In considering the ethics issues that will emerge when making decisions about sending humans into harm’s way on long duration and exploration spaceflights, the committee decided to examine some of the health risks that illustrate key ethical challenges and tensions in risk and decision making. It is important to note that the stressors and health risks may vary from mission to mission, depending on the remoteness of the destination and many other factors. For example, missions to locations that will require months to years to return to Earth can have fundamentally different stressors than destinations that are closer but may involve being in space for long periods of time. Although the phrase “long duration and exploration spaceflights” is used throughout the report, the committee acknowledges that the health risks may vary widely between missions.
These examples are meant to illustrate the diversity of risks and, therefore, highlight some of the ethics quandaries arising in decisions regarding uncertain risks, unknowns about the extent of individual variations, and long-term health implications for astronauts. In addition, the examples also demonstrate the range of risk management strategies that are being employed by or considered by the National Aeronautics and Space Administration (NASA), international partners, and commercial companies to prevent, mitigate, or treat these risks to human health. The following sections are brief overviews of the complex issues relevant to:
- the risk of loss of life due to mission operations,
- vision impairment risks,
- behavioral health and performance risks,
- bone demineralization, and
- radiation exposure.
MISSION OPERATIONS RISKS
Although there have been many successful space missions over the past 50 years, the substantial risks undertaken by astronauts are most evident in documented “near misses” and tragic losses of life. During the Apollo 11 mission, Neil Armstrong piloted a lunar module to the Moon’s surface, landing with less than 30 seconds worth of fuel remaining (Garber and Launius, 2005). Apollo 12 was struck by lightning at approximately 36 and 52 seconds after launch, momentarily shutting down the electrical power (Molloy and Petrone, 2013). During the Apollo 13 mission, an oxygen tank exploded while en route to the Moon, and ground crews and astronauts had to improvise to safely return the crew to Earth (Garber and Launius, 2005). The 1997 collision of a resupply vehicle, as well as a fire, jeopardized the lives of crew members aboard the Mir Space Station (NASA, 2014b). In 1967, a cabin fire in an Apollo capsule during a launch pad test in Cape Canaveral, Florida, killed three astronauts.1 Over the course of the NASA space program, 24 crew members have lost their lives in the line of duty, including the 14 individuals aboard the space shuttles Challenger and Columbia, the Apollo 1 astronauts, and 5 individuals who died in training-jet crashes (NASA, 2014a). Due to the nature of the propulsion systems needed to launch, the distance from Earth, the immediacy of some inflight catastrophes and the challenges of landing, options for rescue operations are limited.
The probability of loss of crew due to a catastrophic vehicle accident is generally higher and accompanied by greater uncertainty in early experience with new crew delivery vehicles. As experience is gained through effective use of continuous risk management systems, the expected results are decreased risk and less uncertainty, although risks remain high. Thus, for example, the probabilistic risk assessment for loss of crew and vehicle on the first shuttle flight was 1/10 versus 1/90 on the 135th shut
1On January 27, 1967, the crew of Roger B. Chaffee, Virgil “Gus” Grissom, and Edward H, White, Jr., were killed in a fire that spread quickly in a pure oxygen environment; the astronauts had no opportunity to open the hatch (Garber and Launius, 2005; Williams, 2011).
tle flight (Behnken et al., 2013). The shuttle mission aggregate risk of loss of crew and vehicle was approximately 1/46 (Behnken et al., 2013).
The risks of the loss of the crew and vehicle during a spaceflight mission are calculated to allow comparisons of different approaches and platforms using probabilistic risk assessments. NASA provides detailed briefings for astronaut candidates on mission risks. Comparisons are made in this briefing to the risk of death in other high-risk occupations and to catastrophic events and combat. Astronauts who provided information to the committee emphasized the thorough and ongoing nature of communication throughout their careers regarding risks.
Vision impairment represents a newly identified health risk and exemplifies how NASA approaches a new risk in the current risk management framework. The potential for vision changes has both acute and long-term implications for the astronaut and for current and future missions.
Overview and Risk Identification
Although astronauts have reported vision changes during spaceflight for more than 40 years (Alexander et al., 2012), these were assumed to be transient and isolated. With the Mir Space Station and the International Space Station (ISS), longer tours of duty became possible. A Mir technical report from 2008 described disc edema in 8 of 16 cosmonauts on landing, with one magnetic resonance imaging (MRI) report showing signs suggestive of intracranial hypertension (as cited in Alexander et al., 2012). The spaceflight environment on Mir was noted to be similar to the ISS.
More significant and lasting visual changes have occurred in astronauts on the ISS and have been documented in published case reports (Mader et al., 2011). These changes were primarily a shift toward hyperopia (far-sightedness). Scotomas (blind spots in the visual field) were also reported. One astronaut reported that he needed to shift his head to compensate for scotoma when reading instruments during the mission (Alexander et al., 2012). In a group of astronauts examined pre- and postflight, clinicians observed flattening of the eyeball globe using ultrasound and MRI, supporting the hyperopic shift, edema of the optic nerve, and changes of the optic fundus noted on examination (Mader et al., 2011). In this discussion of the detailed findings on seven astronauts after
spaceflight, the results are presented in an “unattributable” manner and do not include dates or missions to preserve privacy and confidentiality (Mader et al., 2011). Refractive changes have persisted after return to Earth for these astronauts.
Upon identifying the vision-related issues, NASA rapidly implemented pre- and postflight ocular testing protocols and convened a Papilledema Summit in July 2009 to bring together experts in space medicine and terrestrial analogs and to suggest avenues for research (Watkins and Barr, 2010). The summit examined potential physiological causes of the vision alterations and suggested changes in preflight, inflight, and postflight testing.
