Historical Perspective—X-15 To The Iss And Beyond

There have been very few suborbital* flights in the history of human space flight (Table 114-1, A). These early flights in the “Space Race” demonstrated human survivability in the space environment and tested early technology. Once these objectives were accomplished, attention shifted to orbital and lunar flights.

Until recently, U.S. suborbital flights were limited to two 1961 NASA Mercury flights and two 1963 USAF North American Aviation X-15 program flights that reached an altitude of 100 km (62 miles). In 2004, after a 40-year hiatus, the X-Prize flights by Scaled Composites LLC’s SpaceShip One were heralded as ushering in a new era of commercial space flight to suborbital heights.

Back in the 1960s, the Mercury program was designed to evaluate physiologic responses to the space-flight environment, including:

The suborbital Mercury flight of Alan B. Shepard Jr. on May 5, 1961, reached a peak of 116 statute miles. He was weightless for less than 5 minutes. The second suborbital flight was made by NASA Astronaut I. Virgil “Gus” Grissom on July 21, 1961, to an altitude of 118 statute miles, and distance of 303 miles downrange.

During these suborbital flights, parameters measured were body temperature, respiration (measured by several different methods, none of which gave reliable respiration traces), and electrocardiogram (ECG).

During the Mercury program, life sciences questions tended to focus purely on survivability in the space environment and ignored many of the normal physiologic functions deemed not (yet) relevant. For example, Alan Shepard’s spacesuit for the first Mercury flight Freedom 7 (Figure 114-1 and Table 114-1, B, online) didn’t even provide him with a way to relieve himself (a perhaps forgivable oversight, because the flight was only scheduled to last 15 minutes; unfortunately there was a lengthy delay on the launch pad).⁶¹ It was not until the NASA Mercury Project MA-6 mission of John H. Glenn that blood pressure readings were taken.

FIGURE 114-1 The Mercury-Redstone 3 rocket Freedom 7, carrying Alan Shepard on the first American manned suborbital flight, launches on May 5, 1961.

(From NASA Image Exchange. http://nix.larc.nasa.gov/info;jsessionid=1snd99vh206gk?id=GPN-2000-000859&orgid=12.)

TABLE 114-1, B Space Exploration Vehicles and Crew Compliment

Note: All ISS missions have had visiting vehicles, increasing the crewmembers to a maximum of 13 in March 2009 with the simultaneous docking of STS 119 and Soyuz TMA 14.

Research determined that peak physiologic responses were closely related to critical in-flight events. The Mercury astronauts showed themselves capable of normal physiologic function and performance during launch, in-flight, and reentry. They tolerated the acceleration and vibration well and gave no evidence of motion sickness. The imposed heat loads caused discomfort upon occasion, but did not become a limiting factor during the missions.

All six NASA Mercury crewmembers returned to Earth in healthy condition. They were able to conduct complex visual-motor coordination tasks proficiently in the weightless state, and no evidence of physiologic dysfunction occurred during the flights. The principle findings were weight loss due to dehydration with mild cardiovascular impairment. There was some indication of post-flight orthostatic intolerance and hemoconcentration; however, frank signs of orthostatic hypotension were not noted in these suborbital flights.^53,96

To carry out the Gemini program’s (1965-1966) flight durations from under 5 hours to nearly 14 days, researchers needed to know how best to help humans survive in the microgravity environment for longer periods, and investigations began into what activities humans could successfully do in that environment. Mission objectives for the Gemini astronauts included subjecting humans (and their equipment) to space flights of up to 2 weeks in duration, accomplishing rendezvous and docking maneuvers with orbiting vehicles, and conducting the first spacewalk (or extravehicular activity [EVA]).

The main physiologic effects experienced and observed in the NASA Gemini missions were:⁷¹

During the Apollo program (1968-1972), the body’s reaction to partial gravitational states (i.e., forces between microgravity and Earth’s 1G) became important for the first time, along with questions as to whether the physiologic adjustments to the Moon’s G would impair the astronauts’ ability to do meaningful work while on the surface. In point of fact, the crew were not only able to drive the lunar rover but also to collect geological samples and to play golf!

The Apollo program findings were similar to those of Gemini, with the addition of:⁵¹

Skylab (1973-1974) was America’s first space station, as well as its first attempt to investigate the effects of long-duration (up to 84 days) exposure to microgravity. Associated issues, such as exercise programs, menu variety, and psychological concerns, received heightened attention.

Although the Apollo-Soyuz flight in July 1975 was as much for political purposes as for scientific ones, it added to the knowledge base by allowing comparisons between the Soviet and American methods for handling physiologic adaptations and combining the environmental systems of two very different vehicles.

More recently, the success of the Shuttle program has led to feelings of familiarity and confidence with short-duration (approximately 2-week) flights to low Earth orbit (LEO), but tragically there have also been two total mission failures, resulting in the deaths of 14 crewmembers and loss of the Challenger (OV-99) and Columbia (OV-102) shuttles.

An important legacy of the Shuttle program will be numerous medical and physiologic experiments conducted on board, including those during the four Shuttle/Spacelab Module missions.

