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Safe Passage: Astronaut Care for Exploration Missions (2001)

Chapter: 2 Risks to Astronaut Health During Space Travel

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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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Suggested Citation:"2 Risks to Astronaut Health During Space Travel." Institute of Medicine. 2001. Safe Passage: Astronaut Care for Exploration Missions. Washington, DC: The National Academies Press. doi: 10.17226/10218.
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2 Risks to Astronaut Health During Space Travel . . . we must assume that for a long time to come (although not forever), weight- lessness will be an obligatory condition of space flight. For this reason, all aspects of this issue must be considered from the point of view of the possibility of func- tioning in microgravity. G. I. Meleshko, Y. Y. Shepelev, M.M. Averner, and T. Volk, 1994 OVERVIEW Over the life of the U.S. space program, generations of astronauts have learned how to live and work in weightlessness. Humans evolved in gravity; how the body would function in its absence or near absence was an unan- swered question. Would it be possible to eat and drink in microgravity? Would it be possible to perform complex tasks? The early answers were affirmative. Thus, selected fit and healthy humans have been sent into space for three decades and have functioned well (Lane and Schoeller, 2000). Although humans have adapted to weightlessness, readapting to Earth’s gravity is problematic. Exposure to microgravity affects the body in many ways. Some effects are severe and long lasting, such as loss of bone mineral density. Others are minor and temporary, such as facial puffiness due to fluid shifts (Nicogossian et al., in press). It is unlikely that all effects of 37

38 SAFE PASSAGE microgravity are known, and surprises may yet be in store as humans ven- ture longer and farther into space. This chapter is about human physiologi- cal adaptation to space travel. In it, the Institute of Medicine (IOM) Com- mittee on Creating a Vision for Space Medicine During Travel Beyond Earth Orbit examines what is known about the effects of microgravity and space travel on the human body. This is the starting point for generating priorities in clinical research and health care for space travel beyond Earth orbit. In developing this chapter, the committee has relied heavily on briefings and published information from the study’s sponsor, published scientific articles, and two recent National Research Council (NRC) reports. The first report, A Strategy for Research in Space Biology and Medicine in the New Century (SSB and NRC, 1998a), provided a science-based assessment of the most important biomedical research topics in 1998 to be pursued over the next decade. The second report, Review of NASA’s Biomedical Research Pro- gram (SSB and NRC, 2000), examined the National Aeronautics and Space Administration’s (NASA’s) biomedical research enterprise 2 years later and measured it against the plan set forth in the earlier report. The IOM com- mittee endorses the findings and recommendations of both NRC reports. The current report extends the vision of the two previous NRC reports to clinical research and clinical care in space. This chapter responds to the portion of NASA’s charge to the IOM committee to “conduct an indepen- dent assessment of the current status of scientific knowledge” relevant to providing optimal health care for astronauts traveling beyond Earth orbit. In so doing, the chapter describes the effects of weightlessness and space travel on the physiology and functioning of the human body. It discusses the evidence on which the findings are based, the steps that need to be taken, and the research challenges and opportunities that lie ahead. Most of what is known about the effects of microgravity on the human body has been learned on short missions into space. NASA is now looking ahead to longer-duration space missions, initially in Earth orbit and later into deep space. Over the next decade, a number of astronauts will have 3- to 6-month tours of duty aboard the International Space Station (ISS). These may be followed by extended stays on the Moon or exploration-class mis- sions to Mars, or both. Before the United States and its international space partners commit to any such plans, however, there needs to be a better and fuller understanding of the risks to astronaut well-being and the safety of long-duration space travel in and beyond Earth orbit. The chapter presents numerous examples of the effects of exposure to microgravity and space travel on human physiology (Box 2-1). The examples

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 39 BOX 2-1 Some Major Human Physiological Changes Resulting from Extended Travel in Earth Orbit Musculoskeletal System Loss of bone mineral density Loss of skeletal muscle Cardiovascular System Orthostatic hypotension Loss of hydrostatic pressure Pulmonary System Changes in pulmonary circulation and gas exchange Alimentary System Ileus Decrease in absorption or malabsorption Nervous System Ataxia Motion sickness Disturbed fine motor and gross motor functions Altered sleep-circadian rhythm and sleep deprivation Reproductive System Effects of radiation on gametes Urinary System Renal calculi Hematological and Immunological Systems Anemia Potential immunologic depression Source: Billica, 2000. are by no means exhaustive, however. The material in this chapter is ar- ranged by organ system, with those for which the physiological effects are best documented presented first. The chapter also includes a discussion of future methods for the monitoring of astronauts’ health status—an impor- tant aspect of detecting, understanding, and countering the untoward physi- ological changes that may affect astronaut well-being and mission perfor- mance.

40 SAFE PASSAGE Finally, the chapter includes a discussion of the comprehensive, long- range approach to clinical research that NASA needs to consider imple- menting to best protect human health and safety during long-duration space travel. Historically, NASA has faced difficulty in conducting clinical research in space medicine. One problem is the small numbers of research subjects (astronauts) available for study. The overriding reason, however, is that microgravity cannot be duplicated on Earth; it can only be approximated. The terrestrial means of research on bone mineral density loss in microgravity are bed rest, immersion in water, or immobilization. All have their own disadvantages. The opening era of the permanent presence of humans in Earth orbit on the ISS in October 2000, however, provides an enduring test bed that will eventually help provide an understanding of hu- man physiology in microgravity. Countermeasures to Solve Physiological Adaptations to Space Faced with the necessity to maintain astronauts’ health during periods of exposure to microgravity and other extreme conditions of spaceflight, NASA has pursued the development of preventive and counteracting mea- sures (i.e., countermeasures) to guard against or reverse the potential patho- physiological effects of space travel. A variety of countermeasures have been used in longer-duration spaceflights (Mikhailov et al., 1984; Bungo et al., 1985; Greenleaf et al., 1989; Fortney, 1991; Arbeille et al., 1992; Cavanagh et al., 1992; Charles and Lathers, 1994; Hargens, 1994; Convertino, 1996b). The American and Russian space programs use different strategies. Some examples of countermeasures that had been developed as of 2000 include subcutaneous injections of erythropoietin to prevent decreases in erythro- cyte mass and vigorous in-flight exercise regimens to reduce loss of bone mineral density. So far, countermeasures appear to be largely ineffective, but the data are sparse (Bungo et al., 1985; Buckey et al., 1996b; Convertino et al., 1997; Lane and Schoeller, 2000). NASA’s general approach to the development of countermeasures was presented to the IOM committee at the Johnson Space Center (Paloski, 2000; Sawin, 2000). The rationale (Figure 2-1) outlined a number of steps that have been incorporated into NASA’s Countermeasure Evaluation and Validation Project, which can be summarized as follows: 1. conduct research to understand the basic nature of the physiological problem,

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 41 Operational Review Processes: Medical Policy Board, Safety Office, Astronaut Office CM USE CM VALIDATION Review Process, [flight] Integrated Testing Regimen CM EVAL UATION [ground-based] CM DEVELOPMENT RESEARCH Peer-Reviewed Research physiology & environmental operational & radiation Process behavior health clinical medicine health FUNDAMENTAL B IOMEDICAL RESEARCH Critical Path, Surgeons, Crew, REQUIREMENTS Advisors FIGURE 2-1 Countermeasure (CM) evolution. Source: Paloski, 2000. 2. formulate a countermeasure strategy based upon that physiological understanding, 3. test the countermeasure and demonstrate its efficacy on the ground, and 4. validate the countermeasure in space. It has been difficult for NASA to design and test effective countermea- sures, and no single countermeasure has yet to be validated as clinically effi- cacious. The potential for better design and evaluation of countermeasures improved dramatically on October 30, 2000, with the arrival of the first multinational astronaut crew to inhabit space as residents on the ISS. The ISS offers NASA and its international partners a longer-term orbiting clini- cal research laboratory in microgravity to investigate—and ultimately pre- vent—the adverse changes in human physiology described in the pages that follow. To assume that a terrestrial model duplicates the physiological effects of microgravity is a logical flaw that could lead to reliance on ineffective countermeasures. The plan outlined by NASA therefore demands that a significant amount of physiological research be conducted on the ISS and

