Risks

The human exploration of space inevitably involves exposure to radiation. Associated with this exposure are multiple risks, i.e., probabilities that certain aspects of an astronaut’s health or performance will be degraded. Thus, risk is not only the probability of something happening—it is the probability of something bad happening: injury or loss. The decision to embark on a mission, or to continue with a mission, depends on how bad, as well as on how probable, this risk is. Table 1 is an example of how the impact of risks tends to be quantified.

Table 1 Typical risk scoring criteria
Full size table

Radiation risks depend on the amount of radiation received, commonly described in terms of dose equivalent, colloquially abbreviated to “dose”. Figure 1 shows schematically that, for any dose, there is a probability density distribution leading to a range of possible effects. The most probable effect is proportional to the dose, but a range of outcomes is possible at any dose. For any given endpoint, the risk is a random variable associated with a probability density; risks substantially higher or lower than the most probable risk need to be taken into account. Figure 2, drawn after the work of Cucinotta et al. (2001), illustrates this concept for a particular case, using the risk of radiation-induced death (REID) as the endpoint.

Fig. 1
figure 1

Radiation risks change with dose accumulated throughout exposure, but at every dose a range of effects is possible with different probability

Full size image
Fig. 2
figure 2

Risk of exposure-induced death (REID) for a 40-year-old male on the International Space Station, calculated for an absorbed dose of 100 mGy, using a quality factor QF of 2.52. The risk is calculated as the product of an age- and gender-dependent coefficient, the tissue-dependent dose, and QF. The 95% confidence interval CI corresponds to a risk going from 0.42 to 3.2% (after Cucinotta et al. 2001)

Full size image

For every mission, a de facto decision has been made to accept a certain level of risk. Regardless of whether this decision was made deliberately or not, and whether it was made knowingly or not, the fact that the mission is executed means that those responsible have accepted the associated risk and determined that it is “safe”.

There are multiple perspectives from which to consider whether a mission is “safe”. Prior to the mission, the perception of risk raises the question, “Is it safe?” Risk assessment is the process of answering the question, “Will it be safe?” Risk management addresses the issue of how to deal with risk, “If it is not safe, how can we make it safe?” Finally, in order to assess the effectiveness of risk management and, if necessary, to improve it, the question that must be asked is “Was it safe?”

The major goal of NASA’s space radiation research is “to enable the human exploration of space within acceptable risks from space radiation”. In order to proceed with a rational determination of whether the radiation risks associated with any particular mission are “acceptable”, it is necessary to consider that humans cannot be allowed to assume the risks of space until they are deemed “acceptable”, until the question “Will it be safe?” has been answered in the affirmative. Thus, acceptability is an a priori determination, not a posteriori statement of the level of risk that has been accepted. Acceptability determines the extent to which missions are constrained in order to maintain the risk within given probability boundaries.

NASA quantifies risk in terms of REID as the risk of an exposed individual dying from a certain cancer as a function of the effective dose. This is currently the preferred measure of cancer risk, based on probability densities of the type shown in Fig. 2. 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. Limits apply only to flight crews during actual performance of a mission and there are a number of different PELs: a thirty-day limit, a one-year limit, a career limit, etc.

Career limits are based on a REID of 3%. Figure 3 provides an example of calculated current space radiation risks. For Shuttle missions and a 180-day Space Station mission, both the point estimates and the uncertainty denoted by the confidence interval seem to be below the 3% career PEL. However, for a Mars mission, the point estimate for most probable risk is barely at the maximum acceptable level, and the confidence interval spans a range significantly beyond it.

Fig. 3
figure 3

Recent estimates of human space exploration radiation risk status based on individual fatal risk. Point estimates (filled triangles) correspond to most probable risk; 95% confidence intervals are also shown (shaded rectangles around the point estimates), indicating that the uncertainty in radiation risk for a 3-year Mars mission leads to a significant probability of exceeding the maximum acceptable risk of 3%. The intensity of the shading is proportional to the magnitude of the risk (based on unpublished work by F. Cucinotta and the author)

Full size image

What is acceptable?

