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1 Introduction HAZARDS OF RADIATION Space is a harsh environment. Nevertheless, engineering technology is capable of protecting astronauts against vacuum, extreme thermal conditions, and micrometeoroid environments. Protection from radiation, however, is much less straightforward. The radiation environment in space can be very dynamic. While the general climate of galactic cosmic radiaÂ tion (GCR) varies fairly predictably on an 11-year cycle, solar particle events (SPEs) are unpredictable, both in timing and character. Whereas the radiation hazard posed by episodic SPEs can be managed by providing sufficient shielding, galactic cosmic rays pose a radiation hazard that is distinctly different: (1) galactic cosmic rays are always present, and (2) their energy spectra extend to very high energies with sufficient intensity that the hazard cannot be eliminated by shielding. Moreover, both SPEs and GCR contain not only protons but also heavier nuclei (also known as HZE particles, for âhigh Z [atomic number] and energyâ). Not enough is currently known about the biological effects of HZE particles. Risks cannot be measured directly; they are calculated from measured radia- tion properties and computer model predictions. Due to all of the unknowns listed above, these risk calculations carry large uncertainties that make it difficult to set requirements and to evaluate potential mitigation efforts. In turn, it is difficult to determine whether levels of risk occurring on lunar outposts and Mars missions will remain within acceptable bounds. The health risks to be considered are of two kinds: risks to mission success and risks to health following a successful mission. The success of a mission is jeopardized whenever a crew member is unable to perform his or her functions properly, if at all. In such cases, one or more of the mission objectives may be compromised; in e Â xtreme cases, the mission may be lost. In terms of radiation, the mission could be compromised by these short-term consequences or âacute effects,â which may include headaches, dizziness, nausea, fatigue, and illness ranging from mild to fatal. In addition, mission objectives could be missed because measures to avoid excess radiation exposure might restrict crew activity. Risks incurred during a mission may also extend beyond its successful completion. Severe bone loss, muscle loss, or disorientation may last for a long time after astronauts return to Earth. Radiation risks are of even greater concern; these risksâin particular, the increased risk of fatal cancerâlast for the entire life of the crew member. Astronauts may also face other dangers, including cataracts, skin damage, central nervous system damage, and impaired immune systems. Although these effects are not immediate enough to be classified as acute, they have the potential to impact very long missions or an astronautâs future missions.
MANAGING SPACE RADIATION RISK IN THE NEW ERA OF SPACE EXPLORATION Under the Vision for Space Exploration, the National Aeronautics and Space Administration (NASA) is currently planning to return humans to the Moon by 2020 and then to continue on to Mars around 2035. Under NASAâs currently planned architecture, early lunar sortie missions will last about 1 week, and with the buildup of infrastructure for lunar base operations missions may eventually lengthen to 6 months. Radiation protection must become a matter of constant vigilance if explorers are ever to be replaced by settlers. Research may hold many of the answers to these problems. In some cases, there is existing knowledge that simply needs to be transitioned from research grade to a viable product. In other cases, it may take decades of attenÂtion before answering a fundamental question or developing an enabling capability. This report identifies some of these key research topics and offers recommendations on how to manage them in order to generate the greatest amount of progress in ameliorating the radiation hazards faced by the Vision for Space Exploration. STUDY PROCESS At the request of NASAâs Exploration Systems Mission Directorate, the Aeronautics and Space Engineering Board of the National Research Council formed a committee to evaluate the radiation shielding requirements for lunar and martian missions, and to recommend a strategic plan for developing the necessary radiation mitigation capabilities to enable the planned lunar architecture. The Committee on the Evaluation of Radiation Shielding for Space Exploration was tasked to review current