42 SAFE PASSAGE immediately following long-duration space missions on the ISS. The amount of research required, the duration of the research, and the extensive nature of the research will have to be considered in the planning phases of ISS missions. This chapter also discusses new and future methods for the diagnosis and monitoring of astronaut health status in space and NASA’s health risk assessment and management process that includes countermeasure develop- ment. The chapter concludes with a discussion of the comprehensive long- range approach to clinical research that NASA needs to consider to prepare for successful long-duration missions with humans beyond Earth orbit. MUSCULOSKELETAL SYSTEM Loss of Bone Mineral Density in Microgravity Changes in bone mineral density, muscle mass, and muscle function are the best-documented physiological effects of human space travel. The loss of bone mineral density in microgravity is well documented (Vico et al., 2000). Serious acute consequences of bone mineral density loss (i.e., frac- ture and the formation of renal stones) as well as long-term morbidity may complicate long-duration space travel beyond Earth orbit. Working in microgravity within a spacecraft, during extravehicular activity, and upon a low-gravity moon or planet presents many increased risks for bone fracture and the necessity for wound healing. From a practical viewpoint, virtually nothing is known about how microgravity will affect fracture management and healing during long-duration space missions. For example, is it better to cast, internally fixate, externally fixate, or electrically stimulate a fracture sustained on Mars? The committee was unable to locate data from studies with animals or humans or from basic, translational, or clinical studies on these clinical treatments issues; but knowledge about such clinical treatments issues will be important to sustain human health and performance should a bone fracture occur during space travel beyond Earth orbit. At the basic science level, little is known about the fundamental mecha- nisms underlying the loss of bone mineral density in microgravity; hence, scant progress has been made on the development of effective countermea- sures. This must be an extremely high priority before long-duration space travel can be deemed reasonably safe with regard to the risk of fractures, the associated increased risk of renal stones, and basic skeletal support. Small numbers of subjects and, in many cases, incomplete data have hindered clinical studies, resulting in a lack of reliable databases (SSB and

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 43 NRC 1998b; Lane et al., 1999; Lane and Schoeller, 2000). Thus, it is diffi- cult, if not impossible, to generate reliable conclusions that can be applied to individual astronauts. A recent review (Smith et al., 1999) focusing on calcium metabolism after a 3-month space mission showed an approximately 50 percent increase in the level of calcium absorption accompanied by a 50 percent increase in the levels of both calcium excretion and bone resorption, as determined by calcium kinetics and bone marker analyses, respectively. Subjects lost approximately 250 milligrams of bone calcium per day during space travel in Earth orbit and appeared to regain it at a slower rate after their return to Earth. There is a suggestion from limited studies of bone density markers in serum and urine, which are used to approximate the relative rates of bone formation in relation to rates of bone resorption, that an uncoupling of the two processes led to an imbalance, with bone resorp- tion predominating (Caillot-Augusseau et al., 1998). Again, because of the small numbers of subjects, this conclusion cannot be generalized and may not be applicable to all astronauts. As shown in Table 2-1, mainly weight-bearing bones (spine, neck, fe- mur, trochanter, and pelvis) lost bone mineral density during space missions in Earth orbit: on average, greater than 1 percent per month for cosmonauts on the Russian Mir space station. In contrast, there was no significant loss from bones in the upper extremity (arm). Additional measurements (n = 40) that include shorter-term (<3 weeks) U.S. space shuttle flights show that loss of bone mineral density begins within a few days and continues for the longest period measured (1 year) without showing signs of leveling off. It is noteworthy that the standard deviations in all studies are high, which may indicate wide variations in the responses of individuals. This suggests that there may be substantial phenotypic (and pre- sumably genotypic) variations in susceptibility to microgravity-induced bone mineral density loss. If shown to be true, this concept would have important implications for the selection of crewmembers for long-duration missions (e.g., are menopausal female astronauts or specific male or female astronauts phenotypically at greatly enhanced risk?). The collection and analysis of clinical data for a comprehensive data- base to determine whether individuals with phenotypes that make them “re- sistant” to and “at risk” for space travel-induced bone mineral density loss exist represent but one set of important challenges for NASA that has been identified. Understanding these patterns in relation to basic patterns of up- and downregulation of gene arrays as a result of exposure to microgravity could lead to the development of specific interventions to prevent microgravity-induced bone mineral density loss.

44 SAFE PASSAGE TABLE 2-1 Average Bone Mineral Density Loss on Mir Number of Mean Loss Standard Variable Crewmembers (Percent/Month) Deviation Spine 18 1.07a 0.63 Neck of femur 18 1.16a 0.85 Trochanter 18 1.58a 0.98 Total body 17 0.35a 0.25 Pelvis 17 1.35a 0.54 Arm 17 0.04a 0.88 Leg 16 0.34 0.33 ap < 0.01. SOURCE: LeBlanc et al., 1996. Associated with the bone mineral density loss is a rather consistent hypercalciuria, which increases the risk of formation of renal stones (Schneider et al., 1994). Importantly, this limits pharmacological options such as treatment with normal dietary supplements (e.g., calcium and vita- min D). Reversibility, Genetic Variability, and Mechanism of Bone Mineral Density Loss Although data are limited, it appears that changes in calcium metabo- lism and bone mineral density are reversible. There are suggestions, how- ever, that reversal of the changes is slower than their evolution and that the rate and extent of reversal are highly variable (Vico et al., 2000). A mission to Mars, for example, would involve a period of low or nearly zero gravity during space travel, a second period of time in gravity well lower than that on Earth while the expedition was on the surface of Mars, a third period of time again spent in low gravity during the return flight, and the ultimate return to Earth’s gravity. How these sequential changes in gravity loading will influence bone mineral density loss is unknown. It is possible that they may exacerbate and accelerate the incidence of Earth-based diseases such as

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 45 osteoporosis. As such, it may be appropriate to consider terrestrial refer- enced 6- and 9-year windows of disease incidence instead of the disease incidence for a 3-year period. Limitations in data collection and analysis, the small sizes of databases, the lack of precise bone mineral density measurements (which have a 1 to 2 percent coefficient of variation), and the very high natural variations of mark- ers of bone mineral density turnover all contribute to the difficulty of ob- taining reliable data that would be useful for clinical decision making for space travel. Therefore, the most reliable and efficient clinical studies must probably use individual astronauts as their own controls. The etiology of bone mineral density loss is multifactorial and likely polygenic, like most common diseases. The basis for or the implications of individual variability is not known, further complicating interpretation of the limited clinical data. Long-duration space missions in Earth orbit offer the opportunity to obtain crucial data by careful clinical research. These data could then be used to generate hypotheses and to guide measures to protect subsequent astronauts from unnecessary morbidity and even death. Additional factors that may affect the relative risk of bone mineral den- sity loss during space missions are bone mass as measured by bone densi- tometry and bone turnover rates as measured by markers of bone density in serum and urine determined before the initiation of a space mission. Al- though the selection of astronauts with higher bone masses may not prevent bone mineral density loss, it may prevent the consequences of decreased bone mineral density; that is, bones with higher mineral densities may be at decreased risk of fracture. Thus, because of unknown genetic components and the variability among astronauts with regard to the rate of bone mineral density loss, it might be increasingly important to identify individuals whose bodies are able to resist bone mineral density loss on prolonged space mis- sions (Vico et al., 2000). This can be accomplished only by extensive premission, intramission, and postmission analyses of bone mineral density and markers of bone mineral density turnover in serum. It may be possible to identify individuals who are less prone to the effects of microgravity on bone mineral metabolism and bone mineral density loss. This knowledge not only might have implications for space travel, but it also may well pro- vide important information about diseases such as osteoporosis. A number of animal models, including models of unloading, have been used to simulate weightlessness (SSB and NRC, 1998a; Lane and Schoeller, 2000). It is unknown whether such models reflect the physiology of humans during space travel. Rats have been used in microgravity experiments, but the results of those experiments may be of limited value given the poor cor-