Because the optic changes bore a striking similarity to those seen with elevated intracranial pressure (i.e., papilledema), this hypothesis was the first to be extensively explored (Alexander et al., 2012). Lumbar puncture has been performed on four U.S. astronauts (at single intervals ranging from 12 to 60 days after flight), and opening pressures were found to be elevated in two individuals and borderline in the other two (Alexander et al., 2012). Other hypotheses regarding causes of vision alterations include elevated carbon dioxide in the ISS atmosphere, increased sodium content of the astronaut diet, fluid shifts due to increases in resistance exercises, radiation exposure, and individual susceptibility (Alexander et al., 2012).
Since 1989, as part of postflight eye examinations, U.S. astronauts have been asked about improvements or degradations in their distant or near vision. Compilation of that information on 300 U.S. astronauts who had flown in space found that approximately 29 percent of shuttle astronauts and 60 percent of ISS astronauts noted a degradation in distant and near visual acuity, and some of these changes remained unresolved years after flight (Mader et al., 2011). With the exception of the report from Mir previously cited, data from international partners are lacking.
Risk Management: Clinical Practice Guidelines, Surveillance, and Research
NASA is monitoring vision changes and intracranial pressure, and research efforts in this area are fully under way. NASA has developed clinical practice guidelines for treatment of astronauts with postflight refractive changes, which include a classification system from class 0 (least severe) through class 4 (most severe) based on the results of imaging studies
and indicating the follow-up testing and monitoring that are required (Alexander et al., 2012).
In 2012, the NASA Human Research Program released the evidence report Risk of Spaceflight-Induced Intracranial Hypertension and Vision Alterations, which summarizes research evidence and raises questions for future study (Alexander et al., 2012). Current research efforts are exploring ground-based analogs and include studies of normal individuals in supine and head-down bed rest, hind-limb-suspended rodent models, and various pathologic situations in which elevated intracranial hypertension and papilledema occur. The evidence report details four major gaps requiring future research:
- Etiological mechanisms and contributing risk factors for ocular structural and functional changes seen inflight and postflight;
- Validated and minimally obtrusive diagnostic tools to measure and monitor changes in intracranial pressure, ocular structure, and ocular function;
- Ground-based analogs and/or models to simulate the spaceflight-associated visual impairment and increased intracranial pressures; and
- Preventive and treatment measures to mitigate changes in ocular structure and function and intracranial pressure during spaceflight (Alexander et al., 2012).
Individual Variation Issues
As with many health risks, individual factors (including, but not limited to, age and sex) may account for some of the variability in manifestation of vision and ocular alterations. The evidence report on this risk concludes with this statement, “In summary, 15 long-duration male astronauts ranging in age from 45 to 55 years have experienced confirmed inflight and postflight visual and anatomical changes” (Alexander et al., 2012, p. 87).
Health Standards and Risk Profile
A NASA health standard has not yet been developed for vision impairment and increased intracranial pressure and the extent, if any, to which these are linked is not fully known (IOM, 2014). At present, this
risk is listed as “unacceptable” on the Human Research Roadmap (see Table 2-2) for missions to the ISS for 12 months, to a near-Earth asteroid, or to Mars, and “insufficient data” for a lunar mission (NASA, 2013).
Issues for Long Duration and Exploration Spaceflights
In 2013, Mader and colleagues reported on a single astronaut who was carefully studied after two long-duration missions, documenting progressive vision changes associated with repeat spaceflight, raising concerns that changes occurring during the first flight “may have set the stage for recurrent or additional changes when the astronaut was subjected to physiological stress of repeat space flight” (Mader et al., 2013, p. 249). The prevalence and severity of the observed visual changes raised significant concerns about the ability of an affected pilot to successfully land a spacecraft. This is an active area of research with many remaining unknowns that could affect future crew selection, mission success, and astronaut health.
Ethics issues raised by this example focus on how best to address uncertainty and unknowns including determinations of long-term responsibilities for monitoring, preventing, and treating the health condition moving forward. Issues of individual susceptibility are still being determined, but this example points to the importance of diverse participation, including women, to obtain population-based information (as all examples to date of vision problems have occurred in men).
BEHAVIORAL HEALTH AND PERFORMANCE RISKS
NASA has identified three categories of behavioral health and performance risks associated with long duration and exploration spaceflight: (1) adverse behavioral conditions and psychiatric disorders; (2) performance errors due to fatigue resulting from sleep loss, circadian desynchronization, extended wakefulness, and work overload; and (3) performance decrements due to inadequate cooperation, coordination, communication, and psychosocial adaptation within a Team Gap (Schmidt et al., 2009; Slack et al., 2009; Whitmire et al., 2009).
Overview and Risk Identification
Much of the evidence regarding behavioral health risks associated with long duration spaceflight is derived from anecdotal evidence (Aldrin, 1973; Lebedev, 1988; Burrough, 1998; Linenger, 2000), archival and observational data collected during spaceflight (Kanas et al., 2007; Stuster, 2010), and observational and experimental studies conducted in analog settings, such as polar expeditions (Gunderson, 1974; Sandal et al., 1996; Palinkas and Suedfeld, 2008), submarines (Tansey et al., 1979; Sandal et al., 1999; Thomas et al., 2000), and space simulations (Gushin et al., 1996; Sandal, 2001; Basner et al., 2013). The Mars 500 study, with participants from four nations, was a 17-month isolation experiment in preparation for a mission to Mars (ESA, 2011).