knowledge of the space radiation environment, assess the under- standing of risks associated with lunar exploration activities, review shielding approaches and capabilities, and recommend a strategy, including technology investments, for reducing these risks. These strategies were expected to address the radiation exposure limits specified by NASA and to be consistent with NASAâs current timelines for Constellation Program development. The committee was also to consider the likely radiation mitigation needs of future Mars missions and to place emphasis on research and development alternatives that would enhance NASAâs ability eventually to meet those future needs. The committeeâs full statement of task appears in Appendix A. The committee held a 2-day meeting in Washington, D.C., during December 2006. At this meeting, it received briefings from NASA on the mission architecture and plans for human exploration of the Moon and Mars, as well as briefings on the current knowledge of the radiation environment, health risks from radiation, and current and projected shielding approaches. The committeeâs second meeting took place in Houston, Texas, during February 2007. Representatives from NASA and Lockheed Martin (the Orion contractor) briefed the committee on current plans for the vehicles, habitats, and space suits involved in a lunar mission. The committee also received briefings on space weather monitoring and materials research. At its third meeting, held in Washington, D.C., in May 2007, the committee completed its information gather- ing, with presentations on NASAâs past and current biological and materials research programs, shielding research at other institutions, operational decision making in the face of radiation events, and possible architectures for a mission to Mars. A final, fourth meeting was held in Washington, D.C., in June 2007; at that meeting the committee finalized the findings and recommendations contained in this report. See Appendix C for committee meeting agendas and a full list of speakers. ORGANIZATION OF THIS REPORT The first chapter of this report outlines some background information on the Vision for Space Exploration, NASAâs work in radiation protection, and permissible radiation exposure limits. Chapter 2 reviews the current knowledge of the radiation environments likely to be experienced by astronauts, as well as the knowledge gaps. Chapter 3 reviews the effects of radiation on biological systems, electronics, and missions. Chapter 4 discusses National Aeronautics and Space Administration, The Vision for Space Exploration, NP-2004-01-334-HQ, NASA, Washington, D.C., â 2004. See below for a description of the components of the Exploration mission architectureï£§Orion being the Crew Exploration Vehicle. â
INTRODUCTION NASAâs current mission architecture and current protection plans. Finally, Chapter 5 outlines the committeeâs risk reduction strategy, including areas where research can have a high impact. Chapter 6 lists all of the reportâs findings and recommendations. OVERVIEW OF MISSION ARCHITECTURE NASAâs programs to implement President Bushâs Vision for Space Exploration are collectively called the Constellation Systems. âMission architectureâ means the overall structure, the components, and the interrelation- ships of a mission; it includes a broad range of projects, programs, concepts, and issues. The Constellation Systems mission architecture includes the complete ensemble of launch vehicles, flight vehicles, ground support, support services, and lunar and planetary surface systems. The habitable architecture consists of the flight vehicles and some of the surface facilities that support the astronauts. The first vehicle to be developed is the Orion Crew Exploration Vehicle (often called the CEV; this report simply uses âOrionâ), for which NASA awarded a contract to Lockheed Martin in August 2006. The Orion Block 1 nominally carries four crew members for up to 14 days in low Earth orbit (LEO), with a growth capacity to six crew members. The Orion Block 2 will carry four crew members to lunar orbit and return them to Earth. The Orion cargo variant will also be pressurized but will operate in automated mode to deliver pressurized cargo to the International Space Station. This report does not consider the cargo variant. The lunar crew in the Orion will rendezvous in LEO with the Lunar Lander. The combined vehicle of Orion, the Lunar Lander, and the Earth Departure Stage (EDS) will inject on a cislunar trajectory. The Lunar Lander per- forms the lunar orbit insertion burn. In