46 SAFE PASSAGE relation between rat bone metabolism and the bone architecture in humans (SSB and NRC, 1998a, 2000). The development of land-based animal mod- els for investigation of the pathophysiology and pharmacotherapeutics of bone mineral density loss during space missions beyond Earth orbit will require more knowledge of bone mineral density loss in humans. Although the advantage of using experimental and natural animal models, for example, hibernating bears (Harlow et al., 2001), is evident—in that it allows more focused investigation into basic molecular and cellular mechanisms—suc- cesses from the study or development of such models will likely lag behind the need to test the effectiveness of therapeutic interventions in humans during space missions beyond Earth orbit. The use of immobilized human subjects on Earth as models may have its place. However, the high priority for the evaluation of the effectiveness of countermeasures in the microgravity environment renders such models less valuable in the short term. The technology for the accurate testing of bone mineral density is im- proving and is becoming increasingly miniaturized. It will probably become necessary to assess bone mineral density changes during space missions both in and beyond Earth orbit with reasonable precision for clinical research purposes. It is anticipated that this technology will be useful in assessments of the need for and testing of the effectiveness of specific interventions dur- ing space travel. In addition, it is anticipated that better documentation of the validity of bone mineral density markers in evaluations of the mecha- nisms of bone mineral density loss would also provide a means for optimiza- tion of highly targeted interventions. Therefore, the development of a means to measure such markers in the long-duration space mission environment should be a high priority in conjunction with an aggressive NASA counter- measure research program. Better measures of bone integrity, for example, three-dimensional assessments, not just bone mineral density, are also needed. Ultimately, intramission tests should be the indicators that guide specific therapies. Clinical Research Opportunities in Astronaut Physiology and Health Clinical Research Opportunity 1. Establishing the course of changes in bone mineral density and markers of bone mineral density turnover in serum and urine before, during, and after space travel. Clinical Research Opportunity 2. Developing a capacity for real-time measurement of bone mineral density and enhanced three-dimensional technology to assess the risk of fracture during space travel.

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 47 Because of the significant current research effort into the prevention and treatment of osteoporosis, new techniques that better assess bone min- eral density turnover and fracture susceptibility can be anticipated within the next decade. These techniques should be evaluated, adapted if neces- sary, and incorporated into NASA’s clinical research program. Clinical Research Opportunity 3. Identifying human phenotypes and genotypes resistant to space travel-induced bone mineral density loss. This may involve analysis of both data from large phenotypic databases and genetic factors as they become available. Recent data suggest a wide spectrum in both the level of acute bone mineral density loss and return to normal bone density upon the return to Earth (Vico et al., 2000). Clinical Research Opportunity 4. Tailoring therapeutic interventions (i.e., countermeasures such as diet, exercise, and medications) as a high priority and then validating the promising countermeasures in studies with astronauts during exposure to microgravity. Pharmacological countermeasures deserve special emphasis. They may need to be tailored to an individual’s response, and ongoing monitoring of effectiveness may be required during long-duration space travel beyond Earth orbit. For most of the clinical studies, astronauts must be their own controls, which again requires the ability to conduct comprehensive analy- ses of bone-related metabolic parameters during space travel. Effects of Microgravity on Skeletal Muscle During space travel, the primary effects of microgravity on skeletal muscles include the deterioration caused by the lack of gravity on slow- twitch muscles, with conversion from slow to fast muscle fiber type; and decreased fiber size in rats (Riley et al., 1990; Jennings and Bagian, 1996). Young rats subjected to 18 days of hind-limb unloading developed an ab- normal gait that persisted, suggesting permanent damage to the neuromus- cular pathway (Walton et al., 1997). Significant atrophy has been observed in human muscle after only 5 days in space, but the time course of deteriora- tion has not been established. Moreover, it is not known when or whether a plateau might be reached. In addition, the foot-drop posture in microgravity shortens the extensor compartment and appears to accelerate loss of the thick filament (Edgerton et al., 1995).

48 SAFE PASSAGE Most of these primary changes to skeletal muscle during space travel appear to represent simple deconditioning without apparent pathology and can be considered appropriate adaptations for functioning in a low-workload microgravity environment. However, an abrupt return to gravity imposes high workloads on weakened muscles and post-space travel pathologies: muscle fatigue, weakness, incoordination, and delayed-onset muscle sore- ness. In rats, changes in microcirculation occur during space travel as a re- sult of the cephalad fluid shift, and reloading-induced edema and ischemic tissue necrosis may occur upon the return to Earth’s gravity (Riley et al., 1996). In addition, adaptation to the lower workload in microgravity may render muscle tissue more prone to structural failure when it is reloaded. A variety of exercise-based protocols have been used, but none have been adequately validated and none have proved to be more than modestly effective. Cycle ergometry has successfully been used to ameliorate cardio- vascular aerobic deconditioning but did not prevent muscle deterioration. However, bicycle ergometry and treadmill exercises counteract the tendency to attain the foot-drop posture in microgravity that shortens the extensor range and that appears to contribute to accelerated loss of the thick filament (Edgerton et al., 1995; Widrick and Fitts, 1997). The Task Force on Countermeasures: Final Report (NASA, 1997) con- cluded that existing cycling, rowing, and treadmill exercise protocols did not maintain muscle mass or a positive nitrogen balance. A significant con- tributing factor to this loss of muscle protein may be inadequate nutrition during space travel. To correct this, lower-body negative pressure coupled with resistance exercises should be tested as a means of maintaining the microcirculation and muscle strength (Koslovskaya et al., 1990). NASA funded a lower-body negative-pressure investigation in 1999 to study a coun- termeasure that uses developments in advanced technology. In addition, the possibility of pharmacological intervention apparently has not been ex- plored, and better monitoring and recording of the history of muscle use and condition (e.g., specific exercise and nutrition history) during space travel need to be established to reduce uncontrolled parameters. CIRCULATORY AND PULMONARY SYSTEMS Orthostatic Hypotension The single most significant problem associated with the cardiovascular and pulmonary systems as a consequence of microgravity appears to be

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 49 orthostatic hypotension, a fall in blood pressure and the associated dizzi- ness, syncope, and blurred vision that can occur when one stands up or simply stands motionless in one position. In the context of space travel, a major cause of orthostatic hypotension is the persistent lowering of periph- eral vascular resistance during space travel, and more than two-thirds of all astronauts experience orthostatic hypotension when they reenter Earth’s gravity (Buckey et al., 1996a,b). In other words, orthostatic hypotension is a physiological adaptation to microgravity that readapts after exposure to grav- ity. Resetting of the baroreceptor reflex occurs over a period of several days, depending upon the individual, the duration of space travel, and the individual’s fluid status (Fritsch-Yelle et al., 1994). Symptoms can be un- comfortable or annoying, or both, but life-threatening complications have not been reported. It is generally accepted that there is at least a general relationship be- tween the duration of exposure to microgravity and the duration of the pe- riod of recovery from the orthostatic hypotension. However, few data are available concerning the severity of orthostatic hypotension, and the avail- able information concerning the relationship between the duration of expo- sure to microgravity and the degree and duration of orthostatic hypotension is inadequate. No information is yet available on how orthostatic hypoten- sion might be affected by the 0.4G gravity on Mars compared with how it is affected by the 1G gravity on Earth. It seems reasonable to assume, how- ever, that on an exploration-class mission to Mars, orthostatic hypotension would represent a major problem if emergency procedures were to require the crew to move about immediately after landing. Clinical Research Opportunity 5. Considering artificial gravity and phar- macological interventions as solutions to orthostatic hypotension. Other Effects on the Cardiovascular System The cardiovascular system undergoes several other physiological adap- tations to microgravity. During weightlessness, there is a loss of hydrostatic pressure, especially in the lower extremities. Fluid shifts from extravascular to intravascular spaces and toward the upper part of the body (Thornton et al., 1987; Leach et al, 1996). This provokes objective and subjective symp- toms, especially in the first days of space travel. The baroreceptors in the carotid arch sense a relative central hypervolemia and induce neurohormonal mechanisms that lead to diuresis and hypovolemia. Central venous pressure