The International Space Station (ISS) program began crewed operations in October 2000, with small but increasing numbers of crew. The program now has six permanent crewmembers, with U.S., Russian, European, and Japanese laboratories. Although the ISS missions, along with historical data from the Skylab, Soviet Salyut, and Russian Mir space station programs, have provided information about long-term effects of microgravity on the human body, much of what is “known” has been extrapolated from short-duration Shuttle missions. In addition to lingering questions about the mechanisms of adaptation to and from microgravity, and the associated time courses and best ways to prevent or recover from undesirable changes, research also continues into how ground-based personnel can best support long-duration crews.

Time Course Of Changes And Adaptation To Microgravity

The microgravity environment provokes numerous changes to the space traveler’s physiology. Some of these alterations occur immediately, whereas others have a more delayed onset, and these effects can be short-lived or last for the duration of the mission. As a result, “space normal” physiology can be difficult to define, and the ability of an astronaut to respond to a physiologic insult, such as injury or illness, can also be challenging to predict. This becomes an even more difficult problem with commercial space flight, where the driving force is the need to fly fare-paying passengers whose health may not be optimal in vehicles that are likely to have minimal medical capabilities.

The physiology of the space traveler is most labile during the transition to or from microgravity. Within a few days, the body adapts to its new environment (as described later), but during the first 72 hours following a change to or from microgravity, most of the physiologic processes are in a state of flux. With space tourists on suborbital flights, there may be a rise in medical events because of subclinical conditions that may not be discovered (or admitted to) preflight.^41,59

Long-Term Effects

Several physiologic systems exhibit microgravity-related effects over a longer time frame (weeks to months) (Figure 114-2). For short-duration missions, these changes may be minor or even undetectable, but on longer flights, the effects can become pronounced. In some systems, it is unclear whether a “space normal” equilibrium is ever achieved, or whether changes continue so long as the crewmember remains in microgravity. Examples include alterations in red cell mass and bone demineralization.

FIGURE 114-2 Time course of physiologic changes.

(From DeHart RL, Davis JR, editors: Fundamentals of aerospace medicine, ed 3, Philadelphia, 2002, Lippincott Williams & Wilkins.)

Effects On Human Physiology

Effects on the Cardiovascular and Pulmonary Systems

The cardiovascular system undergoes several predictable adaptations in response to microgravity. Recalibration of baroreceptor homeostasis occurs after several days in space, likely due to cephalad fluid shifts induced by the microgravity environment²⁷ (Figure 114-3). As a result of these shifts, the carotid baroreceptors sense a (relative) hypervolemia and cause a diuresis. Relative hypovolemia (approximately 10% decrease in total body fluid and a fall in central venous pressure (CVP) from a terrestrial average of 7 to 10 mm Hg to 0 to 2 mm Hg) then exists compared with preflight terrestrial fluid status.¹⁵ The fluid shift may also be associated with development of space motion sickness (SMS), with resultant nausea and vomiting that lead to further loss of fluids.

FIGURE 114-3 The cephalad fluid shift is measured by determining venous pressure in the jugular vein.

(From NP-119 Science in Orbit: The Shuttle & Spacelab Experience, 1981-1986. http://history.nasa.gov/NP-119/ch2.htm.)

Although there is still debate on the effects of microgravity on hemodynamic parameters, the most recent data suggest that heart rate and diastolic blood pressure decrease and cardiac output increases. This implies reduction in peripheral vascular resistance and a similar decrease in sympathetic tone. Pulmonary physiology testing has revealed a decrease in tidal volume and an increase in respiratory rate.¹⁰² There is also a decrease in dead space and improved CO₂ diffusion capacity, possibly because of increased intrathoracic perfusion.⁸³

Although these changes have not been demonstrated to affect human health or cardiovascular stamina during spaceflight, orthostatic intolerance following return to Earth’s gravity remains a concern. Compounding this effect may be a loss of adrenergic responsiveness; in a number of returning astronauts, lower norepinephrine levels have been found to correlate with deficient responses to orthostatic stress.^15,88

The severity of orthostasis is generally proportional to the duration of time spent in space and poses a problem if an emergency situation requires crew mobility immediately on landing, especially following a long-duration flight.¹¹ It is also a significant obstacle to planning a mission to Mars, where the crew will be required to function independently immediately on entering a -G gravity environment after several months’ travel in microgravity. Engineering solutions to these problems may necessitate the use of fully automated landing systems.

Another source of concern is the effect of exposure to the space environment on subclinical pathology, even for very short duration flights. Consider a potential commercial space flight participant with controlled congestive heart failure (CHF). This may manifest itself as clinically significant CHF in microgravity or on return to normal gravity. There are no data about or experience in placing patients with subclinical cardiac dysfunction into space, or treatment of its sequelae in or post flight.

Several strategies are currently used as countermeasures against postflight cardiovascular dysfunction in the fit astronaut population. Five-bladder anti-G suits and liquid cooling garments (LCGs) have become standard equipment during landing, after experience from short-duration Shuttle flights demonstrated that they reduced orthostatic hypotension on landing. The former presumably is effective by minimizing venous pooling in the extremities, whereas the latter reduces heat stress, sweating, and dehydration during the high cabin temperatures of reentry.²² Another standard protocol is oral fluid loading with isotonic solutions (such as bouillon or a sports drink–like concoction called Astro-Ade) performed 2 hours prior to landing. This is a short-acting measure to mitigate the relative hypovolemia of spaceflight.

