«Francis A. Cucinotta1,*, Shaowen Hu2, Nathan A. Schwadron3, K. Kozarev3, Lawrence W. Townsend4 and Myung-Hee Y. Kim2 1,* F. Cucinotta (Corresponding ...»
Space Radiation Risk L imits
and E arth-Moon-M ars E nvironmental Models
Francis A. Cucinotta1,*, Shaowen Hu2, Nathan A. Schwadron3,
K. Kozarev3, Lawrence W. Townsend4 and Myung-Hee Y. Kim2
F. Cucinotta (Corresponding Author)
NASA Lyndon B. Johnson Space Center
2101 NASA Parkway, Houston, Texas 77058,
M.Y. Kim, and S. Hu
USRA Division of Space Life Sciences
Houston TX 77058
N. A. Schwadron, Kamen Kozarev Boston University, Department of Astronomy, 275 Commonwealth Ave, Boston, MA, 01760 firstname.lastname@example.org, email@example.com L. Townsend, University of Tennessee, 211 Pasqua Nuclear Engineering Building 1004 Estabrook Road, Knoxville, TN 37996-2300 firstname.lastname@example.org
We review NASA’s short-term and career radiation limits for astronauts and methods for their application to future exploration missions outside of low Earth orbit. Career limits are intended to restrict late occurring health effects and include a 3% risk of exposure induced death (REID) from cancer, and new limits for central nervous system and heart disease risks. Short-term dose limits are used to prevent in-flight radiation sickness or death through restriction of the doses to the blood forming organs (BFO), and to prevent clinically significant cataracts or skin damage through lens and skin dose limits, respectively. Large uncertainties exist in estimating the health risks of space radiation chiefly the understanding of the radiobiology of heavy ions, and dose-rate and dose protraction effects, and the limitations in human epidemiology data. To protect against these uncertainties NASA estimates the 95% confidence in the cancer risk projection intervals as part of astronaut flight readiness assessments and mission design. Accurate organ dose and particle spectra models are needed to ensure astronauts stay below radiation limits and to support the goal of narrowing the uncertainties in risks projections.
Methodologies for evaluation of space environments, radiation quality, and organ doses to evaluate limits are discussed, and current projections for lunar and Mars missions are described.
INTRODUC TIONMars exploration continues to be the primary goal for human exploration with missions returning to the moon or nearby Earth objects as possible intermediate steps towards this goal (NASA, 2009). As missions progress outside of low Earth orbit and away from the protection of Earth’s magnetic shielding, the radiation exposures that astronauts face change to include higher exposure to the full galactic cosmic ray (GCR) spectrum and solar particle events (SPE). The large uncertainties in projecting the risks from space radiation and the potential for unacceptable risks for long-term exposuresGCR are major scientific challenges to achieving the exploration goal (Cucinotta et al., 2001, Cucinotta and Durante, 2006, Durante and Cucinotta, 2008). Heavy ions produce distinct types of biological damage to biomolecules, cells and tissues compared to X-rays or gamma-rays complicating risks assessments based on human data. Responding to large SPE’s presents a distinct challenge that must rely on knowledge of the space environment and the development of operational procedures for effective real-time responses (NCRP, 2006;
NRC, 2008). An objective of the Earth-Moon-Mars Radiation Environment Module (EMMREM) is to provide a framework to overcome the SPE safety challenges (Schwadron et al., 2010). Our review discusses radiation safety issues and limits for the protection of astronauts that can be supported by the EMMREM framework.
NASA has recognized the importance of the uncertainties in risk projection models for radiation exposures, and uncertainty assessments are requirements for mission design optimization and operational radiation protection methods. Mission safety can only be predicted within a defined confidence level, corresponding to the statistical nature of such a calculation. Large uncertainties limit the value of a median projection or so-called point estimate. Permissible Exposure Limits (PELs) for exploration missions have been implemented at NASA based on the NCRP recommendations (NCRP 2000, 2003) for organ dose methodologies and point estimates for cancer risks, however NASA applies these with an ancillary requirement to protect against the upper-bound of the 95% confidence level of risk projection. In support of the principle of As Low As Reasonably Achievable (ALARA), mission design and operations must include cost versus benefit analyses of approaches to improve crew safety with higher confidence. Such analyses are often limited by the uncertainties in risk projections because the benefit of mitigation measures cannot be adequately stated if uncertainties are large.