While the reported incidence of behavioral health problems encountered during spaceflight has been quite low, the actual incidence may be underestimated due to a reluctance of astronauts to report them (IOM, 2001; Shepanek, 2005). Billica (2000) reported a 2.86 per person-year incidence of such problems among the 508 crew members who flew on 89 space shuttle missions between 1981 and 1989. The most common behavioral symptoms reported by crew members were anxiety and irritability. Data collected for 28.84 person-years of NASA spaceflight identified 24 cases of anxiety, for an incidence rate of 0.832 cases per person-year (Slack et al., 2009). No behavioral emergencies have been reported to date (Slack et al., 2009).
Studies in analog settings have reported much higher rates of behavioral health problems, reflecting longer periods of isolation and confinement and differences in crew member characteristics. For example, the incidence of behavioral health problems after extended stays in Antarctica was estimated in one study at 5.2 percent (Palinkas et al., 2004).
A number of factors may contribute to the risk of behavioral health and sleep problems in space, including mission duration (Slack et al., 2009), disruption of sleep and circadian rhythms (Czeisler et al., 1986; Dinges et al., 1997), physiological changes that occur in microgravity, workload, lack of social and environmental stimulation, cultural and organizational factors, family issues, and personality characteristics (Gunderson, 1974; McFadden et al., 1994; Rose et al., 1994; NRC, 1998; Rosnet et al., 2000). For the ISS, both observational (i.e., Stuster, 2010) and anecdotal evidence indicate that the crew has been successful at thriving.
Risk Management: Countermeasures and Research
Analog and other types of studies have identified measures that can prevent or mitigate behavioral health, sleep impairment, and cognition risks. Countermeasures that have been successfully implemented for ISS operations include working with ground control regarding scheduling, support services from operational psychology personnel, multiple layers of accountability when conducting critical tasks, and education for ground crews (Flynn, 2005; Slack et al., 2009). Existing techniques for monitoring crew behavioral health and providing social and psychological support also have proved effective on the ISS. However, it is unknown whether the effectiveness of these countermeasures can be maintained over longer periods and at greater distances from Earth where delays in communication, delivery of services, and implementation of countermeasures will occur. Policies designed to minimize fatigue, individual stress, and interpersonal tension, such as allocated “time off,” will be critical to preserving and promoting the behavioral health of astronaut personnel.
Astronauts also train in analog environments (including Antarctica, the underwater NASA Extreme Environment Mission Operations [NEEMO], and winter and mountain survival programs). The training environments include both physical capabilities and problem resolution involving group dynamics.
The National Research Council (NRC) Committee for the Decadal Survey on Biological and Physical Sciences in Space noted that “continued research is required to identify individual, interpersonal, cultural, and environmental determinants of crew cohesion, crew performance, and ground-crew interaction” (NRC, 2011, p. 89). Such research may lead to greater specificity of behavioral health standards. The NASA Human Research Program is working on addressing research gaps in this area (see Box 3-1).
NASA Human Research Program’s Research Gaps
Behavioral Health and Performance Risks
BMed 1: What are the most effective methods to enhance behavioral health and prevent decrements before, during, and after spaceflight missions?
BMed 2: What are the most effective methods to predict, detect, and assess decrements in behavioral health (which may negatively affect performance) before, during, and after spaceflight missions?
BMed 3: What aspects, if any, of cognitive performance change in-flight? If there are changes, do they persist post mission? If so, for how long?
BMed 4: What are the most effective methods for detecting and assessing cognitive performance during exploration missions?
BMed 5: What individual characteristics predict successful adaptation and performance in an isolated, confined, and extreme environment, especially for long-duration missions?
BMed 6: What are the most effective methods for treating the individual to remedy behavioral health problems during spaceflight missions (including behavioral health meds)?
BMed 7: What are the most effective methods for modifying the environment to prevent and remedy behavioral health problems during spaceflight missions?
BMed 8: How do family, friends, and colleagues affect astronauts’ behavioral health and performance before, during, and after spaceflight?
Team Gap Risks
Team Gap 1: We need to understand the key threats, indicators, and life cycle of the team for autonomous, long duration, and/or distance exploration missions.
Team Gap 2: We need to identify a set of validated measures, based on the key indicators of team function, to effectively monitor and measure team health and performance fluctuations during autonomous, long duration, and/or distance exploration missions.
Team Gap 3: We need to identify a set of countermeasures to support team function for all phases of autonomous, long duration, and/or distance exploration missions.
Team Gap 4: We need to identify psychological measures that can be used to select individuals most likely to maintain team function for autonomous, long duration, and/or distance exploration missions.
Team Gap 5: We need to identify validated ground-based training methods that can be both preparatory and continuing to maintain team function in autonomous, long duration, and/or distance exploration missions.
Team Gap 6: We need to identify methods to support and enable multiple distributed teams to manage shifting levels of autonomy during long duration, and/or distance exploration missions.
Team Gap 8: We need to identify psychological and psychosocial factors, measures, and combinations thereof that can be used to compose highly effective crews for autonomous, long duration, and/or distance exploration missions.
Team Gap 9: We need to identify spaceflight acceptable thresholds (or ranges) of team function, based on key indicators, for autonomous, long duration, and/or distance exploration missions.
Sleep and Cognition Risks
Sleep Gap 1: We need to identify a set of validated and minimally obtrusive tools to monitor and measure sleep-wake activity and associated performance changes for spaceflight.
Sleep Gap 2: We need to understand the contribution of sleep loss, circadian desynchronization, extended wakefulness and work overload, on individual and team behavioral health and performance (including operational performance), for spaceflight.
Sleep Gap 4: We need to identify indicators of individual vulnerabilities and resiliencies to sleep loss and circadian rhythm disruption, to aid with individualized countermeasure regimens, for autonomous, long duration, and/or distance exploration missions.