Previous experience with longer duration (>4 months) missions, such as those to the Mir space station, suggested that regular in-flight cardiovascular exercise benefited orthostatic stamina on reentry, and a robust exercise regimen is now standard on all ISS missions.

The Russians also employ another in-flight countermeasure to improve postflight orthostatic intolerance: a lower body negative pressure (LBNP) device (or Russian “Chibis” suit) (Figure 114-4, online). This device draws blood away from the central circulation and into the lower extremities through a negative pressure gradient, simulating a gravitational stress on the cardiovascular system.²⁵ The Russians have also employed “brazlets,” or thigh cuffs, to encourage greater blood distribution in the lower extremities. Fortunately, even severe postflight orthostatic intolerance is a temporary event and within a few days, virtually all long-duration flight crews are able to mount an appropriate response to orthostatic stress.

FIGURE 114-4 A Russian “Chibis” suit makes use of lower body negative pressure to offset orthostatic changes.

(From http://vsm.host.ru/r_photos.htm.)

For commercial spaceflight participants, it may be appropriate to take a preventive approach; however, there are no data available to determine who is likely to have a problem and therefore how to pursue more comprehensive tests or screening. The FAA has proposed medical standards for both Crew and Spaceflight Participants.

Effects on the Neurovestibular and Sensory Systems

In the terrestrial environment, gravity plays an important role in neurovestibular homeostasis, providing an internal compass for proprioception. Acute exposure to microgravity disrupts interpretation of linear and angular forces in the vestibular organs and may alter integration of visual and proprioceptive input.

One of the earliest effects of these changes is SMS. This form of motion sickness, associated with exposure to microgravity, often manifests as headache, nausea, vomiting, anorexia, poor concentration, or general malaise. It is usually a minor and self-limited condition (generally lasting for the first 1 to 2 days in orbit), yet it is one of the most common reasons for pharmacologic intervention in space (most commonly treated with parenteral or rectal promethazine, 25 to 50 mg, administered at bedtime on the first in-flight day).⁸³ On occasion, however, SMS has persisted for the duration of a Shuttle flight, significantly impairing the affected crewmember’s performance.

SMS is believed to be related to fluid redistribution patterns and/or alterations in bowel motility. It is only weakly correlated with the motion sickness associated with ship or air travel. The motion sickness experienced during parabolic flights on NASA’s DC-8 Reduced Gravity Research Program (Figure 114-5), during which passengers experience intervals of microgravity, has been anecdotally reported to correlate poorly with in-flight SMS.^34,94

FIGURE 114-5 The Reduced-Gravity Program provides the unique weightless or zero-G environment of space flight for testing and training of human and hardware reactions. The reduced-gravity environment is obtained with a specially modified KC-135A turbojet transport that flies parabolic arcs.

(From NASA Image Exchange. http://nix.ksc.nasa.gov/info;jsessionid=31kakwlvw0uun?id=MSFC-0001123&orgid=11.)

SMS affects more than 70% of space travelers. Reliable prediction of its occurrence (particularly in first-time flyers) is difficult.²² The incidence is somewhat lower among repeat flyers, suggesting some training effect.⁵

The operational impact of SMS is considerable. The first in-flight timeline change related to a medical cause occurred because a crewmember suffered SMS. To this day, extravehicular activities (EVAs) are not scheduled within the first few days of a mission because of concerns regarding the effects of SMS during a spacewalk (in particular, the potential for aspiration and fouling of a spacesuit).

Even though SMS can be expected to have resolved several days into a mission, other neurovestibular and neurosensory changes persist, including alterations in eye–head coordination, target tracking, and optokinetic reflex function.^5,27 These adaptations can significantly affect delicate technical operations, such as manipulation of the robotic arm or manually controlled docking maneuvers. In fact, neurovestibular dysfunction was implicated as one cause in the collision of the Russian space station Mir with a Progress resupply vessel in 1997, as well as the “bumping” of a satellite during a capture attempt by the Shuttle’s robotic arm.

Neurovestibular dysfunction has also been suggested as a causative factor in the correlation between Space Shuttle mission duration and accuracy of landing speed, position, and touchdown vertical velocity. As mission duration increases, landing accuracy decreases, leading to concerns about the safety of manual landings following long-duration missions, such as exploration missions to Mars. This may necessitate an engineering approach to develop primary automated landing capabilities and demonstrates why physicians and other life scientists need to take part in mission design and planning as well as participate in ongoing operations. The Shuttle has extended flight capabilities to stay in orbit for 30 days. However, it is unlikely that a crew could have landed the Shuttle in its current configuration with the crew sitting upright, after more than 20 days in orbit.

About 80% of space travelers experience perceptual illusions during or after flight. Several different types have been reported: illusory self-motion (both linear and rotational), a sensation of the floor dropping when doing a squat to stand, the sensation of things floating in space, visual streaming (blurring), visual scene oscillation (oscillopsia), object position distortion, visual axis distortion (tilting or inversion), and platform stability illusion. Some crewmembers also experience a sense of being upside-down early in spaceflight.⁵ The term “EVA acrophobia” has been coined by spacewalking astronauts to describe a feeling that they might “fall off” their vehicle or “fall to Earth” (Figure 114-6).

