National Academies Press: OpenBook

Safe Passage: Astronaut Care for Exploration Missions (2001)

Chapter: 2 Risks to Astronaut Health During Space Travel

« Previous: 1 Astronaut Health Beyond Earth Orbit
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.
×

Payload Specialist Jay C.Buckley, Jr., Payload Commander Richard M.Linnehan, and Astronaut Dafydd R. (Dave) Williams (left to right) during pulmonary function tests in support of the Neurolab mission aboard the Earth-orbiting space shuttle Columbia, April 24, 1998, during the STS-90 mission. NASA image.

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.
×

2
Risks to Astronaut Health During Space Travel

…we must assume that for a long time to come (although not forever), weightlessness 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 functioning 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 unanswered 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

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.
×

microgravity are known, and surprises may yet be in store as humans venture longer and farther into space. This chapter is about human physiological adaptation to space travel. In it, the Institute of Medicine (IOM) Committee 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 Program (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 committee 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 independent 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 missions 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

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.
×

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 arranged 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 important aspect of detecting, understanding, and countering the untoward physiological changes that may affect astronaut well-being and mission performance.

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.
×

Finally, the chapter includes a discussion of the comprehensive, long-range approach to clinical research that NASA needs to consider implementing 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 human 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 measures (i.e., countermeasures) to guard against or reverse the potential pathophysiological 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 erythrocyte 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,

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.
×

FIGURE 2–1 Countermeasure (CM) evolution. Source: Paloski, 2000.

  1. formulate a countermeasure strategy based upon that physiological understanding,

  2. test the countermeasure and demonstrate its efficacy on the ground, and

  3. validate the countermeasure in space.

It has been difficult for NASA to design and test effective countermeasures, and no single countermeasure has yet to be validated as clinically efficacious. 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 clinical research laboratory in microgravity to investigate—and ultimately prevent—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

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.
×

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 development. 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., fracture 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 mechanisms underlying the loss of bone mineral density in microgravity; hence, scant progress has been made on the development of effective countermeasures. 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

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.
×

NRC 1998b; Lane et al., 1999; Lane and Schoeller, 2000). Thus, it is difficult, 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 resorption 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, femur, 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 presumably 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 database to determine whether individuals with phenotypes that make them “resistant” 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.

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.
×

TABLE 2–1 Average Bone Mineral Density Loss on Mir

Variable

Number of Crewmembers

Mean Loss (Percent/Month)

Standard 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 vitamin D).

Reversibility, Genetic Variability, and Mechanism of Bone Mineral Density Loss

Although data are limited, it appears that changes in calcium metabolism and bone mineral density are reversible. There are suggestions, however, 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

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.
×

osteoporosis. As such, it may be appropriate to consider terrestrial referenced 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 markers of bone mineral density turnover all contribute to the difficulty of obtaining 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 density loss during space missions are bone mass as measured by bone densitometry and bone turnover rates as measured by markers of bone density in serum and urine determined before the initiation of a space mission. Although 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 missions (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 provide 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-

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.
×

relation between rat bone metabolism and the bone architecture in humans (SSB and NRC, 1998a, 2000). The development of land-based animal models 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—successes 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 improving 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 during space travel. In addition, it is anticipated that better documentation of the validity of bone mineral density markers in evaluations of the mechanisms of bone mineral density loss would also provide a means for optimization 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 countermeasure 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.

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.
×

Because of the significant current research effort into the prevention and treatment of osteoporosis, new techniques that better assess bone mineral density turnover and fracture susceptibility can be anticipated within the next decade. These techniques should be evaluated, adapted if necessary, 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 analyses 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 abnormal gait that persisted, suggesting permanent damage to the neuromuscular pathway (Walton et al., 1997). Significant atrophy has been observed in human muscle after only 5 days in space, but the time course of deterioration 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).

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.
×

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 soreness. In rats, changes in microcirculation occur during space travel as a result 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 cardiovascular 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) concluded that existing cycling, rowing, and treadmill exercise protocols did not maintain muscle mass or a positive nitrogen balance. A significant contributing 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 countermeasure that uses developments in advanced technology. In addition, the possibility of pharmacological intervention apparently has not been explored, 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

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.
×

orthostatic hypotension, a fall in blood pressure and the associated dizziness, 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 peripheral 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 gravity. 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 uncomfortable or annoying, or both, but life-threatening complications have not been reported.