Sleep Gap 5: We need to identify environmental specifications and operational regimens for using light to prevent and mitigate health and performance decrements due to sleep, circadian, and neurobehavioral disruption, for flight, surface and ground crews, during all phases of spaceflight operations.
Sleep Gap 6: We need to identify how individual crew members can most effectively and safely use medications to promote sleep, alertness, and circadian entrainment, as needed during all phases of spaceflight operations.
Sleep Gap 8: We need to develop individualized scheduling tools that predict the effects of sleep-wake cycles, light and other countermeasures on performance, and can be used to identify optimal (and vulnerable) performance periods during spaceflight.
Sleep Gap 9: We need to identify an integrated, individualized suite of countermeasures and protocols for implementing these countermeasures to prevent and/or treat chronic partial sleep loss, work overload, and/or circadian shifting, in spaceflight.
Sleep Gap 10: We need to identify the spaceflight environmental and mission factors that contribute to sleep decrements and circadian misalignment, and their acceptable levels of risk.
NOTE: Team Gap 7 merged with Team Gap 3. Sleep Gap 3 is closed. Sleep Gap 7 merged with Sleep Gap 2.
SOURCE: NASA, 2014c.
Individual Variation Issues
Several studies have pointed to phenotypic and genotypic variations to sleep disruption and its behavioral consequences (Van Dongen et al., 2004; Landholt, 2008; Kuna et al., 2012), suggesting the identification of predictive biomarkers (Czeisler, 2011) that could prove useful in identifying astronauts likely to experience sleep-related performance decrements and for managing sleep-wake regulation during exploration spaceflight (Goel and Dinges, 2012).
Individual characteristics identified as predictors of social compatibility in analog studies and surveys of astronaut personnel include low extraversion and high introversion (Palinkas and Suedfeld, 2008), high positive instrumentality (goal oriented, active, self-confident) and expressiveness (kind, aware of others’ feelings), and low negative instrumentality (arrogant, hostile, boastful, egotistical) and communion (self-subordinating, subservient, unassertive) (McFadden et al., 1994; Rose et al., 1994).
Crew heterogeneity with respect to social (e.g., age, sex, cultural background), psychological (need for achievement, aggressiveness, autonomy), and other characteristics (e.g., interest in leisure activities) also predicts crew cohesion and conflict, Team Gap decision making, and response to crisis (NRC, 1998, 2011; Kanas and Manzey, 2008; Kanas et al., 2009).
Health Standards and Risk Profile
According to NASA’s Human Research Roadmap, the research ratings for risk of adverse behavioral conditions and psychiatric disorders has been determined to be “Controlled” for the ISS and lunar missions, “Acceptable” for near-Earth asteroid missions, and “Unacceptable” for the Mars design reference mission (see Table 2-2) (NASA, 2013). The risk of fatigue-related performance errors is considered “Controlled” for all four design reference missions. The research rating for the third risk related to Team Gap issues is currently listed as “Controlled” for the lunar design reference mission and “Acceptable” for the ISS, near-Earth asteroid, and Mars missions. For a mission to a near-Earth asteroid or Mars, mission duration and distance from Earth are among the major stressors. A NASA health standard for behavioral health and performance has been developed that addresses all of these risks (see Box 2-1).
Cognitive and psychiatric assessments and screening criteria are used in the initial astronaut selection process and in the annual astronaut re-certification medical examinations. Furthermore, processes have been developed by the ISS partner agencies that detail neurocognitive and behavioral health baseline assessments and follow-up.
Issues for Long Duration and Exploration Spaceflights
Few established policies regarding limits to long-duration isolation and confinement exist. The National Science Foundation’s Division of
Polar Programs requires that all candidates for winter-over duty at Antarctica’s Amundsen-Scott South Pole and McMurdo stations undergo a psychiatric evaluation conducted by a civilian contractor. The U.S. Antarctic Program also places limits on the number of continuous seasons (e.g., summer and winter) a person can stay at the same station, essentially mandating that personnel either spend a subsequent year or part of a year at another station or go home for a season prior to returning.
Astronauts need to be fully informed that individual and interpersonal issues that typically are of little concern on Earth or during short duration missions have the potential to become clinically and operationally significant in conditions of prolonged isolation and confinement.
While more is known about controlling the risk of fatigue-related performance errors for shorter flights, much remains to be learned about the chronic stressors of remote missions. Those missions will require autonomous operations that will change the ways crews conduct tasks, leaving them potentially more vulnerable to fatigue-related performance errors and more open to challenges regarding crew dynamics and cohesion.
Ethics issues raised in considering these risks include appropriateness of crew selection criteria (e.g., genetic, sex, and cultural differences), the necessity of informed decision making, and challenges and tensions between astronaut privacy and the need to continuously learn about the impact of spaceflight on behavior and performance. However, as with other areas of human performance, there remains a great detail of uncertainty in the field of clinical psychology related to the ability to predict individual behavior or group behavior on the basis of individual characteristics, especially in isolated and confined extreme environments (Palinkas and Suedfeld, 2008). Clinical assessments have proved to be more successful at “screening out” individuals who are unsuited to living and working in such environments; however, they have been shown to have less utility in “screening in” individuals who are best qualified for such assignments (Grant et al., 2007). Similarly, the adaptation of countermeasures found to be effective on Earth or on the ISS to long duration and exploration space missions remains controversial given their limited external validity due to context-specific influences (Shepanek, 2005). As with other health issues, establishing ethics principles for these missions must take into consideration the feasibility and desirability of doing so in the face of uncertainty.
Bone demineralization during exposure to microgravity illustrates the multiple parameters of a set of related health risks; how these risks are assessed, studied, and managed by NASA; countermeasure development; and interaction with engineering systems.