early sortie missions, the crew then transfer to the Lunar Lander to land on the Moon. The crew members perform their surface mission while living in the Lunar Lander descent and ascent stages. The descent stage may include a small living module, as well as an airlock that helps to conserve atmosphere and exclude dust from the crew cabin. When the surface mission is complete, the crew secures the descent stage and launches in the ascent stage to rendezvous with Orion in lunar orbit. In later, outpost missions the crew will transfer to a surface facility composed of habitat and laboratory modules and surface transportation vehicles. NASAâS SPACE RADIATION PROGRAM A research and development program is currently underway at NASA to investigate, evaluate, and mitigate the effects of space radiation on astronauts. This program supports several projects, as well as directed research by individual investigators. A vigorous interagency cooperation effort has led to joint support of numerous research and development projects by the Department of Energy (DOE), the National Cancer Institute, and the Armed Forces Radiobiology Research Institute. Through its radiation program, NASA also participates in multi-agency efforts in order to benefit from applications of radiological defense efforts as they evolve. NASA determines its research portfolio in consultation with the science research community, including the National Council on Radiation Protection and Measurements (NCRP) and the Space Studies Board and other orga- nizations within the National Research Council. Within NASA priorities, the concurrent development of Âradiation- transport computer codes and of experimental investigations has provided a large, systematic database of shielding properties of materials, tests of shielding calculations, and a toolbox of computer applications used for spacecraft, space suit, and habitat design. Radiobiological research provides a statistically significant determination of the time that individuals may be exposed to radiation in space without exceeding career and mission limits, or so-called âsafe daysâ in space. In the longer term, the scientific consensus is that radiobiological research is the only way to reduce the uncertainties in risk estimation that limit the number of safe days and, eventually, to mitigate risk. Prior to the Exploration initiative, the Space Radiation program was housed within NASAâs Human Systems Research Technology theme. In 2006, the theme consisted of three programs: Human Health and Performance, Human Systems Integration, and Life Support and Habitation, with a total annual budget of $807 million. Between 2006 and 2008, this program was reduced and condensed into one program, Human Research, with a total annual budget of $183 million, of which $36.2 million is for the Space Radiation program (Pawelczyk, 2007). Radiation health research has been reduced, and shielding research has been nearly eliminated. These changes are reflected
10 MANAGING SPACE RADIATION RISK IN THE NEW ERA OF SPACE EXPLORATION TABLE 1-1â History of the NASA Space Radiation Laboratory Year Event 1958 NASA congressional submission calls for a future facility to simulate space radiation on Earth to study the biology and physics of interactions. 1970 Apollo astronautâs retinal light flash observations cause concern for low fluences of GCRï£§high charge and energy ions. 1973 The National Research Council (NRC, 1973) issues a report concluding, âWe recommend most strongly that at least one accelerator be modified to be capable of accelerating particles of atomic numbers up to  iron, and preferably higher, with energies of at least 500 MeV/nucleon. Such accelerators . . . must have closely associated facilities to provide support for advanced biological and medical research.â 1979-1993 NASA âpiggybacksâ on the Department of Energyâs BEVALAC program at the Lawrence Berkeley National Laboratory. 1991 The Synthesis Group, chaired by T.P. Stafford (1991), concluded that a âmultidisciplinary radiation issues research program . . . will have a major influence on spacecraft design, habitats and mission planning.â 1995-2002 Limited use of the Brookhaven Alternating Gradient Synchrotron is allowed for radiobiology and physics experiments (nine runs for total of ~1,300 hrs). 1996 The National Research Council recommends that NASA fund a Booster Application Facility to achieve radiation safety goals for the International Space Station and exploration. 