It is generally accepted that there is at least a general relationship between the duration of exposure to microgravity and the duration of the period of recovery from the orthostatic hypotension. However, few data are available concerning the severity of orthostatic hypotension, and the available information concerning the relationship between the duration of exposure to microgravity and the degree and duration of orthostatic hypotension is inadequate. No information is yet available on how orthostatic hypotension 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, however, 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 pharmacological interventions as solutions to orthostatic hypotension.

Other Effects on the Cardiovascular System

The cardiovascular system undergoes several other physiological adaptations 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 symptoms, 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

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.
×

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 restoration 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 maintenance of cerebral flow at rest. During orthostatic testing, lower-limb vascular resistance does not compensate for the fluid shift (Arbeille et al., 1996).

The changes in intravascular volume during space travel lead to corresponding changes in stroke volume and cardiac output. For example, corresponding 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 weightlessness (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 potentially 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 “serious” at this time. Substantial new research is required to define the degree of risk, the likelihood and severity of cardiac alteration, and reasonable therapeutic options for long-duration space travel.

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.
×

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 ventilation (Prisk et al., 1995b, West et al., 1997). There is decreased dead space with normal oxygen uptake and an improved carbon dioxide diffusion capacity (Prisk et al., 1995a,b). The overall improvement in lung function cannot be completely explained by what had been supposed to be gravity-induced 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 academic 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 treatment in a microgravity environment is space motion sickness (SMS) (Putcha et al., 1999). SMS is a syndrome consisting of headache, malaise, disorientation, 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

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.
×

al., 1997) are factors that have been implicated. Although SMS is largely considered a “minor” disorder, its effects are serious enough that extravehicular 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 patients 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, particularly if there is no intake of solid food. This gastrointestinal problem appears to be self-limiting, and overtreatment with laxatives may cause diarrhea, which can be a difficult problem to deal with in the space environment. No additional predictable problems with ileus would be expected on long-term spaceflights. Valuable information may be gathered from astronauts with extended stays on the ISS, and further clinical data may be obtained from studies investigating the etiology of SMS.

Clinical Research Opportunity 6. Investigating the relationship between space motion sickness and absent bowel sounds including pharmacological and adaptive countermeasures.

NERVOUS SYSTEM

Useful data on neurovestibular function, sleep (circadian rhythms), eyehand coordination, fine and gross motor functions, and visual perception and reorientation have been collected in real-time, post-space travel studies

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.
×

and from studies with animals. However, little or no readily accessible data are available on sensation and proprioception. Some data are available, however, 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 results of controlled trials, dose-response trials, or comparative efficacy studies 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 duration 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 disruptions and associated abnormalities. These can result from disruption of the hypothalamic-pituitary axis, with the resultant release of growth hormone, and changes in cortisol peaks and valleys. Disruptions of the circadian rhythm may result in abnormal stress responses, diminished performance because of fatigue, and mood and behavioral changes (Mullington et al., 1996). Although astronauts frequently use sleep medication, it has been

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.
×

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 vigilance 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 architecture 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 accommodate 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 accomplished 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 coming 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 contribute to the development of safe and effective countermeasures for associated performance 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.

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.
×

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 during 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 during 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 cognition, 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 functioning 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 vigilance 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 mention of sensorimotor status during or upon the return from space missions, yet sensorimotor status is an important component of muscle activation.

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.
×

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. Systematic study of sensorimotor status in microgravity is important to understanding the contributions of neurological and behavioral factors to human performance 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 occupied 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. Exposure 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 genetic consequences of exposure of the gonads to radiation are of greater concern to many.

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.
×
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 effective enzymatic repair systems, the ovary is more resistant to radiation-induced genetic effects; however, the effect of radiation on oocytes is cumulative. 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 exposure 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 hypophyseal-pituitary-gonadal axis, some evidence for reversible testicular dysfunction has been found. Reductions in testosterone levels have been reported 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 luteinizing hormone, a decrease in testosterone levels may indicate a disturbance in the hypophyseal-pituitary-gonadal axis. If this is the case, one might expect abnormalities of germinal epithelial function and, thus, diminished sperm production as well.

Clinical Research Opportunity 11. Determining female and male reproductive hormone levels during space travel.