Overview and Risk Identification
Bone demineralization in microgravity is a well-recognized and well-studied phenomenon that still is not completely understood. It overlaps significantly with areas of highly active research for terrestrial medicine, but it is not fully known in what ways microgravity-induced bone loss might be similar to, or different from, osteoporosis.
Bone normally remodels in response to physical load, becoming stronger where that load is stronger and weaker where there is less load. Approximately a 10th of the human skeleton is normally renewed annually (Sibonga et al., 2008b). In microgravity, bone and muscle experience less loading, and bone mineral density loss rates of approximately 1 to 1.5 percent per month have been observed. This is compared to the 2 to 3 percent loss in bone mineral density per year in postmenopausal females during the first decade after the onset of menopause, which is considered a time of rapid bone loss (Sibonga et al., 2008b). Bone loss is primarily due to increased resorption of bone with subsequent release of calcium into the bloodstream and excretion by the kidneys.
Risk Management: Countermeasures and Research
Because of significant overlap with important clinical questions for terrestrial health care (e.g., osteoporosis and metabolic bone diseases), human and animal models, monitoring methods (e.g., biochemical markers, bone ultrasound, and dual-energy x-ray absorptiometry scan), and a variety of countermeasures (pharmacologic, dietary, and exercise) have been developed and are being implemented for astronauts (Sibonga et al., 2008a,b). However, much remains to be learned about the effects of microgravity, particularly for long duration exposures.
Bone loss in microgravity is exacerbated by dietary factors. Documented undernutrition (“space anorexia”), lack of adequate vitamin D, a diet relatively high in sodium (which increases kidney urine calcium concentration), and the particular relative composition of amino acids in
the diet have all been implicated as contributing factors and are all amenable to careful nutritional adjustment (Smith et al., 2012).
Potential countermeasures include a combination of aerobic and resistive exercise, nutrition, vitamin D supplementation, and pharmacologic agents such as bisphosphonates. Most recent studies in a small number of astronauts on the ISS have combined optimal nutrition (adequate caloric intake, low dietary sodium, and appropriate amino acid balances) and vitamin D supplementation with an aggressive exercise program (Smith et al., 2012). These relatively successful countermeasures increase the rate of new bone formation, so bone mass is maintained, but there is increased bone-cell turnover. Pharmacologic agents that are used to combat bone loss associated with menopause (in women) and aging (in both men and women) have been studied to a very limited extent in the space environment. Both bone mass and bone architecture contribute to bone strength and fracture risk. New bone that is deposited as a result of countermeasures appears to exhibit altered architecture compared with the bone that has been lost (Sibonga et al., 2008b). Moreover, the increased calcium turnover may increase the risk of kidney stones.2 Drinking more water dilutes the calcium concentration and decreases this risk.
Thus, demineralization poses risk to an individual astronaut during and after the mission, and risk to the mission if bone loss leads to fracture. A combination of countermeasures was tested on 13 crew members during several ISS missions and was associated with increased formation of new bone (without a corresponding decrease in resorption) and overall maintenance of bone mass (Smith et al., 2012).
Although multiple knowledge gaps remain, one of particular interest is development of an easy, noninvasive way to monitor bone health in microgravity. Research gaps are identified in the NASA Human Research Program’s evidence reports on this health concern (Sibonga et al., 2008a,b).
Individual Variation Issues
The significant individual variation in bone demineralization is so wide as to obscure differences due to biological sex characteristics or any
2An unexpected environmental support system hazard occurred during the summer of 2009 when a new type of urine processing assembly (a critical unit that allows water reclamation) broke down shortly after being brought on line in the ISS. The cause was eventually traced to precipitation of calcium from urine in a mechanism strikingly similar to the biological process of renal stone formation (Smith et al., 2012).
other associated predisposing factors (Ploutz-Snyder, 2013). No predictive or explanatory model exists for understanding this range in individual variation.
Health Standards and Risk Profile
The risk of fracture due to bone demineralization is classified as “Controlled” for missions to the ISS or to a near-Earth asteroid, and “Acceptable” for lunar and Mars design reference missions (see Table 2-2). The risk of early-onset osteoporosis due to spaceflight is ranked “Acceptable” for all four design reference missions (see Table 2-2). A NASA health standard for bone mineral loss has been developed that addresses both of these risks (see Box 2-1).
Issues for Long Duration and Exploration Spaceflights
An unknown variable is whether the rate of bone loss for astronauts during prolonged missions will remain consistent with rates observed in shorter (6 months or less) missions, whether it will plateau, or whether it may accelerate as space environment exposure lengthens. The long-term effects of bone loss are relatively well known for Earth-based populations. Both menopause-related and senile osteoporosis are associated with an increased risk of fracture (primarily hip, vertebral, and wrist). While bone demineralization theoretically could increase the risk of actual fracture during a long duration spaceflight, this has never occurred during a mission, although obviously, such an occurrence could jeopardize successful completion of the mission or lead to morbidity affecting astronaut well-being after the mission.
Bone density is closely monitored in the active astronaut corps as well as in those astronauts who participate in the Longitudinal Study of Astronaut Health. During the ISS flights, astronauts exercise extensively, and their diet is tailored for maintaining bone density. After each flight, bone scans are done to examine the degree of bone loss, and protocols (both pharmaceutical and physical) are recommended for bone recovery. Postflight rehabilitation aims to return bone mass levels to preflight baselines (NASA, 2007).
Long duration and exploration spaceflights with landings, such as a mission to Mars, could expose astronauts to additional risks beyond those associated with a long stay on the ISS. Such an exploration-class mission, as currently envisioned, would include a period of exposure to
microgravity, the physical stress of landing, exposure to and activities in the fractional gravity of Mars (0.375 of the gravity of Earth; NASA, 2014d), physical stress during takeoff from Mars, exposure to microgravity again, and finally physical stress during landing on Earth. Essentially nothing is known about how exposure to fractional gravity environment on Mars might influence the time course of microgravity-induced bone loss or fracture during the mission.