50 SAFE PASSAGE drops from the normal range of 7 to 10 mm Hg to 0 to 2 mm Hg (Kirsch et al., 1984; Buckey et al., 1996a). The prolonged reduction in central venous pressure during exposure to microgravity resets the baroreceptors to a lower operating point; this in turn limits plasma volume expansion during attempts to increase fluid intake. Ground-based and in-flight experiments have demonstrated that restora- tion and maintenance of plasma volume before the return to Earth’s gravity and an upright posture may return the central venous pressure to normal levels and reset the baroreceptor reflex (Convertino, 1996a). Microgravity also produces a decrease in renal and femoral vascular resistance, with main- tenance of cerebral flow at rest. During orthostatic testing, lower-limb vas- cular resistance does not compensate for the fluid shift (Arbeille et al., 1996). The changes in intravascular volume during space travel lead to corre- sponding changes in stroke volume and cardiac output. For example, corre- sponding decreases in stroke volume and cardiac output have been observed over a range of atrial pressures. Echocardiographic experiments performed during space shuttle flights showed an average decrease in stroke volume of 15 percent, measured after a period of 3 days of adaptation to weightless- ness (Pantalos et al., 1998). The heart rate is generally unchanged. Thus, although the cardiovascular system demonstrates many changes during space travel, to date few have appeared to be severe enough to affect human health or performance during space travel. NASA has, however, identified poten- tially serious cardiac dysrhythmias, impaired cardiovascular responses to orthostatic stress, diminished cardiac function, manifestation of previously asymptomatic cardiovascular disease, and impaired cardiovascular responses to exercise (Levine et al., 1996) as potentially serious risks during space travel (see Table 2-2 later in this chapter). These alterations in cardiac function could also present serious risks to astronaut health during long-duration space travel. However, the incidence and the severity of the dysfunction in each of these categories during short- and long-duration space missions are unknown. Currently available data (SSB and NRC, 1998c, 2000) have not been critically peer reviewed and do not convince the committee that cardiovascular risks should be labeled “se- rious” at this time. Substantial new research is required to define the degree of risk, the likelihood and severity of cardiac alteration, and reasonable thera- peutic options for long-duration space travel.

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 51 Effects of Microgravity on the Pulmonary System Thus far, no significant problems have been identified on the basis of the observed changes in pulmonary physiology in microgravity. Reported observations include a change in the pattern of ventilation in microgravity that results in decreased tidal volume and an increased frequency of ventila- tion (Prisk et al., 1995b, West et al., 1997). There is decreased dead space with normal oxygen uptake and an improved carbon dioxide diffusion ca- pacity (Prisk et al., 1995a,b). The overall improvement in lung function can- not be completely explained by what had been supposed to be gravity-in- duced ventilation/perfusion ratio inequalities, raising fundamental questions about normal gas exchange physiology (Prisk et al., 1993, 1995a; Lauzon et al., 1998). Two potential risks to pulmonary function are dysbarism and a patent foramen ovale, although the latter is really a potential end risk to the nervous system. NASA is aware of these potential risks to astronaut health and is actively investigating what measures, if any, are needed to decrease the risks. ALIMENTARY SYSTEM Nutrition in Space NASA has conducted extensive research on nutrition issues pertaining to spaceflight and has conducted such studies in collaboration with aca- demic specialists (Lane and Schoeller, 2000). Despite 35 years of nutrition research, however, weight loss, dehydration, and reduced appetite continue to challenge NASA food scientists (Lane and Schoeller, 2000). Space Motion Sickness One of the most common conditions requiring pharmacological treat- ment in a microgravity environment is space motion sickness (SMS) (Putcha et al., 1999). SMS is a syndrome consisting of headache, malaise, disorienta- tion, nausea, or vomiting. Putcha and colleagues (1999) reported that 47 percent of unit doses of medication given during space missions were for the treatment of SMS. These usually consisted of promethazine with or without dextroamphetamine, although the frequency of drug administration and the routes of administration were not reported. The etiology of SMS is not known, although fluid redistribution (Simanonok and Charles, 1994) and alterations in bowel motility (Harris et

52 SAFE PASSAGE al., 1997) are factors that have been implicated. Although SMS is largely considered a “minor” disorder, its effects are serious enough that extrave- hicular activity is prohibited during the first 3 days of a mission to avoid the possibility of vomiting and aspiration into the space suit (Simanonok and Charles, 1994). Given the high frequency of occurrence of SMS, its ability to affect an astronaut’s function, and the high frequency of use of medication with potential adverse effects that affect function, SMS is an area for clinical investigation on longer ISS missions. Many astronauts who develop symptoms of SMS also seem to develop a transient gastrointestinal ileus, diagnosed by an absence of bowel sounds. Although motility may remain decreased throughout flight and bacterial populations may change, data from short-term spaceflights do not suggest that this leads to significant medical problems (Lane and Schoeller, 2000). The etiology is unclear. Microgravity-induced movement of the abdominal contents within the abdominal cavity, simulating a surgical ileus, may be responsible. Another possibility is neurohumoral mediation. Individuals who lack bowel sounds during the first 48 hours in space and who attempt to ingest food will often vomit, just as postoperative pa- tients do. The simplest and most effective treatment appears to be patience. Those individuals who wait until they adjust to zero gravity and develop audible bowel sounds before they ingest solid food seem to do well. Most crewmembers on spaceflights resolve the ileus spontaneously within 48 hours, and adequate hydration should be maintained during that time, par- ticularly if there is no intake of solid food. This gastrointestinal problem appears to be self-limiting, and overtreatment with laxatives may cause diar- rhea, which can be a difficult problem to deal with in the space environ- ment. No additional predictable problems with ileus would be expected on long-term spaceflights. Valuable information may be gathered from astro- nauts with extended stays on the ISS, and further clinical data may be ob- tained from studies investigating the etiology of SMS. Clinical Research Opportunity 6. Investigating the relationship between space motion sickness and absent bowel sounds including pharmacologi- cal and adaptive countermeasures. NERVOUS SYSTEM Useful data on neurovestibular function, sleep (circadian rhythms), eye- hand coordination, fine and gross motor functions, and visual perception and reorientation have been collected in real-time, post-space travel studies

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 53 and from studies with animals. However, little or no readily accessible data are available on sensation and proprioception. Some data are available, how- ever, from isolated and confining analog situations describing predictors of behavior during long-duration missions (Palinkas et al., 1998). Neurovestibular Function Several studies have been performed to assess the effects of microgravity, nonvertical positioning, and simulated gravitational environments (short- arm centrifuge) on the neurovestibular system. These studies have focused on the adaptation to—and the relationship between—the visual system and body position during travel in space (Cohen et al., 1995). Countermeasures have been designed to test the effects of the centrifuge apparatus or body tilt on postural stability and possibly orthostatic tolerance (Black et al., 1999). Disconjugate gaze occurs and persists for weeks after space travel (Markham and Diamond, 1999). Markham and Diamond discuss the use of medication (promethazine) for SMS, including its impact on alertness, the mechanism of delivery, and possible antidotes (amphetamines) for drowsiness. No re- sults of controlled trials, dose-response trials, or comparative efficacy stud- ies have been published. Recent NASA Neurolab studies, however, deemphasize the potential importance of overt neurovestibular disturbance during space travel. Sleep and Circadian Rhythm Evidence is accumulating that sleep is disrupted during space travel and that the circadian rhythm is disrupted (Box 2-2). This may be mediated through the neurovestibular system. Data from Mir suggest that the dura- tion of sleep is reduced, that sleep is not as deep, and that in other ways sleep may be physiologically different from the sleep experienced on Earth (Putcha et al., 1999). Possible explanations include the low levels of light in the spacecraft, changes in light-dark cycles, and the misalignment of work- rest shifts with light cycles. Long-duration space travel may produce even more aberrant sleep dis- ruptions and associated abnormalities. These can result from disruption of the hypothalamic-pituitary axis, with the resultant release of growth hor- mone, and changes in cortisol peaks and valleys. Disruptions of the circa- dian rhythm may result in abnormal stress responses, diminished perfor- mance because of fatigue, and mood and behavioral changes (Mullington et al., 1996). Although astronauts frequently use sleep medication, it has been