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.
×
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 selected into an astronaut class is 32 years, and many have not had children. Of 99 female finalist candidates examined during five selection cycles between 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 development of endometriosis and consequent infertility (Sampson, 1927; Scott et al., 1953; Jennings and Baker, 2000). Many women experience some retrograde menstruation that, at Earth’s gravity, is usually confined to the pelvis. 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 symptoms, 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 hyperplasia 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 associated with decreased bone mineral density. The effect of microgravity, in addition to the effects of stress and an exercise program necessary to maintain cardiovascular and musculoskeletal well-being on long-duration flights, may increase the risk of developing one of these conditions. Exogenous

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.
×

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 reasons 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 analyses. 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 expect sex differences to exist. The report lists other areas in which the prediction 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 concern that are also addressed elsewhere in this report.

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.
×

In addition to identifying areas of concern for female astronaut health care, including pregnancy and the pharmacodynamics of oral contraceptives, 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 design preclude some individuals from performing some tasks (Seddon et al., 1999). A stated goal described in the workshop report of Seddon and colleagues (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 excretion in the urine are well-identified concerns of space travel, with an expected incidence of 0 to 5 percent. The effects of microgravity on the urinary 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 recommended 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 desirable 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

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.
×

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 formation 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 environment. 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 nanotechnology, 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 medicine 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, highprecision, 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 dramatically alter monitoring capabilities during space missions. Technology may be used to minimize the intrusiveness of testing by developing means of obtaining 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 determination of complete blood counts. The application of nanotechnology to human fluid analysis, genomic analyses, and pharmacogenetics will also likely be routine aspects of future space travel. NASA’s hosting of periodic international nanotechnology conferences to bring engineers and biologists to-

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.
×

gether and the agency’s collaborative initiative in cell biology with the National 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-precision, 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-disciplinary strategy to assess, understand, mitigate, and manage the risks associated with long-term exposure to the space environment. Initiated during 1997 and 1998, it is an iterative approach of review, analysis, and deliberations 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 illustrated in Box 2–3. Fifty-five risks have been identified and stratified (Table 2–2). The goal is to identify and validate clinical interventions (countermeasures) 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.

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.
×

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 occurrence 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.

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.
×

TABLE 2–2 Critical Path Roadmap Project: Critical Risks. SOURCE: Charles, 2000

Bone Loss

Cardiovascular Alterations

Human Behavior and Performance

Immunology, Infection and Hematology

Muscle Alterations and Atrophy

Acceleration of age-related osteoporosis

 

Human performance failure because of poor psychosocial adaptation

 

 

Fractures (traumatic, stress, avulsion), and impaired healing of fractures

Occurrence of serious cardiac dysrhythmias

Human performance failure because of sleep and circadian rhythm problems

 

Loss of skeletal muscle mass, strength, or endurance

 

Impaired cardiovascular response to orthostatic stress

 

Inability to adequately perform tasks due to motor 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

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.
×

Neurovestibular Adaptation

Radiation Effects

Clinical Capability

Other

 

Carcinogenesis caused by radiation

Trauma and acute medical problems

 

Severe Risks

Disorientation and inability to perform landing, egress, or other physical tasks, especially during/ after g-level changes

Damage to central nervous system from radiation exposure

Toxic exposure

Inadequate nutrition (malnutrition) (three risks)

Very Serious Risks

Impaired neuromuscular coordination and/or strength

Synergistic effects from exposure to radiation, microgravity and other environmental factors

Altered pharmacodynamics and adverse drug reactions

Postlanding alterations in various systems resulting in severe performance decrements and injuries

 

 

Early or acute effects from radiation exposure

 

Habitation and life support (eight risks)

 

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.
×

Bone Loss

Cardiovascular Alterations

Human Behavior and Performance

Immunology, Infection and Hematology

Muscle Alterations and Atrophy

Injury to connective tissue or joint cartilage, or intervertebral disc rupture with or without neurological complications

Diminished cardiac function

Human performance failure because of human system interface problems and ineffective habitat and equipment design, etc.