Ethics issues raised in consideration of this health risk include potential impact on crew selection, responsibilities for long-term monitoring and health of astronauts, balancing the unknowns regarding this risk with other risks and benefits, and ensuring informed decision making.
For space missions in low Earth orbit (LEO), the major source of radiation exposure results from solar storms. For exploration-class missions beyond LEO, it is the exposure to galactic cosmic radiation that is the significant health concern for both acute and long-term health consequences. High levels of radiation exposure (e.g., during solar storms) can lead to acute effects, including fatigue, nausea, and vomiting. Chronic exposure increases the risk of cancer, tissue degeneration, development of cataracts, and potential effects on the central nervous system, cardiovascular system, immune function, and vision.
Overview and Risk Identification
NASA recognized the potential radiation risk from the beginning of its efforts to send astronauts into space. An extensive radiation research program has been under way and input has been received from a number of independent organizations (see Box 3-2).
Overview of Selected Reports on Radiation Exposure Limits
National Research Council (NRC, 1967), Radiobiological Factors in Manned Space Flight. The working group focused on identifying “immediate or early performance decrement (early responses) occurring within a few hours to 1 month of major exposure;” “progressively increasing performance decrement or serious loss of performance over longer period of
flight as a result of an accumulating exposure (progressive injury to the blood-forming system);” and “probability of late radiation response” (p. 244).
National Research Council (1970), Radiation Protection Guides and Constraints for Space-Mission and Vehicle-Design Studies Involving Nuclear Systems. The NRC committee noted that the risk-benefit decisions depend on “a wide range of general and specific scientific and subjective judgments and should be the responsibility of those most informed about the aims and goals of the nation’s space program” (p. 3). The report noted the value of specific limits to spacecraft design and mission planning. The final recommendations of the NRC committee included a set of short-term guidelines to limit acute and degenerative effects, along with the concept of a “primary reference risk” to limit the exposure to that which “corresponds 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 at risk” (p.16). They estimated this exposure to be 400 REM, which they believed corresponded to about a 2.3 percent risk of developing cancer. However, they noted that acceptance of a higher risk for planetary missions than for space station missions “would seem both realistic and practical” (p. 16).
National Council on Radiation Protection and Measurements (NCRP) Report 98, Guidance on Radiation Received in Space Activities (1989). In addition to recommending short-term limits to minimize acute effects, this study noted that cancer was the principal risk, and career limits were set to limit the risk of fatal cancer. The report limited the risk to 3 percent excess lifetime cancer mortality, based on comparison with other hazardous occupations. The study was also the first to take age and sex into account.
NCRP Report 132, Radiation Protection Guidance for Activities in Low-Earth Orbit (2000). NCRP 132 questioned the use of mortality data from hazardous occupations as the basis for space-related radiation limits, but endorsed the 3 percent lifetime risk of cancer death, as consistent with guidelines for terrestrial radiation workers. The report also briefly but explicitly addressed the challenge of uncertainties in quantifying radiation limits: “It is well known that risk estimation is a difficult field in which there are many sources of potential error and therefore uncertainty…. Given the magnitude of these uncertainties and the problems of dose specification estimates or risk on which dose limits for astronauts are based should be recognized as very conservative and possibly subject to modified values when more precise information becomes available” (pp. 146-147). Report 132 clarified that the NCRP had only been tasked to assess risk to low Earth orbit (LEO) missions: “In NCRP Report No. 98 (NCRP, 1989) a section on mission scenarios with estimates of radiation exposure during missions to the moon and Mars was included. Perhaps because of that inclusion, some have assumed that the guidance on dose limits in that report applied not only to missions in LEO but to all space missions. That was not the intention since the guidance provided was limited to exposures in LEO. In this Report, the guidance is also only intended to be ap-
plied for radiation exposures incurred during missions in LEO. Further NCRP reports will deal with other space situations” (pp. 2-3).
NCRP Report 153, Information Needed to Make Radiation Protection Recommendations for Space Missions Beyond Low-Earth Orbit (2006). “The purpose of this Report is to identify and describe information needed to make radiation protection recommendations for space missions beyond low-Earth orbit (LEO). Current space radiation guidelines pertain only to missions in LEO and are not considered relevant for mission beyond LEO” (NCRP, 2006, p. 1).
NRC (2012a), Technical Evaluation of the NASA Model for Cancer Risk to Astronauts Due to Space Radiation. The NRC committee did not address the underlying assumption of whether the permissible exposure limits were appropriate, rather they looked in detail at the constituent elements of the model used to calculate the risk. They found the model to be consistent with general radiation community approaches to quantify the radiation risk.
SOURCES: NRC, 1967, 1970, 2012a; NCRP, 1989, 2000, 2006.
Current career limits in the NASA standards are designed to keep the astronaut’s lifetime risk of exposure-induced death from cancer to no more than 3 percent (i.e., limiting the additional risk of cancer posed by mission-related radiation exposure). The risk and lifetime exposure limit can be explained as follows: If 100 astronauts were exposed to the upper bounds of the radiation limits, 3 would die of cancer attributable to that exposure. It would be anticipated that life expectancy for astronauts with radiation-induced cancer would be reduced by an average of 12 to 16 years compared to those without radiation-induced cancer (NASA, 2007).