54 SAFE PASSAGE BOX 2-2 Altered Sleep Patterns as Example of Multifactorial Problems Arising During Space Travel Impairment of normal sleep patterns can erode cognitive performance and vigi- lance during spaceflight (Berry, 1969). Astronauts experience sleep disruption, as shown by a survey of 48 astronauts and polygraphic recordings (Stogatz et al., 1987; Santy et al., 1988; Gundel et al., 1993; Monk et al., 1998), with an average deficit of just under 2 hours per day from the programmed time, with a change in the architec- ture of sleep, and with less time spent in stages 3 and 4 of sleep. Sleeping pills account for 45 percent of all medications used by space shuttle crews (Putcha et al., 1999). Sleep problems are multifactorial. They begin with the distorted sleep-wake cycle on launch day and continue with the schedule geared to Mission Elapsed Time rather than local times at takeoff or landing sites. The schedule is further disrupted to accom- modate the arrivals and departures of Earth-to-orbit vehicles. In 1990, NASA issued guidelines (Mission Operations Directorate, 1990) limiting the time that the daily sleep schedule can be changed in each day. The 4- to 7-hour phase advance shift is accom- plished gradually (20 to 40 minutes earlier per day on space shuttle missions STS-90 and STS-95) or in a series of 2-hour jumps. Light is the most powerful synchronizer of the human circadian pacemaker (Czeisler and Wright, 1999). There is an external 90-minute light-dark cycle on orbiting flights, which can disrupt the circadian pacemaker and interfere with sleep. Measurements of light levels on Spacelab and Spacehab showed much higher levels on the flight deck than the middeck, with some high readings in the early evening. Since inappropriately timed light exposure or insufficiently intense lighting can be disruptive, astronauts com- ing from the dimmer middeck to a presleep period on the flight deck may have difficulty with sleep. The carryover effects of the cyclic use of hypnotics and caffeine can intensify any cognitive deficit and should be avoided. Additional research studies aboard the space shuttles and the ISS will further characterize the nature of sleep disturbances and con- tribute to the development of safe and effective countermeasures for associated perfor- mance decrements. Abstracted from Barger and Czeisler, 2000. only modestly effective in correcting the insomnia. Use of melatonin as a possible countermeasure has been discussed, but assessment of effective medication for sleep has not been undertaken in a methodical way. Electroencephalographic changes have been recorded with prolonged wake- fulness, and these correlate with neurobehavioral performance capability (Dijk et al., 1992; Dinges et al., 1997). Clinical Research Opportunity 7. Building a coordinated clinical research program that addresses the issues of neurological safety and care for astronauts during long-duration space travel.

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 55 Clinical Research Opportunity 8. Performing pharmacological trials with dose-response and pharmacokinetic measures to assess the efficacies and toxicities of medications commonly used to treat sleep disturbances dur- ing space travel. Clinical Research Opportunity 9. Considering clinical trials on the use of growth hormone and other countermeasures and developing devices to control ambient light and the core temperature at appropriate levels during space travel to reduce sleep disturbances. Eye-Hand Coordination and Sustained Gross Motor Activity There is evidence of the degradation of task performance over time dur- ing space travel (Manzey and Lorenz, 1998). The tasks studied have included fine motor function, eye-hand coordination (including documentation of the inability to pilot a vehicle for days after landing from short flights), and gross motor activity, especially extravehicular activity. Explanations for this deterioration of performance include difficulty in vigilance, sensory input diminution, and changes in visual acuity (Billica, 2000; Marshburn, 2000a). Neurological recovery of balance and mobility requires 1 to 3 days after short space missions and 10 to 30 days after longer missions. Few data that can shed light on the causes of these observations are available. Astronauts commonly complain of sluggishness, impaired cogni- tion, and disorientation. In fact, the most frequently identified symptom during space travel is fatigue or asthenia. Some of this may be related to sleep-circadian rhythm disturbance; some of it may have to do with the func- tioning of the stimuli from the peripheral to the central nervous system. An important issue is sustained performance during long-duration missions in space. Pharmacological countermeasures have addressed sleep but not vigi- lance or performance-enhancing strategies. Shift assignments have been used to provide more autonomy and more rest and relaxation, with some success. Concerns remain about the possible sequelae of changes in visual acuity, vigilance, balance, and muscular function. PERIPHERAL NERVOUS SYSTEM Published reports and presentations to the committee made little men- tion of sensorimotor status during or upon the return from space missions, yet sensorimotor status is an important component of muscle activation.

56 SAFE PASSAGE The lack of stimulation may be contributing to the decreased muscle mass and possibly the asthenia and fatigue that occur during spaceflight. Devices that stimulate sensory input (vibrators, pneumatic pumps) could be studied to better understand the role that sensorimotor status might play in muscle activity. Because nervous system health is important to crew health and mission success, data could be collected from studies on the ISS on the efficacies of pharmacological agents in improving nervous system performance. System- atic study of sensorimotor status in microgravity is important to understand- ing the contributions of neurological and behavioral factors to human per- formance in deep space. REPRODUCTIVE SYSTEM The committee was unable to find many data on the effects of microgravity on the reproductive system. This is understandable, given that space missions thus far have been of relatively short duration. As the lengths of missions are extended, however, and outposts in the solar system occu- pied by humans become a possibility, the study of the effects of space travel on human reproductive physiology, the risks associated with exposure of the gametes to radiation, and the study of reproduction in space are warranted. Effects of Radiation on Gametes Effects on Male Gametes Spermatogonia are among the most radiosensitive cells in the body. Ex- posure to radiation at levels as low as 10 REM (roentgen equivalent in man) has been known to cause reduced levels of sperm production (ICRP, 1969), and exposure to levels of 50 REM may cause temporary sterility. A single exposure sufficient to produce permanent azospermia would be fatal to the individual. Exposure to a lower dose of radiation over a protracted period of time, however, would not be lethal but could produce sterility (Jennings and Santy, 1990). Although infertility is an important issue, the long-term ge- netic consequences of exposure of the gonads to radiation are of greater concern to many.

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 57 Effects on Female Gametes Unlike the testis, which constantly replaces spermatogonia, the ovary has a fixed supply of oocytes that have been present since birth. Because the oocytes do not actively undergo mitotic division and since they possess ef- fective enzymatic repair systems, the ovary is more resistant to radiation- induced genetic effects; however, the effect of radiation on oocytes is cumu- lative. A single dose of 300 to 400 rads has been sufficient to eliminate oocytes from the ovaries as well as estrogen production. When the exposure is fractionated, there is increased tolerance over a single dose, and primary oocytes show a degree of recovery from the accumulated radiation damage (ICRP, 1969). Assessment of the effects of space travel on the reproductive endocrine system and on ovulatory function should be ongoing, and NASA should consider offering preflight gamete cryopreservation for men and women who may wish to reproduce after a long-duration space mission. Clinical Research Opportunity 10. Determining whether radiation expo- sure during space travel causes genetic damage or altered fertility in men and women and, for women, premature ovarian failure. Human Reproductive Physiology in Space Male Reproductive Physiology Although little is known about the effects of space travel on the hypo- physeal-pituitary-gonadal axis, some evidence for reversible testicular dys- function has been found. Reductions in testosterone levels have been re- ported during flight and postflight in both rats and humans (Plakhuta-Plakutina, 1977; Tigranjan et al., 1982; Deaver et al., 1992). Since testosterone secretion by the Leydig cells in the testis is stimulated by lutein- izing hormone, a decrease in testosterone levels may indicate a disturbance in the hypophyseal-pituitary-gonadal axis. If this is the case, one might ex- pect abnormalities of germinal epithelial function and, thus, diminished sperm production as well. Clinical Research Opportunity 11. Determining female and male repro- ductive hormone levels during space travel.