FIGURE 114-6 Astronaut Franklin Chang-Diaz participates in an EVA on the ISS; some astronauts report feeling that they might “fall off” the space station.

(From NASA Human Space Flight Gallery. http://spaceflight.nasa.gov/gallery/images/shuttle/sts-111/html/s111e5132.html.)

Restoring sensory references is one goal of neurovestibular countermeasures. Research is currently underway using preflight virtual reality training to simulate conflicts between the proprioceptive and visual systems and to help flyers become accustomed to the ISS geometry prior to arrival. ISS modules also have fluorescent directional guides and standardized coloration of “floor” and “ceiling” surfaces.^28,99

Crewmembers tend to choose an external frame of reference to orient themselves. “Down” direction may be toward Earth, the vehicle “floor,” or wherever their feet may be. Crewmembers using the latter strategy (“my feet = down”) seem to experience less disorientation when encountering an unexpected visual stimulus. Additional countermeasures include use of in-flight sensory input stimulators and exercises to reaffirm “up/down” visual cues. For example, watching a video while exercising on a treadmill has anecdotally improved postflight symptomatology on return to a 1G environment.

On return to Earth, the nervous system readapts to a 1G field remarkably well, often returning to preflight states within 48 hours. However, most crewmembers experience some postflight neurovestibular symptoms, including nausea, illusory movements, clumsiness, and vertigo.⁷ These symptoms are usually mild, but in the event of an emergency egress or bailout over the water, they could prove dangerous.⁷ Crewmembers in such an emergency not only will have to overcome any musculoskeletal and aerobic deconditioning, but are also likely to have to cope with unsteadiness, poor coordination, vertigo, and motion sickness. Pharmacologic prophylaxis (anti–motion sickness medications) were used for landings during the Skylab program, but were ultimately felt to be counterproductive because of side-effect profiles that included sedation. With short-duration suborbital flights, even mild SMS might significantly detract from the enjoyment for commercial space travelers.

Effects on the Musculoskeletal System

With exposure to microgravity, mechanical load on the musculoskeletal system lessens dramatically, leading to muscle atrophy (particularly in the postural/antigravity muscles) and bone demineralization (disuse osteoporosis). The latter is one of the high priorities of space physiology research, because osteoporosis remains a limiting factor for astronaut health during and after long-duration flight.⁷⁵ Changes are seen within the first few days of space flight and continue throughout exposure to microgravity, although they do not immediately become clinically significant.

Although the mechanism of this muscle atrophy is not well understood, initial studies suggest that the loss of muscle mass is associated with both decreased protein synthesis (approximately 15%) and increased rate of protein breakdown. As muscle atrophy begins, there is concomitant increase in urinary excretion of nitrogen compounds, 3-methylhistidine, creatinine, and sarcosine. Over the course of a long-duration mission, muscle mass losses of up to 50% can be seen (up to 20% on short-duration missions).

These changes are readily seen in the skinny “chicken legs” of a space traveler; although the cephalad fluid shift seen early in space flight can contribute to this effect, the fluid shift is complete after the first week or so of flight, but leg volume (particularly in the thigh area) continues to decrease throughout the course of a mission. Leg volume measurements do not revert to preflight levels immediately on rehydration and return to Earth, providing further evidence that the changes are primarily associated with muscle atrophy and not merely fluid shifts.

Other changes in muscle morphology include decreases in muscle volume, cross-sectional fiber area (although not fiber number), contractile proteins, oxidative-to–glycolytic enzyme ratio, and capillary density within muscle tissue. This contributes to a more anaerobic metabolic profile and decreased muscle strength and endurance.

Changes in bone structure appear to be caused by increased osteoclastic activity and are particularly pronounced in the spine and legs. Losses of up to 20% of preflight bone mass have been documented and can continue for several weeks (or months) following return to a 1G field. Interestingly, the gender-based differences in bone demineralization rates that are seen terrestrially do not exist in space; men and women lose calcium at approximately the same rate (1% to 2% per month in the lower limbs).

These changes in musculoskeletal structure and function have implications for the amount and type of physical activity that can be performed during space missions and also suggest that crewmembers may be at high risk of injury during and after flight. Animal studies have demonstrated a microgravity-associated reduction in muscle fiber regeneration and repair, which would imply that should an injury occur, recovery may be prolonged. Similarly, the relative reduction in osteoblastic activity could significantly impair healing of an in-flight fracture. Animal studies also reveal microgravity-related alterations in the architecture of bone, leading to further doubts about the body’s ability to properly repair a fracture in-flight.

The persistent diminution in bone and muscle mass is a source of great concern for exploration-class missions. Depending on the mission profile, travel to the destination could require several months, and crewmembers will likely be expected to engage in physical activity on the surface immediately (e.g., specimen collection, habitation module construction). If there have been significant losses in strength and fitness en route, injuries may occur or mission objectives may be compromised. Further exacerbating any physical weaknesses will be neurologic abnormalities in balance, coordination, and gait, all of which increase the risk of falls, fractures, dislocations, and other injuries. It is therefore critical to ensure that space travelers arrive in acceptable physical condition.

To that end, a robust countermeasures exercise program has been initiated on the ISS, making use of both strength and aerobic training. Because enthusiasm for exercise (and compliance with the scheduled program) varies from person to person, it is important for the aerospace physician to educate space travelers about the need for regular exercise, as well as to monitor fitness and strength levels during the course of a mission (see http://www.asc-csa.gc.ca/eng/astronauts/living_exercising.asp).

