. It has been estimated [1] that on a 3-year mission to deep space, a mammalian cell nucleus of area 100 μm2 behind aluminum shielding of 4 g/cm2 would be traversed on the average by 400 protons, 0.6 carbon nuclei, and 0.03 ion nuclei. The latter number may seem small but it does represent 3% of cells in the body. Since the linear energy transfer (LET) of most of the energetic iron nuclei is in the neighborhood of 200 keV/μm and the energy of a primary ionization resulting from the passage of the iron nucleus through matter is somewhat less than 100 eV, 3% of the cell nuclei would have 2000 primary ionizations deposited in them during the 3-year flight. The probability of such a cell surviving unharmed is pretty small. Thus, from a cellular point of view, HZE particles could be devastating.

Shielding is necessary to protect humans on long outerspace exploration trips but the conventional type of shielding that we usually think about, lead or aluminum, is not appropriate. The dose behind a lead shielding actually increases even for densities up to 30 g/cm3 and aluminum does not reduce the dose below 50% for the same mass per unit volume [2]. The reason for the apparent anomaly is that the impact between an HZE particle and a nucleus in shielding, especially a heavy nucleus, will give rise to spallation products of high energy that also have high-LET values. The best type of shielding would contain lots of hydrogen, since its nucleus will not produce spallation byproducts. Hence, the problem of shielding and the dose behind shielding is a difficult one as is the estimation of the biological effects of this complicated radiation field. My introductory summary of the state of knowledge is shown in Fig. 1. The extrapolations from various scientific fields to arrive at the effects on humans, and the uncertainties in estimates and extrapolations, are indicated. Thus, e.g., the particles and fluence rates in deep space are reasonably well-known, so that uncertainty is low. On the other hand, when HZE particles strike shielding, they cause extensive spallation of target nuclei and hence, the doses behind shielding are not well-known, so that these uncertainties are probably in the neighborhood of 20%. There are extensive data in the field of radiobiology but most of the data have been obtained on simple systems, such as the killing of cells in vitro or in vivo and the induction of chromosomal aberrations. Almost all these data indicate that the relative biological efficiency (RBE) increases with LET to a value in the neighborhood of 2–4 at LET values around 100–200 keV/μm. Above this value, the RBE decreases [3]. However, in the one investigation of the effects of HZE particles on tumor induction in experimental animals — the Harderian gland in mice — the RBE does not seem to decrease at high values of LET but remains at an approximate plateau with values of 20–40 [4] as indicated in Table 1. Hence, extrapolation to other tumor types exposed to high-LET radiation has a large uncertainty. The estimation of the effects of high-HZE particles on humans involves several extrapolations. Most of our risk estimates for cancer induction, e.g., come from studies of the Japanese population exposed to atomic bombs [5]. These data represent the effects of a range of low-LET doses delivered in a minute instant of time, i.e., at a high dose rate. But space radiation is not a high dose rate phenomenon, but one in which doses are accumulated over months or years. Hence, one must extrapolate from acute exposures to chronic exposures over relatively long periods of time. This extrapolation is determined, to a large extent, from animal studies. Low dose rates are less efficacious in inducing biological effects than are acute exposures by a factor that ranges, depending upon the endpoint, from 2 to 10, although a nominal value of 2 is often used [5]. The final step is to extrapolate from low chronic LET exposures to high-LET chronic exposures. There are negligible data, but good guesses, for this extrapolation. Nevertheless, this extrapolation has larger uncertainties than the others. There are no very useful data. The net result is that estimates of the health effects of radiation on humans traveling through deep space have very large uncertainties, uncertainties estimated to be in the range of 4- to 15-fold [6].

It should be obvious from a glance at Fig. 1 that the major uncertainties are in the biological, not the physical, parameters. In a rational world — one that attempts to maximize the precision of predications for the level or resources committed — one would conclude that the resources should be proportional to the uncertainties. NASA's budget allocations do not follow this rule.

Section snippets

Formation of the task group

The Committee on Space Biology and Medicine felt that the need for additional data to minimize the present uncertainties in the biological effects of radiation in deep space, especially the HZE particles, would require the formation of a small Task Group to evaluate the state of knowledge and make recommendations for future research. The Task Group was made up of individuals knowledgeable about dosimetry and radiation effects on molecules, cells, tissues, animals, and humans. It received

Summary of the report

This summary is taken from the Executive Summary of “Radiation Hazards to Crews of Interplanetary Missions,” National Academy Press, 1996.

(1) The principal risks of suffering early effects as a result of exposure to radiation in space arise from solar particle events (SPEs). It is not too difficult a task to provide appropriate shielding or storm shelters to protect against exposure during SPEs, but surveillance methods to predict and detect SPEs from both sides of the sun relative to a

Acknowledgements

Brookhaven National Laboratory is operated by Brookhaven Science Associates, LLC for the U.S. Department of Energy. Support for the task group's efforts was provided by a contract between the National Academy of Sciences and the National Aeronautics and Space Administration.

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