1997 Headquarters and Johnson Space Centerâs Space Radiation Program attend the conceptual design review at the Brookhaven National Laboratory for NSRL. 1998 Space Radiation Program provides first funding for NSRL construction. 2003 (June 30) NSRL construction is complete. 2003 (July 7) The first commissioning experiments take place at NSRL. in the funding of external research: in 2004, NASA selected 28 research projects in biology and materials, at a total of $28 million; in 2007, 16 radiation biology projects were selected at a total of $15 million. The Space Radiation program is led by the NASA Johnson Space Center. A small amount of shielding technology develop- ment and demonstration work supports Exploration from within the NASA Langley Research Centerâs Structures, Materials, and Mechanisms project. Ground-based simulation of space radiation is an essential, mission-critical element of this program. For that purpose, the NASA Space Radiation Laboratory (NSRL) at the Brookhaven National Laboratory (BNL) in Upton, New York, is used to obtain beams of all particles and energies in the space radiation spectrum. See Table 1-1 for a history of NSRL. In addition, simulations of the complex SPE and GCR spectrum are being developed for the validation of biological risk predictions and materials testing. The $34 million NSRL facility has been operational since 2003. It was built on schedule and under budget; cooperation with the DOE, the operator of BNL, has enabled the leveraging of resources from this $1 billion BNL facility. The chief health and medical officer of NASA oversees the application and technology transfer of radiation research results to NASA operations and their incorporation into flight rules. Operational and engineering organi- zations within NASA also collaborate closely with the solar science and space weather community on matters of space radiation warning, monitoring, and environmental measurements. OVERVIEW OF RADIATION PROTECTION NASA develops human system standards to ensure an appropriate environment for human habitation, qualified human participants, a necessary level of medical care, and risk mitigation strategies against the deleterious effects of spaceflight. The standards include exposure limits, fitness for duty criteria, permissible outcome limits, and nominal and off-nominal operating bands (intervals of relevant operational parameters within which it is appropriate or not to conduct a mission). The goals of these standards are to ensure mission completion, to limit morbidity, and to reduce the risk of mortality during Exploration-class missions. These standards are established and maintained See http://www.nasa.gov/centers/marshall/news/news/releases/2003/03-120.html; http://www.nasa.gov/home/hqnews/2006/sep/ â HQ_06313_radiation_biology.html.
INTRODUCTION 11 under the direction of NASAâs chief health and medical officer. Research findings, lessons learned from previous space missions and in analogue environments (for example, bed-rest studies), current standards of medical practice, risk management data, and expert recommendations are considered in the process of setting standards. Figure 1-1 shows the historical radiation doses incurred by astronauts, compared with various exposure limits and typical doses related to some terrestrial activities. Figure 1-2 shows astronaut exposure rate by mission year and illustrates the variability of average daily dose rate experience through 2005. This previous human spaceflight experience Other examples: 100 Chest x-ray .005 to .02 cSv Annual limit for astronauts Cross-country round trip .005 cSv in low Earth orbit High-altitude dose rate .002 cSv/hour Effective Dose, cSv/year Effective Dose, cSv in a year Dose rates on orbit .01 to .1 cSv/day 10 Recommended limit for âradiation workersâ Astronaut mission dose, measured 1 Recommended annual limit for U.S. public Typical U.S. public annual exposure 0.1 Badge Dose, cGy Effective Dose, cSv 10 Mission Dose, cGy or cSv 1 0.1 0.01 FIGURE 1-1â Radiation doses in context. Astronaut radiation exposure limits and history are compared in this figure with OccupaÂtional Safety and Health Administration limits for U.S. radiation workers and for the U.S. public, and with typical public annual exposure, including exposure from medical sources and natural background radiation. Insert on left shows e Â xposures from typical activities. Insert on lower right is a more detailed breakout of U.S. astronaut mission doses through R01155, Fig 1-1, prints in color 2002, organized sequentially by astronautâs order of flight. NOTE: cGy, centigray; cSv, centisievert. SOURCE: Modified from Cucinotta et al., 2002.