Mechanical countermeasures, such as low-intensity vibrations (to stimulate bone maintenance) or the Russian “penguin suit” (which makes use of elastic bands to force use of extensor muscles) (Figure 114-7, online), have been studied to determine efficacy. Unfortunately, none has as yet proved sufficiently useful to warrant routine use.¹¹ Pharmacologic and nutritional interventions, such as bisphosphonates and other antiosteoclastic agents or high-calcium diets, are currently under investigation as countermeasures, but none is (yet) routinely used. One concern is that use of drugs to alter calcium balance may have unintentional effects on renal physiology and enhance creation of kidney stones. Artificial gravity infrastructures could some day offer a solution to this problem, but current designs remain impractical for space travel in the near future.⁷²

FIGURE 114-7 The Russian “penguin suit,” with built-in resistance bands.

(From Russian/Soviet Space Suits. http://www.globaleffects.com/C_b06_frameset.html?Page=rentalC01_b06_S_russian.html.)

In the immediate postflight period, crewmembers experience muscle weakness, fatigue, and soreness. Whether this is due to readaptation of cellular processes (i.e., readjustment of protein synthesis and degradation pathways), unaccustomed strain associated with reexposure to gravity, or micro injuries within the muscle tissue is unclear.

Muscle mass and strength tend to recover rapidly on return to Earth and initiation of a postflight exercise regime. Within two weeks, muscle strength is generally recovered, although further research is needed regarding the possibility of any associated changes to connective tissue structures (such as tendons and ligaments).

Despite rigorous rehabilitation programs, full recovery of preflight bone mass can take years. In part, this is due to the slower time course for terrestrial mineralization (100 mg of calcium per day) compared to space-associated demineralization (250 mg per day), which suggests that it will take two to three times as long to recover bone mass as it did to lose it. In other words, it may take up to 18 months to recover the amount of calcium lost from bones during a 6-month mission. In addition, different parts of the skeleton lose and recover mass at different rates. For example, demineralization of the lumbar spine has been noted to linger for up to 6 months postflight, even as other parts of the spine were recovering and adding mass. As a result, there is a desire to maximize bone density before flight, and many crewmembers engage in preflight exercise programs to increase bone density. This is particularly important for groups at highest risk for the development of osteoporosis (e.g., white women, women with a positive family history).

Effects on the Immune System

Space travelers demonstrate a significant increase in the number of many types of circulating white blood cells (neutrophils, monocytes, T-helper cells, B cells), but a decrease in natural killer cells. It is not clear whether these changes are due to increased production or decreased destruction of the cells. Studies have shown that the increased plasma norepinephrine levels measured on landing correlate with increased white blood cells, suggesting that landing stresses produce sympathetic redistribution of circulating leukocytes.

Although studies to date have produced conflicting results, anecdotal data persistently indicate that immune function is compromised in microgravity. In vitro studies show that lymphocytes in-flight are largely unaffected by stimulating agents, suggesting that the white blood cells circulating in the blood, although more numerous than terrestrial levels, may be unable to mount an effective immune response.^21,153,154 Changes in leukocyte morphology have also been reported, further suggesting that immune function may be impaired (Figure 114-8, online).

FIGURE 114-8 White blood cells cultured in microgravity (left) did not reproduce at normal rates, but white blood cells cultured in space on the 1G centrifuge (right) proliferated normally.

(From NP-119 Science in Orbit: The Shuttle & Spacelab Experience, 1981-1986. http://history.nasa.gov/NP-119/ch2.htm.)

Some researchers who have documented an increase in plasma cortisol levels posit that it is associated with in vivo corticosteroid-induced immunosuppression. Because microorganisms may be more highly concentrated in the confined environment of a spacecraft, the issue of immunocompetence could be important. Whether the findings demonstrate changes related to the microgravity environment or merely stress-related effects on the immune system remains unclear. To date, there have been reports of (usually minor) illnesses during space flight, but no evidence that illnesses are more severe or longer lasting in space.

Subjects in analog environments, such as crews spending the winter in Antarctica, often display latent viral reactivation,⁵⁴ so it is not unreasonable to imagine that such an effect could be present in long-duration space flight.

Other than ensuring that space travelers are in good health and have had all standard vaccinations, no countermeasure program for the immune system is currently in place. The crew health care systems of the Shuttle and ISS contain medications, including antibiotics and antivirals, for treatment of disease, and the “health stabilization program” (or quarantine period) immediately before launch is intended to further diminish the likelihood of communicable disease during a mission. Unfortunately, the effects of space flight on the immune system are insufficiently understood to permit development of additional countermeasures at this time.

Although in-flight changes to the immune system remain unclear, current work focuses on identifying any alterations in structure and function, along with the associated time course(s). Until these trends have been clearly established, the aerospace physician must assume that immunocompromise, if present in-flight, may persist in the postflight period. As a result, space travelers should be cautioned about the risks of postflight exposure to infectious diseases and the importance of seeking medical help promptly should symptoms develop.