Estimates of the uncertainties for cancer risk from low linear energy transfer (LET) radiation, such as X-rays and gamma-rays, have been reviewed several times in recent years, and indicate that the major uncertainty is the extrapolation of cancer effects data from high to low dose-rates (NCRP, 1997; BEIR 2006). Other projection model uncertainties include the transfer of risk across populations, and sources of error in epidemiology data including dosimetry, bias, and statistical ones. Additional uncertainties contribute to protecting against the cancer risks from the protons and heavy ions and secondary radiation in space, and in space dosimetry (Cucinotta et al., 2001). The limited understanding of heavy ion radiobiology has been estimated to be the largest contributor to the uncertainty for space radiation effects (NAS 1996, Cucinotta and Durante 2006, Durante and Cucinotta, 2008). Understanding heavy ion risk is difficult because of the absence of epidemiology data for humans exposed to heavy ions, and has been impeded by the lack of a dedicated facility to perform experiments with heavy ions on biological models until 2003 (Cucinotta and Durante, 2006). SPE’s present distinct challenges since their time of onset, size and spectral characteristics cannot be predicted reliably (NRC 2008). SPE challenges include specifying mission design criteria based on a well defined worse-case (Kim et al., 2009, 2010), and the development of real-time response models (Schwadron et al., 2010).
In this paper we review the basis for radiation limits for astronauts and the recently revised limits implemented at NASA for planning towards exploration missions returning humans to the moon or voyaging beyond. The historical bases for acceptable levels of risks and astronauts radiation limits are first described, and changes in radiation epidemiology data in recent years summarized. We then outline methodologies appropriate for the application of space radiation environment and transport models to exploration missions. Example risks and organ specific and Effective dose (E) projections for lunar and Mars GCR and SPE exposures are then described.
A C C E P T A B L E L E V E LS O F R IS K S- H IST O R I C A L P E RSP E C T I V EPermissible exposure limits (PEL) for radiation exposure of astronauts have the primary functions of preventing in-flight risks that would jeopardize mission success, and limiting chronic risks to acceptable levels based on legal, ethical or moral, and financial considerations. Early radiation effects usually are related to a significant fraction of cell loss, exceeding the threshold for impairment of function in a tissue. These are “deterministic” effects, so called because the statistical fluctuations in the number of affected cells are very small compared to the number of cells required to reach the threshold (ICRP 1991). Maintaining dose limits can ensure that no occurrence of early effects occurs. Late effects can result from changes in a very small number of cells, so that statistical fluctuations can be large and some level of risk is incurred even at low doses. Referring to them as a “stochastic” effect recognizes the predominance of statistical effects in their manifestation.
NASA has followed several distinct recommendations on radiation limits since the Apollo era until the Constellation program of today due to the evolving understanding of space radiation environments inside spacecraft and tissue, new epidemiology data, and the age and gender makeup of astronauts. Recommendations by the National Academy of Sciences (NAS) in 1967 (NAS 1967) noted that radiation protection in manned space flight is philosophically distinct from protection practices of terrestrial workers because of the high-risk nature of space missions. The 1967 NAS report did not recommend “permissible doses” for space operations, noting the possibility that such limits may place the mission in jeopardy and instead made estimates of what the likely effects would be for a given dose of radiation.