An excellent discussion of this topic can be found in an article by Krewski (2002). He asserts that “acceptable risk” is “the likelihood of an event whose probability of occurrence is small, whose consequences are so slight, or whose benefits (perceived or real) are so great, that individuals or groups in society are willing to take or be subjected to the risk that the event might occur”. This concept evolved from the realization that human activity cannot be conducted with absolute safety, and that risks commonly, and implicitly, found to have been accepted by common practice, could be used to provide explicit safety guidelines.

Krewski presents two ways of establishing acceptable risk. One of these, the “revealed preference” approach, is the method used by the National Council on Radiation Protection and Measurements (NCRP 2000) to make the recommendations used by NASA. It is based on a review of life shortening attributed to occupations in which humans engage regularly, and which appear to be condoned by society. The other method is based on public consultation and the NCRP has occasionally had recourse to this method as well (NCRP 1997).

Older approaches to determinations of what is acceptable risk, e.g., cost/benefit considerations, do not seem to be either ethical or practical (or both). Extreme circumstances, such as the recurrent plot of science fiction movies, that humanity is threatened with extinction by impact from an asteroid, may require heroism to save mankind from the direst astrophysical threats. Similarly, soldiers, firefighters, or even passengers on a sinking ship may be put in a situation where they will risk their lives to save those of others. In those cases, the benefits are clearly presumed to vastly exceed the cost of individual sacrifices. However, space exploration cannot be understood as taking place at the ragged edge of survival. It can only be understood as an ongoing human activity conducted as a normal occupation, regardless of the mystique with which it is associated. Cost/benefit estimates are bound to be uncertain with respect to an effort whose benefits are not always immediately clear, even if they are significant in the long run.

Acceptable to whom?

The next question that needs to be asked is the corollary, “Acceptable to whom?” The answer to that question depends on who is asked. While a potential astronaut might be willing to go on a one-way trip to any extraterrestrial destination, his or her spouse and children might have a different perspective; so might the astronaut’s mother and, almost certainly, the astronaut’s lawyer. The astronaut’s offer might stem from sincere abnegation, but could not be accepted except in some of the direst circumstances discussed above. Otherwise, such an offer would have to be rejected by a human society that does not, as a matter of principle, engage in suicide missions to further its policies.

The purpose of space exploration is the pursuit of knowledge and the development of space as an economic, political and military resource. The whole point of sending human beings on the exploration of space is for them to come home safely, delivering information and treasure. There is no place in such a program for individuals looking upon space as an opportunity for adventure and personal aggrandizement. While such “space cowboys” are a popular figment of the media imagination, it would be impossible to justify the use of public monies to satisfy the megalomaniacal urges of such individuals.

Risk, to be accepted, needs to take into consideration contractual rights and obligations of individuals, as well as societal rights and obligations. Individuals who engage in any occupation assume a certain amount of risk. These risks are not only assumed by the individual for personal profit, but also in order to meet societal rights and obligations. However, there are limits to the risks to which individuals may consent in a civilized society, and even risks considered to be well within traditionally accepted magnitudes require that individuals provide their consent to incur them.

Informed consent

Astronauts are not only employees, but also volunteers. They are not conscripted to serve on space exploration, but are participants in what will remain research as long as there are no permanent colonies in space. Thus, the paradigm that applies is the same one that applies to the risks that research subjects can be expected to assume, the notion of informed consent.

This notion is firmly rooted in the fundamental principle of representative government, that government governs by consent of the governed. It takes into account the interests of the participants, even if they do not wish to or are unable to enforce those interests. As a consequence, astronauts must be accurately informed of the risks they incur, and the benefits they, or society, expect to realize from their willingness to take such risks upon themselves. Furthermore, even if the level of risk they are required to incur is within the range of acceptability, it is understood that their employer, e.g., NASA, is required to make every reasonable effort to reduce the level of risk below any level deemed acceptable.

The following requirements summarize the salient points applicable to the United States (United States Code of Federal Regulations 2009).

  • Risks are comparable to the risks incurred by workers in a range of commonly accepted occupations.

  • Risk assessments are sufficiently accurate that the upper bound of risk estimates falls below the level that is considered acceptable.

  • Risks or discomforts (or to the embryo or fetus, if the crew member is or may become pregnant) have been adequately explained to every crewmember.