Stressful Environment: Psychological and Behavioral Issues

NASA has identified a number of behavioral issues that are thought to have a direct bearing on productivity and habitability during routine operations onboard a permanently manned space station:^68,102

• Sleep

• Clothing

• Exercise

• Medical support

• Personal hygiene

• Food preparation

• Group interaction

• Habitat aesthetics

• Outside communications

• Recreational opportunities

• Privacy and personal space

• Water disposal and management

• On-board training, simulation, and task preparation

• Behavioral and psychological requirements associated with microgravity

External sources of stress during space missions include limited ability to contact family and friends, real or imagined crises at home, supply shortages, a sense of “interference” from ground personnel, and feelings of rejection resulting from delays in relief. Internal sources of stress may include lack of privacy, forced social interaction, boredom, sexual and emotional deprivation, and perceived absence of status and role definition.

Studies show that isolated and confined populations frequently demonstrate mild psychiatric symptoms, including depression, anxiety, increased defensiveness, and belligerence.¹⁸ It has also been suggested that prolonged isolation and confinement harms group dynamics, with social irritability reported among polar expeditioners, submariners, and space-simulator subjects.¹⁸

Interpersonal conflict is a real and concerning threat to crew health and mission performance.³⁹ Cramped and confined spaces, cultural differences (often including language differences), and busy schedules replete with complex physical and mental tasks can often exacerbate stressed relationships. Ground support currently plays a large role in monitoring and providing intervention for crewmember strife, but this strategy will need modification for a mission to Mars, where transmission time delays can be as long as 40 minutes round-trip. This delay will not only diminish reliance on ground support for problem solving but may increase tensions between the crew and mission control. History is replete with strife between those on the “front” and the decision makers in the rear. During World War II, submariners on 2- to 3-month patrols complained about the “unrealistic” orders sent from rear echelon officers thousands of miles away. There have already been reports of animosity toward mission control from astronauts on long-duration missions, most famously during the first Skylab Mission. Indeed, ISS initially presented similar issues with unrealistic task timelines for crewmembers; much more emphasis is now placed on “free time” for crewmembers.

The main psychological effects on space travelers can be summarized as:⁴³

1 Impaired intellectual functioning: Isolation and confinement lead to decreases in alertness, concentration, and memory. It is worth noting that there are numerous anecdotal reports from astronauts that it is harder to think clearly in space.

2 Motivational decline: Prolonged isolation and confinement can result in loss of motivation. Polar expeditions have shown general decline in work satisfaction and group accomplishment, with fatigue, inertia, and general apathy occurring as the mission wears on.⁸⁶

3 Somatic complaints: Psychosomatic symptoms, including insomnia, headaches, sleep disturbance, digestive problems, toothache, and fears of acute surgical emergencies, have occurred both in space and analog environments. Anecdotal reports from Russian long-duration space crews have shown concern (bordering on obsession) with physical well-being at various phases of a mission.

4 Morale: Substantial evidence suggests that regardless of mission length, morale reaches its nadir between the one-half to two-thirds mark.

The consequences of psychological dysfunction have resulted in early abandonment of at least one Salyut mission, when cosmonaut Vladimir Vasyutin began showing signs of severe stress and depression after 2 months in space. He was hospitalized on his return. In another example, chronic high stress among the crew may have contributed to the 1997 collision between Mir and a resupply vehicle during a docking maneuver.

In-flight communications remain crucial for assessment and improvement of crew behavioral health. Crew access to medical and emotional support from family members and mental health professionals is essential for mission morale. Favorite meals and “care packages” from family are often dispatched on supply vehicle flights.

These issues become more problematic with customer-driven space tourism. It is relatively easy to impose discipline on a team that may in part be military and/or professional civilian crew. However, with a paying customer, the dynamics within a crew will invariably change and not necessarily for the better.

Lack of Privacy

Privacy (or the lack of it) is a feature not only of space exploration but also of other stressful environments. Shuttle crews are arguably at more of a disadvantage than are ISS crews, despite their shorter mission duration. The larger crew size and smaller habitable volume force Shuttle crews to live in extremely close quarters (Figure 114-9). By contrast, the larger ISS, even with its six-person crew, affords significantly more privacy and personal space, with crewmembers having individual “sleeping” quarters in the various modules of the ISS. That said, the tiny crew complement ensures that there are few secrets on board.

FIGURE 114-9 Sleeping astronauts on the crowded Shuttle middeck demonstrate the lack of privacy on board.

(From NASA Human Space Flight Gallery. http://spaceflight.nasa.gov/gallery/images/shuttle/sts-112/lores/sts112-345-028.jpg.)

Privacy may well become an even greater issue in interplanetary travel, with long transit times and larger crews. Unfortunately, the vehicle may not have the size to allow each crewmember the privacy he or she would like; space will be allocated to supplies (particularly food and water) as a matter of priority. Worsening the issue is the fishbowl-like nature of life in space. Not only do members of the ground support team follow the crew’s every action with close attention but so do members of the world media. This will be particularly true of any exploration mission crew bound for the Moon or Mars (Table 114-2).

TABLE 114-2 Space Vehicles and Habitable Volume

Monotony and Boredom

Monotonous tasks that require little cognitive processing or attention are common in flight, and may cause crewmembers’ attention to drift, and their performance to become lethargic. This represents a serious threat to the integrity and safety of the crew, because of the nature of system monitoring and interpretation of warning signals.