12 MANAGING SPACE RADIATION RISK IN THE NEW ERA OF SPACE EXPLORATION FIGURE 1-2â U.S. astronaut exposure rate history, by mission year, 1962 through 2005. The scatter of the exposure rate is the result of the fact that exposure rate varies as a function of altitude, orbit inclination, time in solar cycle, variations in solar activity, the vehicle shielding, orientation of the vehicle, and location within the vehicle. NOTE: mSv, millisievert. SOURCE: Cucinotta, 2007. R01155, Figure 1-2, prints in color, fixed image, not changeable is useful, but it is only partially applicable to the Vision for Space Exploration. Apollo missions were only a few days long, and so the radiation protection involved minimizing travel through the Van Allen radiation belts that encircle Earth in order to avoid solar particle events. There have been longer missions on the space shuttle and the International Space Station, but these took place within the protections of Earthâs magnetosphere. Predicting the risks associated with exposure of biological tissue to a given quantity of radiation is a compli- cated process. Most of the available data come from studies of Japanese atomic blast survivors, animals, and cell cultures. These data Âprimarily concern high-dose-rate exposures to gamma rays. Since the data available are not ideally suited for determining the effects on a modern American of a long, low-dose-rate exposure to GCR, there is significant Âuncertainty in these estimations. Risk of exposure induced death (REID) is the currently preferred measure of risk, replacing the mathematically ill-defined âexcess relative risk.â REID quantifies the risk of an exposed individual dying from a certain cancer as a function of the effective dose. For reference, the American Cancer Society reported that 23 percent of all deaths in 2004 were due to cancer (ACS, 2007). Permissible exposure limits (PELs) follow recommendations of NCRP Report 132 (NCRP, 2000) with modifications, including new epidemiology and uncertainty assessments, estimates of noncancer risk, and acute effects. There are a number of different PELsâa 30-day limit, a 1-year limit, a career limit, and others. Career limits are based on a REID of 3 percent. This value is based on a comparison with other flight risks, a comparison with the risks faced by workers in less-safe industries (such as mining), and terrestrial radiation limits. The effects of exposures corresponding to this REID vary with age and gender. Furthermore, given the uncertainties in the calculation of REIDâboth biological and physicalâthis measure of the risk of exposure is expressed in the form of a probability density function. The PEL is therefore not set to the most probable value of the dose estimate corresponding to a REID of 3 percent. Instead, the PEL is set to the lower 95 percent confidence level of dose estimate, as shown in Figure 1-3. In that way, mission designers are 95 percent certain that REID will be less than 3 percent if the exposure is kept below the PEL.
INTRODUCTION 13 100 90 Unacceptable 80 Estimated Career 70 Effective dose (cSv) Dose Limit 60 50 Remedial actions taken 40 30 95% CI 20 10 Nominal 0 30 35 40 45 50 Age (years) FIGURE 1-3â Permissible exposure limits. Example dose limits for a female astronaut are shown as a function of age. For a s Â elected astronaut (here, a 45-year-old), the expected mission dose falls in the white area. Radiation exposure is measured through- out the missionâif her dose rises to a level represented by the lightly shaded area, mission control decides on various actions R01155, Figure 1-3 that will reduce her dose and ensure that it does not reach the career limit. NOTE: cSv, centisievert; CI, confidence interval. NASA uses the As Low As Reasonably Achievable (ALARA) principle, a legal requirement applicable on Earth as well as in space, intended to maximize public safety. ALARA is particularly important in space, because the uncertainties associated with space radiation are much higher than those associated with terrestrial radiation. AccordÂing to ALARA, the PEL should not be considered a âtolerance valueâ to be used as a design point. Instead, ALARA challenges engineers to include as much radiation protection as possible within the available resources. Setting the PEL to correspond to the lower 95 percent confidence level, as described above, is also in the spirit of the ALARA principle. The ALARA requirement is implemented at several levels. Launch dates, length of mission, and trajectory may be modified during mission design to take into account solar activity and cosmic ray exposure. Spacecraft may be designed to incorporate different materials and