In 1990, NASA agreed to accept the recommendation of NCRP Report 98 to limit the lifetime risk to astronauts to 3 percent and formalized it in the 1995 NASA Health Standards (NASA, 1995). The 1995 standards stated the permissible exposure limit in terms of sex- and age-dependent, dose-equivalent limits expressed in REMs,3 which were developed to be consistent with the 3 percent risk of exposure-induced death (REID) (e.g., 237.5 REM for a 35-year-old male, 177.5 REM for a 35-year-old female) (NASA, 1995). These estimated limits did not include a margin to represent the uncertainty in the calculations. As a result
3REM (rad equivalent man) was used as a unit of dose equivalent in the 1995 NASA standard, but NASA now uses the sievert (Sv), the international standard for dose equivalent (NRC, 2012a). 1 REM is equal to 0.01 Sv.
of NCRP Report 132, published in 2000, NASA proposed to add a 95 percent confidence interval to the 3 percent limit. The current radiation permissible exposure limits with the 95 percent confidence interval was formally accepted in March 2007 (NASA, 2007) (see Box 2-1). The procedure used to estimate the confidence interval is based on extrapolation from a wide range of physical and biological research, much of it conducted by NASA in a peer-reviewed research program open to the wider radiation research community. The model and the underlying methodology and assumptions are periodically reviewed by independent committees, most recently by the NRC Space Studies Board (NRC, 2012a).
Risk Management: Research, Exposure Limits, Monitoring, and Countermeasures
NASA has undertaken a research program to quantify the estimates of the risk of radiation-related cancers and other acute and chronic adverse health effects (Cucinotta and Durante, 2009; Cucinotta et al., 2009; Huff and Cucinotta, 2009; Wu et al., 2009). Progress based on a peer-reviewed, science-based approach has significantly reduced the uncertainties associated with the estimates of the risk, and the long-term program goal is to reduce it further.
Radiation levels are monitored in real time on the ISS by a suite of sensors distributed throughout the station. The reports from these sensors are used to estimate astronaut exposure throughout the mission. When the rate of exposure exceeds threshold levels, mission control is informed. For short periods, when radiation levels are significantly above normal, astronauts may be instructed to stay in better shielded areas of the ISS. In addition, each astronaut has a personal dosimeter that is read after the mission by the NASA Space Radiation Analysis Group to confirm and record exposure. For exploration missions, research is under way to develop real-time personal dosimeters.
NASA’s Space Radiation Analysis Group works with the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center to monitor the radiation levels near Earth and monitor for evidence of solar storms. The center issues alerts when a radiation storm is under way, and the Space Radiation Analysis Group alerts ISS mission operations.
Countermeasures for radiation exposure are limited. The primary countermeasure is shielding built into the spacecraft. However, due to the highly penetrating nature of galactic cosmic rays, shielding is only mar-
ginally effective at reasonable thicknesses; increasing the thickness of the shielding adds substantial mass with minimal additional reduction in exposure (NRC, 2008). Innovative approaches to reduce the risks also are being investigated, including pharmacological countermeasures. To date, these methods have not been shown to be effective.
Thirty-day exposure limits have been established to limit the potential for acute health outcomes due to radiation exposure, and operational processes and procedures are implemented to ensure that astronauts do not approach these limits. The greatest likely source of acute exposure is solar storms. Providing access to modest shielding and a system for timely warning of a pending or ongoing storm are critical elements of a risk management strategy to mitigate this risk (NRC, 2008). Pre-mission planning includes estimates of the radiation exposure to minimize the risk that the crew will exceed the career limits (with consideration of the crew member’s prior exposure).
Radiation exposures also may include risks of chronic, degenerative effects that could affect health during a lengthy mission. Only recently has quantitative progress been made in understanding these risks, particularly those affecting the central nervous and circulatory systems (Cucinotta et al., 2013b). An ongoing research effort is under way to improve this understanding.
Health Standards and Risk Profile
NASA has long recognized the threat of radiation, and has established space permissible exposure limits to protect its astronauts (see Box 2-1). The three tiers of radiation exposure limits in the NASA standards address: career limits, designed to limit the lifetime risk of exposure-induced death from cancer; short-term limits, designed to limit the risk of acute affects; and operational processes and procedures which emphasize that “in-flight radiation exposures shall be maintained using the ‘as low as reasonably achievable’ (ALARA) principle” (see Box 2-1; NASA, 2007, p. 20).
Radiation limits are significant in planning for long duration and exploration missions. Existing radiation standards, developed for the Space Shuttle and the ISS, would limit missions to durations of 150 to 250 days (Cucinotta et al., 2013a). The radiation standards are written to apply to all NASA human spaceflight missions and are not developed for any specific program (NASA, 2007). However, while some of the existing programs, such as the Space Shuttle and the ISS programs, can be con-
ducted within the standards, these standards impose potential limitations on long-duration missions (e.g., a 1-year stay on the ISS) or missions with architectures and objectives outside of LEO. The standards are based on recommendations to NASA from NCRP in a report that focused on LEO missions (NCRP, 2000). The NCRP report noted, “In this Report, the guidance is also only intended to be applied for radiation exposures incurred during missions in LEO” (NCRP, 2000, pp. 2-3).
Individual Variation Issues
A specific individual’s cancer risks vary by age, sex, and race, as well as genetic factors and life-style choices. National databases allow calculation by age, sex, and other factors, emphasizing the role of individual variability. NASA’s standards limit the additional risk of cancer posed by radiation exposure, not the total risk of dying from cancer. If one assumes a generic baseline lifetime risk of 23 percent for males and 19 percent for females (ACS, 2013), and leaves aside any differences in individual susceptibility, then male astronauts exposed to the 3 percent radiation exposure limit would have an estimated lifetime risk of death from cancer of 26 percent, while female astronauts would have an estimated risk of 22 percent.