58 SAFE PASSAGE Female Reproductive Physiology At birth, women possess all the gametes that they will ever have. Thus, as women age, so do their gametes, and advanced oocyte age is associated with infertility. Many women, including women who are astronauts, delay childbearing until after they have completed their education and have achieved some of their career objectives. The average age of women se- lected into an astronaut class is 32 years, and many have not had children. Of 99 female finalist candidates examined during five selection cycles be- tween 1989 and 1997, only 18 had been pregnant. Once they are admitted into the space program, given the constraints of training on a pregnancy, female astronauts commonly further delay childbearing until after the completion of one or two spaceflights. As a result, female astronauts are often in their 40s when they attempt pregnancy (Jennings and Baker, 2000). Retrograde menstrual flow is considered an etiologic factor in the devel- opment of endometriosis and consequent infertility (Sampson, 1927; Scott et al., 1953; Jennings and Baker, 2000). Many women experience some ret- rograde menstruation that, at Earth’s gravity, is usually confined to the pel- vis. However, in microgravity there is concern that the level of retrograde menstrual flow might be increased and that instead of being confined to the pelvis it would disperse throughout the abdominal cavity. Abdominal symp- toms, shoulder pain, or an obvious reduction in the amount of menstrual flow has not been observed among women during spaceflights. However, retrograde menstrual flow has not been subjected to systematic study (Seddon et al., 1999). Because of the short durations of space missions so far, coupled with the pulsatile nature of hormone secretion by the hypophyseal-pituitary-ovarian axis, the effect of space travel on ovulatory function has not been studied (Seddon et al., 1999; Strollo, 1999). The effects on the menstrual cycle of stress and exercise during space travel also have not been studied (Seddon et al., 1999). On Earth, stress and exercise can be associated with anovulation, and continuous estrogen exposure can be associated with endometrial hy- perplasia and excessive vaginal bleeding. Alternatively, stress and excessive exercise can be associated with hypogonadotropic hypogonadism, resulting in reduced estrogen levels and amenorrhea. The latter condition is associ- ated with decreased bone mineral density. The effect of microgravity, in addition to the effects of stress and an exercise program necessary to main- tain cardiovascular and musculoskeletal well-being on long-duration flights, may increase the risk of developing one of these conditions. Exogenous

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 59 hormone therapy in the form of oral contraceptives or some other therapy may be both preventive and therapeutic. Clinical Research Opportunity 12. Determining the effect of microgravity on menstrual efflux and retrograde menstruation. Sex Differences The National Space Biomedical Research Institute held a workshop on sex-related issues in spaceflight research and health care in August 1999 (Seddon et al., 1999). Focusing on sex-related issues, the group reviewed existing demographics and epidemiological information, identified areas of needed research, and identified ways to accelerate research on the ground and in space to ensure the health of space crews and to provide the best medical care to diverse crewmembers. The report from the workshop indicates that in most areas there are insufficient data from which to draw valid conclusions about sex-specific differences in physiological responses among astronauts. One of the rea- sons for the lack of data is the relatively small female astronaut population compared with the size of the male astronaut population, which precludes performance of a study with sufficient statistical power for adequate analy- ses. In addition, in many areas, the individual differences in physiological responses among members of the same sex are as great or greater than those among individuals of different sexes. This adds to the difficulty in doing the analysis, which occurs in many areas of research involving astronaut health because of the small sample sizes. This is an important research problem in clinical trials with small numbers of participants, such as those involving astronauts (IOM, 2001e). Other than orthostatic hypotension after space shuttle missions, in which women have a greater likelihood of presyncope during postmission “stand” tests than men (Fritsch-Yelle et al., 1996), the workshop panel members identified few areas in which there were differences by sex. The report points out that there are no data for several systems (postmenopausal bone loss, iron intake, muscle strength, and endurance) in which one might ex- pect sex differences to exist. The report lists other areas in which the pre- diction of sex differences is not possible and for which no data exist but that are worthy of study. These include susceptibility to decompression sickness, pharmacokinetics and pharmacodynamics, immune function, sensitivity to radiation, and psychosocial adaptation. These are important areas of con- cern that are also addressed elsewhere in this report.

60 SAFE PASSAGE In addition to identifying areas of concern for female astronaut health care, including pregnancy and the pharmacodynamics of oral contracep- tives, the panel looked at issues dealing with the human-machine interface that may affect women during space travel. Apparently, existing equipment, including extravehicular activity suits and shuttle egress suits, and task de- sign preclude some individuals from performing some tasks (Seddon et al., 1999). A stated goal described in the workshop report of Seddon and col- leagues (1999) is assurance that all individuals selected to be astronauts are able to perform all tasks associated with the job, regardless of their size or sex. The panel asked that a firm commitment be made to achieving this goal. The present IOM committee supports that recommendation. Clinical Research Opportunity 13. Collecting clinical data for both men and women when anatomically possible and physiologically sensible for all individuals in the space program and, on a regular basis, subjecting the data to analysis for sex-related differences. URINARY SYSTEM Renal stone formation secondary to bone calcium mobilization and ex- cretion in the urine are well-identified concerns of space travel, with an ex- pected incidence of 0 to 5 percent. The effects of microgravity on the uri- nary system also include changes in urodynamics (unknown incidence) and urinary hesitancy (reported in seven cases). Significant changes in the pH of urine and in urine calcium and citrate levels increase the risk of renal stone formation. Countermeasures for urinary problems are primarily oriented toward the prevention of nephrolithiasis through adequate hydration. The recom- mended daily fluid intake volume for astronauts during spaceflight is greater than 2.5 liters (Lane and Schoeller, 2000). Therefore, the availability of a more than adequate supply of water must be ensured so that crewmembers do not hesitate to drink adequate volumes of water to prevent the formation of renal calculi. This may necessitate daily monitoring of water consumption levels, as adequate in-mission treatment of calculi during extended missions may be impossible. As ultrasound devices become smaller, it is possible that an ultrasound or other imaging device will be standard medical equipment for all long-duration space missions. This would make it possible and desir- able to perform intramission screening for nephrolithiasis to identify those requiring increased hydration levels to prevent the growth of calculi. As effective countermeasures for the problem of bone mineral density

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 61 loss in microgravity are developed, it must be ensured that the solutions to the problem do not result in significantly increased rates of renal stone for- mation secondary to alterations in calcium metabolism. PHYSIOLOGICAL MONITORING NASA has recognized not only the need to measure various parameters to evaluate astronauts during space travel but also the need to store blood and urine for future analysis upon the return to Earth (SSB and NRC, 2000). Both are important requirements for the clinical research program for long- duration space travel. Because of the unique environmental stresses of space travel (gravity forces, radiation), when a new technology is developed, there must be an adequate lead time to test the new technology in the space en- vironment. With the evolution of new technology, NASA will continually be challenged to evaluate the newest methods and instruments in a microgravity environment. For example, the rapid application of nanotech- nology, nonivasive biosenors, new imaging techniques, and informatics will provide diagnostic and treatment assessment capabilities vastly different from those available today. Validation of new technologies in the space medi- cine clinical research program and a transition to the routine application of those technologies in the space medicine clinical research program must be high priorities and continuous challenges for NASA. Monitoring During Space Travel: Development of Technology Priority should be given to the development of high-resolution, high- precision, yet minimally invasive or noninvasive methods for the monitoring of important physiological parameters and for biological imaging during all periods of space travel. Technologies are evolving rapidly and will dramati- cally alter monitoring capabilities during space missions. Technology may be used to minimize the intrusiveness of testing by developing means of obtain- ing transcutaneous measurements or substituting urine or saliva samples for blood samples. Examples of technologies under commercial development are noninvasive blood glucose monitoring kits and kits for the determina- tion of complete blood counts. The application of nanotechnology to hu- man fluid analysis, genomic analyses, and pharmacogenetics will also likely be routine aspects of future space travel. NASA’s hosting of periodic inter- national nanotechnology conferences to bring engineers and biologists to-

62 SAFE PASSAGE gether and the agency’s collaborative initiative in cell biology with the Na- tional Cancer Institute are appropriate means of gaining wider expertise to understand and deal with methods that can be used to monitor risks to human health during future expeditions beyond Earth orbit. Research Opportunity 14. Giving priority to high-resolution, high-preci- sion, yet minimally invasive or noninvasive methods for the monitoring of physiological parameters and for imaging of the human body during space travel. A STRATEGY FOR A SPACE MEDICINE CLINICAL RESEARCH PROGRAM The Critical Path Roadmap project is NASA’s integrated, cross-disci- plinary strategy to assess, understand, mitigate, and manage the risks associ- ated with long-term exposure to the space environment. Initiated during 1997 and 1998, it is an iterative approach of review, analysis, and delibera- tions among intramural and extramural investigators focused on a worst- case scenario: long-duration, highly autonomous interplanetary missions such as a human expedition to Mars. The project consists of seven elements (risks, risk factors, critical questions, risk mitigation, the Critical Path Roadmap project, requirements, and deliverables [countermeasures]), as il- lustrated in Box 2-3. Fifty-five risks have been identified and stratified (Table 2-2). The goal is to identify and validate clinical interventions (countermea- sures) for half of the risks by 2006 and for all of the risks by 2010 (Charles, 2000). Continuing revision of the Clinical Path Roadmap is available at http:/ /criticalpath.jsc.nasa.gov. Eleven discipline risk areas have been identified: • advanced life support; • environmental health; • radiation effects; • human performance; • bone loss; • cardiovascular alterations; • food and nutrition; • muscle alterations and atrophy; • immunology, infection, and hematology; • neurovestibular adaptation; and • space medicine.