different interior material distributions, including âstorm sheltersâ and extra shielding of crew sleeping quarters, to optimize shielding against solar particle events. Pre- dicted shielding properties of a spacecraft may be validated and their accuracy improved by the partial exposure of components to simulated space radiation on the ground. Extravehicular activities (EVAs) may be scheduled for periods of reduced solar activity and no transit through radiation belts; radiation measurements are used to verify radiation exposure and provide warning for retreat to storm shelters. Once a mission has been designed, it will include an estimated exposure. Crew members are selected so that the sum of their previous radiation exposure and their predicted radiation exposure are below the PELs. If excessive exposures are predicted or do occur, the
14 MANAGING SPACE RADIATION RISK IN THE NEW ERA OF SPACE EXPLORATION mission may be altered: an EVA may be canceled or the astronauts brought home early. The development of improved early diagnostic methods and radiation effect modifiers, such as radioprotectant pharmaceuticals, will improve the ability to react to unexpected radiation doses. Following a successful mission, medical surveillance and control of further radiation exposures are required. The thickness of the lightly shaded area in Figure 1-3 is proportional to the mean. Therefore, increasing the career dose limit for an astronaut leads to a higher value of the mean permissible exposure level, but it also widens the distribution, because all the known uncertainties in the calculation of radiation risk are relative errors of the multiplicative factors of a product risk. Thus, accepting a higher career dose limit leads to larger values of uncertainty in terms of radiation exposure and does not necessarily increase the amount of ânominalâ area (shown as white in Figure 1-3)ï£§it only widens the shaded region. As a consequence, the most significant gains in the cost-benefit ratio of ALARA application are to be found in the reduction of the uncertainty in the risk calculation, that is, in reducing the width of the 95 percent confidence interval. This report uses a number of terms common to the field of radiation protection. The most frequently used are defined in Box 1-1; this report also contains a glossary in Appendix D. BOX 1-1 Common Radiation Terms and Their Definitions absorbed dose (D): average amount of energy imparted by ionizing particles to a unit mass of irradiated material in a volume sufficiently small to disregard variations in the radiation field but sufficiently large to average over statistical fluctuations in energy deposition, and where energy imparted is the difference between energy entering the volume andÂ energy leaving the volume. The same dose has different consequences depending on the type of radiation delivered. Unit: gray (Gy), equivalent to 1 J/kg. As a default, this report uses cGy, because 1 cGy is equal to 1 rad (a deprecated unit still used occasionally). ALARA (As Low As Reasonably Achievable): a safety principle, as well as a regulatory requirement, that emphasizes keeping doses of and exposure to radiation as low as possible using reasonable methods, and not treating dose limits as âtolerance valuesâ; defined at NASA as limiting radiation exposure to a level that will result in an estimated risk below the limit of the 95 percent confidence level. biological end point: effect or response being assessed, e.g., cancer, cataracts. cross section (Ï): measure of the probability per unit particle fluence of observing a given end point. Unit: cm2. deterministic process: process whereby a given event will occur whenever its dose threshold is exceeded. dose equivalent (H): estimate of radiation risk that accounts for differences in the biological effectiveness of different types of charged particles that produce the absorbed dose. H = Q Ã D, where Q is a quality factor based on the type of radiation (Q = 1 for x-rays). NASA uses Q as specified in ICRP Publication 60 (ICRP, 1991). Unit: sievert (Sv), equivalent to 1 J/kg. As a default, this report uses cSv because 1 cSv is equal to 1 rem (a deprecated unit still used occasionally). effective dose (E): estimate of radiation risk given in ICRP Publication 60 (ICRP, 1991). It sums the individual effects of all types of radiation present over all of the individual types of tissue in the body. Unit: cSv. E = â w T â w RD T,R T R where wR is a weighting factor for the type of radiation (NASA uses wR as specified in ICRP Publication 60; ICRP, 1991); wT is a weighting factor for the type of tissue; and DR,T is the average dose from radiation R in tissue T.