Measures to counter this situation have proved difficult to develop in both ground and flight operations, although intelligent computer-controlled monitoring systems relieves the crew of much of this duty.

Circadian Patterns and Sleep Disturbances

Sleep and circadian rhythm disturbances may also contribute to in-flight disorientation. Sleep hygiene is encouraged through limited exposure to light, pharmacologic aids (45% of all medicines used by Space Shuttle crews are sleep aids⁸⁴), and guidelines that limit sleep-shifting schedule changes.

Throughout the Apollo 9 mission, it was common practice to employ staggered sleep schedules for crewmembers. Even during later flights when crewmembers slept simultaneously, hectic schedules frequently required displacement of the sleep phase by several hours, leading to complaints of fatigue and sleep problems. The hazard of having all crewmembers sleeping simultaneously lies in the inability of most subjects to function optimally immediately on awakening. During some of the past space missions, events have occurred that required the crew to arise from sleep and respond immediately.

Workload Issues

Managing workload is a challenging task; activity levels have to be delicately balanced between too much (stress of overwork) and too little (frustration from boredom, time to brood). In this respect, the differences between Russian and U.S./European procedures are marked. Shuttle activity is planned by ground personnel in great detail, to maximize efficient use of time on orbit. Russian practice involves more flexible schedules, with greater crew discretion. These differences in part reflect cultural differences and the longer-duration flight times of most Russian missions.

The Russian Space Agency (RSA) operated Mir on a 5-day-per-week/8-hour-per-day schedule, with planned time off. Failure to do this in Skylab resulted in near-revolt when after 3 weeks, the crew refused to work any further without a day off. With Shuttle missions, NASA generally tries to pack in the maximum amount of activity through a tightly planned schedule. However, anecdotal reports show that time overruns on the Shuttle were common, with crews working an extra 4 hours per day just to keep up. Although crewmembers on ISS missions have been given greater flexibility in determining their own work schedules, both American and Russian space crews continue to ask for additional options in their work and leisure schedules.

In summary, the missions that have been completed to date have shown that the psychological needs of crewmembers are very important as mission length increases. The dominance of trained professional crew in a disciplined structured environment has helped to ensure mission success. However, introduction of civilian commercial spaceflight will add an unknown element that may have negative consequences.

Future Research

Just as long-duration ISS missions differ from short-duration Shuttle missions, exploration missions differ still further from both. Future exploration goals require additional preparation and research before we can be ready to move beyond LEO on a permanent basis. Planning for a lunar colony or mission to the asteroid belt requires additional knowledge of the dangers and best practices facing humans in these aerospace environments.

Analog Environments

There are numerous models for space flight exposure, ranging from parabolic flight to underwater operations, from polar expeditions to a bedrest laboratory. Each type mimics a different aspect of the space flight environment and each therefore has applicability for different studies. Microgravity itself (i.e., free fall) can be experienced through parabolic flight. The NASA C9B Reduced Gravity Research Program aircraft offers alternating intervals of zero gravity (approximately 25 seconds) followed by 1.8G (approximately 20 seconds) per parabola, 40 to 60 parabolas per flight. Other gravitational forces (including lunar and Martian gravities) can also be simulated, although the interval time course remains generally similar.

For research into muscle atrophy and off-loading of gravitational forces, rat models use tail-suspension methods, whereas human subjects undergo prolonged bedrest. Head-down tilt (usually 4 degrees) can be added to the bedrest, or “dry immersion” methods can be used to create cephalad or intrathoracic fluid shifts similar to those seen on orbit. Research into fluid and electrolyte balances, musculoskeletal changes, and endocrinologic changes have all been studied in this way.

To investigate the effects of prolonged isolation and small group dynamics, several analog environments have been used. Examples include polar station and submarine crews, NASA Extreme Environment Mission Operations (NEEMO) projects in the Aquarius underwater laboratory, and the NASA-Haughton-Mars project on Devon Island. Astronauts in their spacesuits can practice EVA tasks in a space analog environment at the NASA-Johnson Space Center Neutral Buoyancy Lab (NBL).

Valuable conclusions can be inferred from looking at health data from these analog environments. These populations have prolonged absences from proximity to health care, and have limited resources. The Australian National Antarctic Research Expeditions (ANARE) Register noted 5103 illnesses and 3910 injuries over the 9 years from 1988 to 1997.⁵⁴ Although most of these individual medical events posed little risk to the crew and mission, there were several of significant severity that needed emergency intervention.

Each of these analogs has benefits and limitations. No single model is suitable for all microgravity research, and investigators must carefully select the one most appropriate to the work. Both water immersion and bedrest studies have proved ineffective at predicting changes to cardiovascular parameters in space, the periodic microgravity of the C9B is quite a different stress than the sustained microgravity of space flight, and motion sickness aboard the KC is not predictive of in-flight SMS. Submarines and polar stations typically have much larger crews than those seen on space stations (to date), and even the NBL cannot entirely emulate the extravehicular environment for spacewalkers, because materials often behave differently under water than they do in a vacuum. At the same time, much can be learned from working in these analogs, as long as the differences and confounding factors are clearly and prospectively identified.