In 1970, the NAS Space Science Board made recommendations of guidelines for career doses to be used by NASA for long-term mission design and manned operations. At that time, NASA employed only male astronauts and the typical age of astronauts was 30-40 years. A “primary reference risk” was proposed equal to the natural probability of cancer over a period of 20-years following the radiation exposure (using the period from 35 to 55 years of age) and was essentially a doubling dose. The estimated doubling dose of 382 rem (3.82 Sv), which ignored a dose-rate reduction factor, was rounded to 400 rem (4 Sv). The NAS panel noted that their recommendations were not risk limits, but rather a reference risk and that higher risk could be considered for planetary missions or a lower level of risk for a possible space station (NAS 1970). Ancillary reference risks were described to consider monthly, annual, and career exposure patterns. However, the 1970 NAS recommendations were implemented by NASA as dose limits used operationally for all missions until 1989.
At the time of the 1970 NAS report the major risk from radiation was believed to be leukemia. Since that time the maturation of the data from the Japanese atomic bomb (AB) survivors has led to estimates of higher levels of cancer risk for a given dose of radiation including the observation that the risk of solid tumors following radiation exposure occurs with a higher probability than leukemia’s although with a longer latency period before expression. Along with the maturation of the AB data, re-evaluation of the dosimetry of the AB survivors, and inclusion of data from other exposure cohorts, scientific assessments of the dose response models and dose-rate dependencies have contributed to the large increase in the risk estimate over this time period (1970-2009), and these continue to be modified (BEIR 2006; UNSCEAR 2006). A newer finding is the large risk of heart disease death from radiation that appears in many exposed cohorts (Little et al., 2009), albeit data for low dose-rate exposures is inconsistent. The mortality risk for heart disease may approach that of solid cancers at least at older ages (Preston et al. 2003) and research in this area will be important in the future.
By the early 1980’s several major changes in epidemiology data had occurred leading to the need for a new approach to define dose limits for astronauts. At that time NASA requested the U.S. National Council on Radiation Protection and Measurements (NCRP) to re-evaluate dose limits to be used for low Earth orbit (LEO) operations. Considerations included the increases in estimates of radiation-induced cancer risks in the Japanese Abomb survivors, the criteria for risk limits, and the role of the evolving makeup of the astronaut population from male test pilots to a larger diverse population (~100) astronauts including mission specialists, female astronauts, and career astronauts of older ages that often participate in several missions. In 1989, the NCRP Report No. 98 recommended age and gender dependent career dose limits using as a common risk limit of a 3% increase in cancer mortality. The limiting level of 3% excess cancer fatality risk was based on several criteria including comparison to dose limits for ground radiation workers and to rates of occupational death in the less-safe industries. It was noted that astronauts face many other risks, and adding an overly large radiation risk was not justified. It also is noted that the average years of life loss from radiation induced cancer death, about 15 years for workers over age 40-y, and 20 years for workers between 20-40 y, is less than that of other occupational injuries. A comparison of radiation-induced cancer deaths to cancer fatalities in the US population is also complex because of the smaller years of life loss from cancers in the general population where most cancer deaths occur above age 70-y.
In the 1990’s, the additional follow-up and evaluation of the AB survivor data led to further increases in the estimated cancer risk for a given dose of radiation.
Recommendations from the NCRP (NCRP, 2000), while keeping the basic philosophy of risk limitation in their earlier report, advocated significantly lower limits than those recommended in 1989 (NCRP, 1989). The NCRP Report No. 132 (NCRP 2000) notes that the use of comparisons to fatalities in the less-safe industries advocated by the NCRP in 1989 was no longer viable because of the large improvements made in ground-based occupational safety; indeed the decreased rate of fatalities in the so-called less safe industries, such as mining and agriculture would suggest a limit well below the 3% fatality level estimated in 1989. The most recent reviews of the acceptable levels of radiation risk for LEO, including a 1996 NCRP symposium (NCRP 1997a) and the report on LEO dose limits from the NCRP (NCRP 2000), instead advocate that comparisons to career dose limits for ground-based workers should be used. On one-hand it is widely held that the social and scientific benefits of space flight continue to provide justification for the 3% risk level for astronauts participating in exploration missions. On the otherhand improvements in other aspects of space safety (NASA 2009) place pressure on radiation protection to also improve. The recent report from the National Research Council (NRC) (NRC 2008) reinforces the need to uphold radiation limits at NASA for safe mission design and astronaut health.