In general, for females the effective-dose radiation exposure is about three quarters that of males before reaching career limits (NASA, 2007). Females, on average, are more susceptible to radiation-induced cancer, in part because of the baseline higher risk of breast cancer in females; body size also is a factor as individuals with smaller builds have less body self-shielding. However, an individual’s risk of cancer is dominated by a number of factors (including genetic susceptibility, lifestyle choices, and prior exposures, both natural and medical), most of which may be highly individualized or unknown and, therefore, not currently possible to factor in calculations of cancer risk.
Issues for Long Duration and Exploration Spaceflights
As noted above, the number of days that astronauts could be exposed and stay within the current permissible exposure limits (safe days) will depend on assumptions about shielding and the radiation environment. The exposure varies with the 11-year solar cycle and with the number and intensity of solar storms (Cucinotta et al., 2013a). The existing radiation exposure limits would limit long duration or exploration missions to
150 to 250 days (Cucinotta et al., 2013a). The estimated time in the zero-g interplanetary environment (“deep space”)4 for a Mars mission with current propulsion systems is estimated at 400 to 600 days (NASA, 2009). The design reference missions to Mars that are being considered include those that would include a long stay on the Martian surface (500 or more days) with a shorter stay in deep space, as well as those that would have a longer deep space duration and shorter Mars surface stays, nominally 30 to 90 days (NASA, 2009). Technology research into alternative propulsion systems could significantly reduce the Mars transit time (NRC, 2012b).
Other factors also could affect the number of safe days in space with regard to radiation exposure. For example, noting that astronauts, in general, are healthier than the average U.S. population, NASA recently calculated the number of safe days in space using the data on cancer risks for non-smoking, normal weight Americans (see Table 3-1). This change in the population data increases the estimates of safe days in space by 30 to 90 percent, depending on an astronaut’s age and sex (Cucinotta et al., 2013a).
The NASA current precautionary decision to set exposure limits to protect astronauts with 95 percent probability (given the level of uncertainty) itself limits the number of safe days in space. If a less precautionary approach were taken by NASA using a lower confidence interval, the number of allowable days in space would be greater; alternatively, if greater precaution were exercised by using a higher confidence interval, fewer days would be permissible. More precise information about the dose of radiation-inducing fatal cancers could also change the number of permissible days in space that would not exceed the health standard.
Ethics issues raised in the consideration of radiation exposure include informed decision making by astronauts, individual variation and other factors that might impact crew selection, and assessing and balancing the unknown risks of radiation exposure with other risks and benefits of the mission. While NASA has analyzed health risks of radiation based on age and sex (see Table 3-1), other factors influencing individual variation and radiation risk must also be considered. Existing data on such variation should be factored into analyses, and new data should be collected to better predict individual risk from exposure to radiation.
4Calculations regarding radiation exposure factor in the higher exposures to radiation in interplanetary space outside Earth’s magnetosphere and the relatively lower exposure levels on a planetary or lunar surface.
TABLE 3-1 Estimates of Safe Days in Deep Spacea
|Average solar minimum GCR||Average solar maximum GCR and one significant solar storm (similar to that which occurred in August 1972)|
|Age at Exposure (years)|
|35||209 (205)||271 (256)||306 (357)||395 (458)|
|45||232 (227)||308 (291)||344 (397)||456 (526)|
|55||274 (256)||351 (335)||367 (460)||500 (615)|
|35||106 (95)||187 (180)||144 (187)||276 (325)|
|45||139 (125)||227 (212)||187 (232)||319 (394)|
|55||161 (159)||277 (246)||227 (282)||383 (472)|
NOTE: Solar minimum is a 2- to 3-year period of low solar activity in the 10- to 11-year solar cycle. Solar maximum is a corresponding 5- to 7-year period of enhanced solar activity. Galactic cosmic radiation is at a peak during solar minimum and somewhat reduced during solar maximum. Values in parentheses for solar minimum are for the deep solar minimum of 2009. Values in parentheses for solar maximum are for the case where a storm shelter is available to reduce the solar storm exposure to a negligible amount. GCR = galactic cosmic radiation; REID = risk of exposure-induced death.
aSafe days in deep space (zero-g interplanetary environment) is a concept defined as the maximum number of days with 95 percent confidence interval to be below the NASA 3 percent REID limit, assuming nominal shielding of 20 g/cm2 aluminum.
SOURCE: Adapted from Cucinotta et al., 2013a.
DISCUSSION: Because the highly penetrating GCR dominates the effective dose, and because GCR is at a peak at solar minimum, the safe days are lowest at solar minimum. The deep solar minimum of 2009 had a significantly higher GCR than previously experienced, thus further reducing the safe days. There are more safe days near solar maximum because the GCR is reduced relative to solar minimum, and because solar storm radiation is significantly attenuated by nominal shielding. If a storm shelter (a smaller volume within the vehicle with additional shielding) is available to the astronauts, the exposure from a solar storm can be reduced to a negligible amount, so the total exposure would be from the GCR alone. This could further increase the number of safe days at solar maximum, assuming adequate solar storm warning.
The examples provided in this chapter illustrate the range of issues that are faced as NASA makes decisions about health standards for long duration and exploration spaceflights. The potential short- and long-term health impacts encompass many systems of the human body as well as behavior and performance issues. Decisions about health standards are complicated by the depth of uncertainty regarding what will happen with extended stays in space, high exposures to galactic cosmic radiation, and other risks and challenges. Additionally, data are minimal or non-existent for variations in individual susceptibility based on factors such as ethnicity, sex, age, etc. A major challenge is the lack of information about the interaction of risks and the extent to which these interactions alter the overall level of risk. The following chapter outlines the ethics principles and responsibilities that can be brought to bear on decisions regarding health risks.
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