INTRODUCTION 15 OVERVIEW OF GUIDANCE ON RADIATION LIMITS PROVIDED TO NASA Historical Guidance A detailed history of space radiation exposure limits can be found in Townsend and Fry (2002). The earliest standards, set in 1970 by a National Research Council (NRC) panel, used the concept of reference risk, which âshould correspond to an added probability of radiation-induced neoplasia over a period of about 20 years that is equal to the natural probability for the specific population under consideration.â The career limit was set to 400Â cSv, with additional limits for bone marrow, skin, ocular lens, and testes (NRC, 1970). Throughout the 1970s and 1980s, as more data became available, risk estimates for cancer induction per unit dose of radiation began to rise. The National Council on Radiation Protection and Measurements (NCRP) recom- mended new limits in 1989 in NCRP Report No. 98 (NCRP, 1989), replacing the previous reference risk with 3Â percent excess risk of cancer, a quantity comparable to the risks faced by terrestrial radiation workers and workers in other less-safe occupations. In addition, the NCRP limits varied with age and gender. The 400 cSv risk (which had been set for a 30- to 35-year-old male) dropped to 250 cSv. Dose rates for various organs were also set. fluence, or particle fluence (F): number of particles incident on a small sphere centered at a given point in space, divided by the cross-sectional area of that sphere. Mathematically, it is given as dN/da, where N is the number of particles and a is the cross-sectional area. Unit: m2. fluence rate (dF/dt): change in fluence over a given small time interval, or the time derivative of the fluence. Unit: m2/s. flux (Î¦): term used historically by the nuclear community for fluence rate and also used for particle flux density, but deprecated by the ICRU convention to eliminate confusion between the terms âparticle flux densityâ and âradiant flux.â The term âfluxâ is used in this report because it is common within the space weather and radiation protection communities. See fluence rate. linear energy transfer (LET): measure of the average local energy deposition per unit length of distance traveled in the material. Unit: keV/Î¼m. permissible exposure limit (PEL): maximum amount of radiation to which an astronaut may be exposed. For terrestrial workers, PELs are legal limits, defined by OSHA. NASA PELs are set by the chief health and medical officer. relative biological effectiveness (RBE): measure of the effectiveness of a specific type of radiation or particle in producing a specific biological outcome relative to the outcome with the same dose of gamma rays. RBE = Dg /Drad of interest. risk of exposure induced death (REID): measure of risk used by NASA as a standard for radiation protection; reflects a calculation of the probability of death due to exposure to radiation in space. stochastic process: process whereby the likelihood of the occurrence of a given event can be described by a probability distribution.
16 MANAGING SPACE RADIATION RISK IN THE NEW ERA OF SPACE EXPLORATION Even as NCRP Report No. 98 (NCRP, 1989) was being released, it was out of dateï£§the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 1988) and the BEIR V committee (the Committee on the Biological Effects of Ionizing Radiation) (NRC, 1990) issued reports that included new esti- mates of risks of stochastic effects of radiation: estimates of excess cancer mortality rose from 1.5 percent per sievert to 5Â percent per sievert. These revised risk estimates were used by the NCRP (1993) to set new limits for occupationally exposed individuals in terrestrial occupations; new limits for astronauts followed in 2000 (NCRP, 2000). In addition to the career limits, other short-term limits were recommended in order to avoid deterministic effects in critical organs, such as the skin, ocular lens, and bone marrow. Space Radiation Research Recommendations of 2006 Because of the unique nature of the space radiation environment beyond LEO, for which no epidemiological data on cancer incidence or mortality exist, guidance on radiation exposures to limit excess cancer mortality to some desired level, such as 3 percent, could not be provided. At the request of NASA, the NCRP published NCRP Report No. 153, Information Needed to Make Radiation Protection Recommendations for Space Missions Beyond Low- Earth Orbit, in November 2006 (NCRP, 2006). This lengthy report, over 400 pages, provided recommendations on research needs to enable future radiation protection guidance for missions beyond LEO to be developed. Current Radiation Limits and Guidance The current radiation exposure limits for NASA, listed in NASA Space Flight Human Standard Volume 1: Crew Health (NASA, 2007, p. 65), are as follows: Career Cancer Risk Limits Career exposure to radiation is limited to not exceed 3 percent REID (Risk of Exposure Induced Death) for fatal cancer. NASA assures that this risk limit is not exceeded at a 95 percent confidence level using a statistical assess- ment of the uncertainties in the risk projection calculations to limit the cumulative effective dose (in units of Sievert) received by an astronaut throughout his or her career. Dose Limits for Non-Cancer Effects Short-term dose limits are imposed to prevent clinically significant non-cancer health effects including performance degradation, sickness, or death in-flight. For risks that occur above a threshold dose, a probability of <10 â3 is a practi- cal limit if more accurate methods than dose limit values are to be implemented. Lifetime limits for cataracts, heart disease, and damage to the central nervous system are imposed to limit or prevent risks of degenerative tissue diseases (e.g., stroke, coronary heart disease, striatum aging, etc.). Career limits for the heart are intended to limit the REID for heart disease to be below approximately 3 to 5 percent, and are expected to be largely age and sex independent. Average lifeloss from gamma-ray-induced heart disease death is approximately 9 years. Example age-dependent career effective dose limits for a 1-year mission and calculated days in deep space to stay below permissible exposure limits are shown in Table 1-2. Dose limits for noncancer effects are listed in Table 1â3. For comparison, Table 1-4 shows the estimated REID for some sample missions.