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 63 BOX 2-3 Elements of the Critical Path Roadmap Project ✓ Risks: Likelihood of an undesirable event occurring as a result of exposure to the space environment. Risk assessment includes the probability of the risk’s occurrence, the severity of the consequences of the occurrence, and the current status of mitigation of the risk. Examples: Occurrence of serious cardiac dysrhythmias Fracture and impaired fracture healing ✓ Risk Factors: A condition or precipitating factor that must be present for the risk to occur. Such conditions can operate singly or in combination to contribute to the occur- rence of risk. Examples: Poor nutrition In-flight work schedule overload ✓ Important Questions: What research and technology need to be developed to further assess the risk and address its mitigation? Example: Will bone mass loss continue unabated for missions longer than 6 months, or will it plateau at some time consistent with absolute bone mineral density? ✓ Risk Mitigation: Strategies, devices, interventions, and requirements that need to be in place to modify the occurrence or impact of the risk. Examples: Resistive exercise regimens Medications Preflight fitness requirements ✓ Critical Path Roadmap Project: Graphic representation(s) of the essential set of research and technology development tasks required to address the risks associated with exploration-class space missions with humans, specifically denoting ➡relationships (causal pathway among risks, risk factors, and consequences) ➡priorities ➡temporal sequencing (predecessor questions or tasks) ✓ Requirements Examples: Crew screening and selection requirements for minimization of bone loss Habitability requirements Nutritional requirements Medical care systems for diagnosis, monitoring, and treatment of illness and injury ✓ Deliverables: Specific end items that can be identified, completed, and available at known dates. Examples: Validated, preflight training countermeasures (see Table 2-2) for psychosocial adaptation Validated, in-flight virtual reality-based training for emergency egress Exercise countermeasures for bone loss, muscle atrophy, and cardiovascular and neurovestibular adaptation Source: Charles, 2000.

64 SAFE PASSAGE TABLE 2-2 Critical Path Roadmap Project: Critical Risks. SOURCE: Charles, 2000 Immunology, Muscle Bone Cardiovascular Human Behavior Infection and Alterations Neurovest Loss Alterations and Performance Hematology and Atrophy Adaptation Acceleration of Human age-related performance osteoporosis failure because of poor psychosocial adaptation Fractures Occurrence of Human Loss of Disorienta (traumatic, stress, serious cardiac performance failure skeletal and inabili avulsion), and dysrhythmias because of sleep muscle mass, perform la impaired healing and circadian strength, or egress, or of fractures rhythm problems endurance physical ta especially after g-lev changes Impaired cardio- Inability to Impaired vascular response adequately neuromus to orthostatic perform tasks coordinatio stress due to motor and/or stre performance problems, poor muscle endurance, and disruptions in structural and functional properties of soft and hard connective tissues of the axial skeleton Inability to sustain muscle performance levels to meet demands of performing activities of various intensities

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 65 es, 2000 scle erations Neurovestibular Radiation Clinical d Atrophy Adaptation Effects Capability Other Carcinogenesis Trauma and acute Severe caused by medical problems Risks radiation ss of Disorientation Damage to Toxic exposure Inadequate Very letal and inability to central nervous nutrition Serious scle mass, perform landing, system from (malnutrition) Risks ength, or egress, or other radiation exposure (three risks) durance physical tasks, especially during/ after g-level changes bility to Impaired Synergistic effects Altered Postlanding equately neuromuscular from exposure to pharmaco- alterations in form tasks coordination radiation, dynamics various systems e to motor and/or strength microgravity and and adverse resulting in formance other environ- drug reactions severe blems, mental factors performance or muscle decrements durance, and and injuries ruptions in uctural and ctional perties of t and hard nnective sues of the al skeleton bility to Early or acute Habitation and stain muscle effects from life support formance radiation exposure (eight risks) els to meet mands of forming vities of ious intensities continued

66 SAFE PASSAGE TABLE 2-2 Continued Immunology, Muscle Bone Cardiovascular Human Behavior Infection and Alterations Neurovest Loss Alterations and Performance Hematology and Atrophy Adaptation Injury to Diminished Human Immuno- Propensity to Impaired connective tissue cardiac function performance deficiency/ develop cognitive a or joint cartilage, failure because of infections muscle injury, physical or intervertebral human system connective performan disc rupture with interface problems tissue to motion or without and ineffective dysfunction, ness symp neurological habitat and and bone or treatme complications equipment design, fractures due especially etc. to deficiencies after g-lev in motor skill, changes muscle strength, and muscular fatigue Renal stone Manifestation Human Carcinogenesis Vestibular formation of previously performance caused by contributio asymptomatic failure because of immune cardioregu cardiovascular neurobehavioral system dysfunctio disease dysfunction changes Impaired Altered hemo- Impact of Possible c cardiovascular and cardio- deficits in impairmen response to dynamics from skeletal muscle orientation exercise stress altered blood structure and balance fu components function on due to other systems micrograv radiation Altered wound healing Altered host- microbial interactions Allergies and hypersensitivity reactions

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 67 scle erations Neurovestibular Radiation Clinical d Atrophy Adaptation Effects Capability Other pensity to Impaired Radiation effects Illness and Serious velop cognitive and/or on fertility, ambulatory health Risks scle injury, physical sterility and problems nnective performance due heredity sue to motion sick- sfunction, ness symptoms d bone or treatments, ctures due especially during/ deficiencies after g-level motor skill, changes scle strength, d muscular gue Vestibular Development contribution to and treatment cardioregulatory of decompression dysfunction illness complicated by microgravity- induced deconditioning act of Possible chronic Difficulty of icits in impairment of rehabilitation letal muscle orientation or following landing ucture and balance function (two risks) ction on due to er systems microgravity or radiation

68 SAFE PASSAGE Short-term Historical Physiological Ground Animals spaceflight data models studies Proposed CM Package Evaluation on Space Station Missions Obtain Performance Indices (physiological) YES Acceptable NO Accept CM 1. Identify ineffective CM(s) 2. Change and continue testing FIGURE 2-2 Countermeasure (CM) development and evolution. Sources: Feiveson, 2000; IOM, 2001e. These risk areas have been grouped into four categories: • environmental and technological, • human behavior and performance, • human health and physiology, and • medical care capabilities. It is expected that clinical research will be an integral component in the successful prevention or mitigation of the risks to astronaut health in each of the discipline risk areas. The Critical Path Roadmap project, with its clinical research program on countermeasures, is NASA’s mechanism for the devel- opment of priorities and validation of the means of mitigating the risks that humans may be expected to encounter during exploration-class missions into deep space. As a mechanism for gathering information, the Critical Path Roadmap project is an appropriate model for clinical research in space medicine, with use of the ISS as the critical platform for clinical research in space medicine and for NASA to validate countermeasures in the space travel environment as it prepares to go beyond Earth orbit (Figure 2-2). NASA has, however, not integrated a prospective and continually reevalu- ated understanding of clinical research on astronaut health into the Critical Path Roadmap project in any of the information that it provided to the com- mittee or that the committee was able to find. It should be noted that the environment on Mars, at 0.4G and with some atmosphere, is different from that of the spacecraft. Whether the presence of significant gravitational pull

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 69 will tend to ameliorate the adverse effects of microgravity is unknown but is worth consideration in the planning of research. A Clinical Research Program for NASA IOM has conducted a number of studies focused on various aspects of clinical research (IOM, 1998, 1999a, 2001c,d,e). Although there are many definitions of clinical research (IOM, 1994), IOM recently endorsed the broad definition of clinical research set forth by the Graelyn Conference and National Clinical Research Summit (Box 2-4) and formed the Clinical Research Roundtable to periodically discuss continuing developments and needs in clinical research. The varied, significant, and potentially harmful changes in physiology associated with space travel require health care interventions to protect the well-beings of the participating astronauts and the integrity of the mission. However, traditional clinical research with astronauts is difficult because of the small number of participants available for study, the inability to replicate microgravity and its effects on Earth, restrictions on the use of control groups, and limitations on the substitution of results from studies with ani- mals. A large portion of NASA’s human clinical research to date has been in the form of countermeasure development and evaluation, and this research has been conducted through trials of single interventions directed at specific physiological changes. This largely precludes, however, the classic approach of comparing alternative treatments in clinical trials with adequate statistical power (adequate sample sizes for statistical analysis) to demonstrate safety and efficacy (IOM, 2001e). Individual variations in both physiological BOX 2-4 What Constitutes Clinical Research? Clinical research embraces a continuum of studies involving interaction with patients, diagnostic clinical materials or data, or populations in any of these categories: disease mechanisms; translational research; clinical knowledge, detection, diagnosis, and nat- ural history of disease; therapeutic interventions, including clinical trials; prevention and health promotion; behavioral research; health services research, including out- comes; epidemiology; and community-based and managed care-based research. Source: AAMC, 1999, p. 4.