Immuno-deficiency/ infections

Propensity to develop muscle injury, connective tissue dysfunction, and bone fractures due to deficiencies in motor skill, muscle strength, and muscular fatigue

Renal stone formation

Manifestation of previously asymptomatic cardiovascular disease

Human performance failure because of neurobehavioral dysfunction

Carcinogenesis caused by immune system changes

 

 

Impaired cardiovascular response to

 

Altered hemo-and cardio-dynamics from altered blood components

Impact of deficits in skeletal muscle exercise stress structure and function on other systems

 

Altered wound healing Altered host-microbial interactions Allergies and hypersensitivity reactions

 

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.
×

Neurovestibular Adaptation

Radiation Effects

Clinical Capability

Other

Impaired cognitive and/or physical performance due to motion sickness symptoms or treatments, especially during/ after g-level changes

Radiation effects on fertility, sterility and heredity

Illness and ambulatory health problems

 

Serious Risks

Vestibular contribution to cardioregulatory dysfunction

 

Development and treatment of decompression illness complicated by microgravity-induced deconditioning

 

 

Possible chronic impairment of orientation or balance function due to microgravity or radiation

 

Difficulty of rehabilitation following landing (two risks)

 

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.
×

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 development 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 reevaluated understanding of clinical research on astronaut health into the Critical Path Roadmap project in any of the information that it provided to the committee 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

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.
×

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 animals.

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 natural history of disease; therapeutic interventions, including clinical trials; prevention and health promotion; behavioral research; health services research, including outcomes; epidemiology; and community-based and managed care-based research.

Source: AAMC, 1999, p. 4.

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.
×

changes and responses make it even more difficult to evaluate results because 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 account for the limited success of countermeasures to date. This does not imply 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 numbers 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 efficacies 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 increased 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 countermeasures, can largely be conducted only during space missions. There does not appear to be a clearly visible and transparent plan of who oversees, designs, reviews, and is responsible for this clinical research. NASA’s organizational charts, lines of authority and responsibility, administration, and budgeting 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 fragmented, nonuniform process of strategic planning and oversight that is insufficient 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

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.
×

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 clinical 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 opportunities 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 priorities; 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 environment 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 analysis 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

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.
×

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, exercise, and medications) as a high priority and then validating the promising countermeasures in studies with astronauts during exposure to microgravity.

Cardiovascular

  1. Considering artificial gravity and pharmacological interventions as solutions to orthostatic hypotension.

Gastrointestinal

  1. Investigating the relationship between space motion sickness and absent bowel sounds including pharmacological and adaptive countermeasures.

Nervous System

  1. Building a coordinated clinical research program that addresses the issues of neurological safety and care for astronauts during long-duration space travel.

  2. 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.

  3. Considering clinical trials on the use of growth hormone or other countermeasures and developing devices to control spacecraft ambient light and the core temperature at appropriate levels during space travel to reduce sleep disturbances.

Reproductive Health

  1. Determining whether radiation exposure during space travel causes genetic damage or altered fertility in men and women and, for women, premature ovarian failure.

  2. Determining female and male reproductive hormone levels during space travel.

  3. Determining the effect of microgravity on menstrual efflux and retrograde menstruation.

Physiological Monitoring

  1. 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.

  2. 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.

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.
×

measures for pathophysiological changes including those that are associated with aging; and the care of astronauts during space missions.

The strategic research plan should be systematic, prospective, comprehensive, 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 mechanisms 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, universities, and industry, including pharmaceutical, bioengineering, medical device, and biotechnology firms; and

  • developing the health care system for astronauts as a research database.

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.
×
Page 36
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.
×
Page 37
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.
×
Page 38
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.
×
Page 39
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.
×
Page 40
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.
×
Page 41
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.
×
Page 42
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.
×
Page 43
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.
×
Page 44
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.
×
Page 45
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.
×
Page 46
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.
×
Page 47
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.
×
Page 48
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.
×
Page 49
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.
×
Page 50
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.
×
Page 51
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.
×
Page 52
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.
×
Page 53
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.
×
Page 54
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.
×
Page 55
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.
×
Page 56
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.
×
Page 57
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.
×
Page 58
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.
×
Page 59
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.
×
Page 60
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.
×
Page 61
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.
×
Page 62
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.
×
Page 63
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.
×
Page 64
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.
×
Page 65
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.
×
Page 66
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.
×
Page 67
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.
×
Page 68
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.
×
Page 69
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.
×
Page 70
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.
×
Page 71
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.
×
Page 72
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.
×
Page 73
Next: 3 Managing Risks to Astronaut Health »
Safe Passage: Astronaut Care for Exploration Missions Get This Book
×
Buy Paperback | $80.00 Buy Ebook | $64.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!