INTRODUCTION 17 TABLE 1-2â Example Age-Dependent Career Effective Dose (E) Limits for a 1-Year Mission and Calculated Days in Deep Space to Stay Below 3 Percent REID with 95 Percent Confidence Â 3 Percent REID Males Females Age E (cSv) Days E (cSv) Days 30 â 62 142 â 47 112 35 â 72 166 â 55 132 40 â 80 186 â 62 150 45 â 95 224 â 75 182 50 115 273 â 92 224 55 147 340 112 282 NOTE: REID, risk of exposure induced death; cSv, centisievert. SOURCE: NASA, 2007. TABLE 1-3 Dose Limits for Short-Term or Career Noncancer Effects Organ 30-day limit 1-year limit Career Lens a 100 cGy-Eq 200 cGy-Eq 400 cGy-Eq Skin 150 300 400 Blood forming organs 25 50 Not Applicable Heartb 25 50 100 Central nervous system (CNS)c 50 100 150 CNSc (Z â¥ 10) â 10 cGy 25 cGy NOTE: Relative biological effectiveness for specific risks is distinct as described in this table. a Lens limits are intended to prevent early (<5 yr) severe cataracts (e.g., from a solar particle event). An additional cataract risk exists at lower doses from cosmic rays for subclinical cataracts, which may progress to severe types after long latency (>5 yr) and are not preventable by existing mitigation measures; however, they are deemed an acceptable risk to the program. b Heart doses calculated as average over heart muscle and adjacent arteries. c CNS limits should be calculated at the hippocampus. SOURCE: NASA, 2007. TABLE 1-4â Estimated REID with 95 Percent Confidence Interval (CI) for Sample Exploration Missions Solar Minimum Solar Maximum REID (%) REID (%) Point CI Point CI Sample Mission Estimate [lower, upper] Estimate [lower, upper] Long lunar mission, 6 days in deep space, 84 days on surface Â Â Â Â â Male 0.28 [0.09, .95] 0.36 [0.12, 1.2] â Female 0.34 [0.11, 1.2] 0.43 [0.13, 1.4] Mars swing-by, 600 days in deep space â Male 3.2 [1.0, 10.4] 2.0 [0.60, 6.8] â Female 3.9 [1.2, 12.7] 2.5 [0.76, 8.3] Mars surface mission, 400 days in deep space, 600 days on surface â Male 3.4 [1.1, 10.8] 2.4 [0.76, 7.8] â Female 4.1 [1.3, 13.3] 2.9 [0.89, 9.5] NOTE: Assumes 20 g/cm2 aluminum shielding and 40-year-old astronauts. Solar maximum includes an August 1972 event in addition to GCR during deep-space portion. REID, risk of exposure induced death. SOURCE: Cucinotta et al., 2005.
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