  • Best practices and good faith efforts are made to ensure that risks are minimized below the acceptable level, including operational procedures that provide warning levels adequate to allow evasive actions.Footnote 1

ALARA

These requirements, especially the last one, are consistent with the so-called ALARA (as low as reasonably achievable) principle in radiation protection. They establish an ethical basis on which space exploration risks, in general, as well as space radiation risks, in particular, may be incurred.

Human system standards 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 space flight. NASA standards are established and maintained under the direction of the Chief Health and Medical Officer. They include PEL exposure limits, fitness for duty criteria, and permissible outcome limits.

ALARA is implemented by establishment of nominal and off-nominal operating bands (intervals of relevant operational parameters within which it is appropriate or not to conduct a mission) and by defining action levels that ensure that PEL is not reached, so that PELs are never used as an operational limit. A formal appraisal of radiation hazards is made before each mission including detailed calculations of mission risk; detailed radiation exposure records are maintained for each crew member, based on in-flight dosimetry; and, formal operational procedures and rules covering any radiation exposure contingency have been developed and documented.

Risk mitigation

The most important consequence of ALARA is the requirement to support a scientific research program to reduce risk uncertainty and develop science-based risk mitigation. Improvements in the absolute magnitude of risk assessment are achieved by studies of the relationship between radiation type and risk, ranging from epidemiology to detailed modeling of the biochemical pathways and networks involved in the processing of radiation injury. Such studies lead to predictions of likely targets for intervention, and provide guidelines for the development of biological model systems and organisms, and for the proper extrapolation of results obtained with them to humans.

Knowledge of the actual radiation environment during a mission requires measurements of the space radiation environment, inside spacecraft, space suits, and habitats, and the accurate calculation of radiation dose to internal organs. The information that needs to be included comprises identification of the types of particles, their energies, direction, dose, and quantities related to the structure of their tracks. In addition, biomarkers of risk, such as certain types of chromosome aberrations, can provide useful comparisons and complement physical dosimetry.

The most common techniques for risk reduction involve avoidance and shielding. Avoidance is accomplished by mission scheduling, orbit selection, forecasting and warning. Successful avoidance depends on radiation measurements of the type described above. The usefulness of shielding depends on knowledge of how the radiation environment is modified by spacesuits, spacecraft materials, and tissue; it is on the basis of this information that risk is calculated. However, the amount and disposition of shielding is severely constrained by spacecraft mass limits and habitat resources.

Individuals may not be subjected to discrimination on the basis of genetics during selection and screening. However, there are individual variations in radiosensitivity that has a genetic basis and knowledge of their effect on radiation risk must be provided to inform individuals when requesting their consent to assume the risks of exposure to space radiation. Limits on mission duration and on the number of missions permitted may be different for radiosensitive individuals in order to conform to risk-based career radiation limits.

Prevention and intervention are still dependent upon discoveries in biology. However, the last few decades have witnessed a string of breakthrough understandings including fundamental aspects of DNA repair and misrepair, apoptosis, cell cycle control and radiation damage, cytokines and signal transduction. This history leads to optimism that pre- and post-exposure radioprotection may be achievable in the near future. At the very least, the improved knowledge will lead to early diagnostic tools which, by themselves, provide significantly improved success rates for treatment of diseases such as cancer.

Summary

Risk is not measured directly; only properties of radiation can be measured. However, the probability distribution of observing all significant health effects arising out of exposure to radiation in space, a stochastic variable, can be calculated in order to provide a quantitative risk prediction. This prediction is not complete without the determination of a margin of safety. A substantial assurance that risk limits will not be exceeded, as required by ALARA, requires accurate risk predictions (a narrow distribution reflecting adequate scientific information). Operational measures and radiation shielding are currently the main means of reducing radiation risk; improved biological markers have the potential to enable individual risk prediction; discovery of means of biological prevention and intervention may lead to significantly more powerful methods to overcome the biological consequences of exposure to radiation.

Societal rights and obligations rightfully require individuals employed in all occupations to assume a certain level of risk; however, the acceptability of risk depends on the informed consent of these individuals, and risks must be kept as low as possible within radiation protection, this is the ALARA principle, but similar margin-of-safety requirements apply in every field. It is only within the limits established by ethical concerns that the goal to assure that we can safely live and work in the space radiation environment, anywhere, any time, can be achieved.