70 SAFE PASSAGE changes and responses make it even more difficult to evaluate results be- cause of the small number of participants (i.e., astronauts). Because of these difficulties, NASA has had to select the most promising clinical intervention for its studies of countermeasures to avoid further dilution of its study group and possible ethical breaches. In addition to this empirical approach, more basic research is sorely needed to define the mechanisms that produce the physiological changes that result from space travel, which, it is hoped, will improve the focus of countermeasure clinical trials. The present set of NASA priorities may ac- count for the limited success of countermeasures to date. This does not im- ply deficiencies on the part of NASA researchers but recognizes the severe limitations under which they work. Potential methods for improvement in clinical research with small num- bers of participants are described in a recent IOM study report, sponsored by NASA, Small Clinical Trials: Issues and Challenges, which reviews study designs and statistical methods for determination of the validities and effica- cies of studies with small numbers of participants (IOM, 2001e). NASA’s request for the study is one example of the agency’s attempt to reach out in new directions to better use the limited opportunities during space missions to conduct clinical research with astronauts. Formation of the National Space Biomedical Research Institute and in- creased collaboration with the National Institutes of Health (e.g., the new initiative with the National Cancer Institute) are additional directions that will provide NASA with wider expertise. It is not clear, however, how well clinical research design, strategies, and so forth fit into these initiatives, as clinical research with astronauts relevant to space medicine beyond Earth orbit, particularly validation of counter- measures, can largely be conducted only during space missions. There does not appear to be a clearly visible and transparent plan of who oversees, de- signs, reviews, and is responsible for this clinical research. NASA’s organiza- tional charts, lines of authority and responsibility, administration, and bud- geting processes and its levels of accountability presented to the committee (in official NASA policy documents and during briefings by NASA officials throughout the committee’s information-gathering process) describe a frag- mented, nonuniform process of strategic planning and oversight that is in- sufficient to provide an effective means of understanding and mitigating the risks to human health from traveling in space. A comprehensive strategic plan for the prevention and amelioration of the many potential risks to well-being during long-duration travel beyond

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 71 Earth orbit and treatment of the conditions that result from those risks is vital to the future of space medicine. This requires an understanding of the relationship between human physiology and adaptation to space travel. It also requires a broad, evidence-based, collaborative, and coordinated clini- cal research effort to ensure that NASA has the ability to provide health care commensurate with the expectations of the medical community and society at the time of launch beyond Earth orbit. Some of the clinical research op- portunities in space medicine are listed in Box 2-5. CONCLUSION AND RECOMMENDATION Conclusion NASA has devoted insufficient resources to developing and assessing the fundamental clinical information necessary for the safety of humans on long-duration missions beyond Earth orbit. • Although humans have flown in space for nearly four decades, a paucity of useful clinical data have been collected and analyzed. The reasons for this include inadequate funding; competing mission priori- ties; and insufficient attention to research, analysis including insufficient investigator access to data and biological samples, and the scientific method. • Although NASA’s current approach to addressing health issues through the use of engineering design and countermeasures has been successful for short-duration missions, deep space is a unique environ- ment that requires a different approach. • A major problem of space medicine research is the small number of astronaut research participants, which requires special design and analy- sis of the data from clinical trials with small numbers of participants. This necessitates a strategy focused on maximizing opportunities for learning. Recommendation NASA should develop a strategic health care research plan designed to increase the knowledge base about the risks to humans and their physiological and psychological adaptations to long-duration space travel; the pathophysiology of changes associated with environmental forces and disease processes in space; prediction, development, and validation of preventive, diagnostic, therapeutic, and rehabilitative

72 SAFE PASSAGE BOX 2-5 Clinical Research Opportunities for Astronaut Health Musculoskeletal System 1. Establishing the course of changes in bone mineral density and markers of bone mineral density turnover in serum and urine before, during, and after space travel. 2. Developing a capacity for real-time measurement of bone mineral density and enhanced three-dimensional technology to assess the risk of fracture during space travel. 3. Identifying human phenotypes and genotypes resistant to space travel-induced bone mineral density loss. 4. Tailoring therapeutic interventions (i.e., countermeasures such as diet, exer- cise, and medications) as a high priority and then validating the promising counter- measures in studies with astronauts during exposure to microgravity. Cardiovascular 5. Considering artificial gravity and pharmacological interventions as solutions to orthostatic hypotension. Gastrointestinal 6. Investigating the relationship between space motion sickness and absent bowel sounds including pharmacological and adaptive countermeasures. Nervous System 7. Building a coordinated clinical research program that addresses the issues of neurological safety and care for astronauts during long-duration space travel. 8. Performing pharmacological trials with dose-response and pharmacokinetic measures to assess the efficacies and toxicities of medications commonly used to treat sleep disturbances during space travel. 9. Considering clinical trials on the use of growth hormone or other countermea- sures and developing devices to control spacecraft ambient light and the core temper- ature at appropriate levels during space travel to reduce sleep disturbances. Reproductive Health 10. Determining whether radiation exposure during space travel causes genetic damage or altered fertility in men and women and, for women, premature ovarian failure. 11. Determining female and male reproductive hormone levels during space travel. 12. Determining the effect of microgravity on menstrual efflux and retrograde men- struation. Physiological Monitoring 13. Collecting clinical data for both men and women when anatomically possible and physiologically sensible for all individuals in the space program and, on a regular basis, subjecting the data to analysis for sex-related differences. 14. Giving priority to high-resolution, high-precision, yet minimally invasive or noninvasive methods for the monitoring of physiological parameters and for imaging of the human body during space travel.

RISKS TO ASTRONAUT HEALTH DURING SPACE TRAVEL 73 measures for pathophysiological changes including those that are as- sociated with aging; and the care of astronauts during space missions. The strategic research plan should be systematic, prospective, com- prehensive, periodically reviewed and revised, and transparent to the astronauts, the research community, and the public. It should focus on • providing an understanding of basic pathophysiological mecha- nisms by a systems approach; • using the International Space Station as the primary test bed for fundamental and human-based biological and behavioral research; • using more extensively analog environments that already exist and that have yet to be developed; • using the research strengths of the federal government, univer- sities, and industry, including pharmaceutical, bioengineering, medi- cal device, and biotechnology firms; and • developing the health care system for astronauts as a research database.

Astronaut Charles Conrad, Jr., commander of the first manned Skylab mission, undergo- ing a dental examination by Medical Officer Joseph Kerwin, M.D., in the Skylab 2 Medi- cal Facility during Earth orbit on June 22, 1973. In the absence of an examination chair, Conrad simply rotated his body to an upside down position to facilitate the procedure. NASA image. 74

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Safe Passage: Astronaut Care for Exploration Missions sets forth a vision for space medicine as it applies to deep space voyage. As space missions increase in duration from months to years and extend well beyond Earth's orbit, so will the attendant risks of working in these extreme and isolated environmental conditions. Hazards to astronaut health range from greater radiation exposure and loss of bone and muscle density to intensified psychological stress from living with others in a confined space. Going beyond the body of biomedical research, the report examines existing space medicine clinical and behavioral research and health care data and the policies attendant to them. It describes why not enough is known today about the dangers of prolonged travel to enable humans to venture into deep space in a safe and sane manner. The report makes a number of recommendations concerning NASA's structure for clinical and behavioral research, on the need for a comprehensive astronaut health care system and on an approach to communicating health and safety risks to astronauts, their families, and the public.

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