Artificial Gravity

The countermeasure most avidly sought as the “solution” to many of the most concerning microgravity-associated physiologic effects is artificial gravity (AG). This could be accomplished through rotation, either of the entire spacecraft or an onboard centrifuge. It is hypothesized that through the use of AG, muscle atrophy, bone demineralization, neurovestibular and neurosensory alterations, and many other physiologically deconditioning effects could be avoided, thus facilitating crewmember return to a gravitational field, whether on Earth, the Moon, or Mars. If space travelers could use an in-flight centrifuge (or other form of AG device) to achieve a dual-adapted state, where they are equally comfortable in microgravity or full gravity, they could potentially avoid reentry disorientation and postlanding postural instability. However, use of a centrifuge could also create vestibular problems through the Coriolis effect, leading to significant motion sickness.

A major consideration of AG is the trade-off between the radius of the centrifuge arm and the rotation rate required to achieve a desired gravity level. Depending on the radius of the rotation, Coriolis and other nausea-inducing forces may persist, and gravitational forces could vary over the length of a human body. The physiologic effects of having different parts of the body at different gravitational gradients are unknown.

Even if such provocative stimuli could be avoided, there is currently little information as to what an “adequate” gravity level might be. Is 1G needed or would a yet to be determined partial gravity be sufficient? In addition, does the AG stimulus need to be continuous, or will intermittent application (as with an onboard centrifuge) prevent the physiologic adaptations? If the latter, will the repeated shifting between microgravity and AG cause neurovestibular difficulties? How much time would be required in the centrifuge? Will it vary among crewmembers? How will the “AG prescription” be determined? One study flew a short-arm centrifuge on the Shuttle (STS-90) and found that astronauts who were subjected to 20 minutes of 0.5G to 1G along the longitudinal axis of their bodies on alternate days during the 16-day mission experienced reduced postflight cardiovascular deconditioning. This is encouraging, but not conclusive, evidence for the value of AG.

Although it might seem simpler to subject the entire vehicle to a steady gravitational field, there are significant barriers to the design of a rotating spacecraft. Financial, operational, and maneuverability challenges are enormous, and it is unlikely that they will be overcome in the near future.

Micrometeoroids And Space Debris

Spacecraft routinely collide with “space dust” of size 1 µm and larger.⁶⁷ Meteors (also known as meteoroids or micrometeoroids, depending on their size) are small bodies of solid matter that move through space, having originated in the interplanetary region, presumably from crumbling asteroids or comets.⁹⁹

Most meteors (61%) are made of stone, with a sizable number (35%) composed of iron.⁶⁷ The vast majority of the micrometeoroid material is <0.1 mm in diameter.⁶⁹ Although their size is tiny, micrometeorites travel at such high velocities that they have significant kinetic energy. When they enter Earth’s atmosphere, they are heated by the friction of the thickening air around them and may become visible as “shooting stars.” About 10 million kilograms of this interplanetary dust reaches Earth’s surface every day as meteorites or micrometeorites.³⁸

Orbital debris is any human-made object in orbit that no longer serves a useful purpose; a great deal of orbital debris is material that comes from other spacecraft. Some estimates suggest that millions of kilograms of debris are suspended in orbit, of which the most numerous component is “fragmentation debris” consisting of old satellite fragments (e.g., dead batteries, unused fuel cells) and deterioration products such as paint chips and aluminum oxide particles left over from rocket fuel exhaust.⁶⁸ The remainder is composed of rocket bodies, nonfunctional spacecraft, and mission-related debris.⁶⁸ Orbital debris can remain in orbit indefinitely, depending on its orbital altitude. Objects at altitudes less than 200 km (124 miles) are likely to fall to Earth within a few days, whereas those above 36,000 km (22,370 miles) will stay in orbit forever.⁸⁰ Debris at altitudes between 200 and 600 km (124 and 373 miles) will likely stay in orbit several years; debris at altitudes between 600 and 800 km (373 and 497 miles) will stay in orbit several decades; and between 800 and 36,000 km (497 and 22,370), will stay in orbit for centuries. Humanity’s short history of spaceflight to date has already produced a remarkable amount of debris (Figure 114-10).

FIGURE 114-10 An artist’s image of Earth showing the currently tracked orbital debris in low Earth orbit.

(From NASA Orbital Debris Program Office. http://www.orbitaldebris.jsc.nasa.gov/photogallery/photogallery.html.)

Collisions with either form of matter (micrometeoroids or orbital debris [MMOD]) take place at very rapid speeds (approximately 10 km/s on average or 16 km/sec for head-on collisions), which releases a great deal of kinetic energy, even for tiny objects such as paint flakes.⁶⁷ As a result, this material poses a danger to space travelers. For Shuttle missions, the risk of a lost mission increases from 1/107 to 1/76 when MMOD is considered.⁴⁰ MMOD was termed the “number one loss of crew vehicle risk driver” in a 2003 probabilistic risk assessment (PRA), although the risk and consequence(s) of the MMOD strike vary greatly depending on where on the orbiter the impact occurs.¹⁰⁰ In a September 2002 briefing, a senior ISS engineer reported that MMOD was predicted to cause one ISS evacuation every 214 years.⁴⁰ This was considered the second most likely cause for an ISS evacuation, with a medical emergency (including the sequelae of a radiation event) being the most likely cause (estimated at one evacuation every 5 years). In the same presentation, data were presented assuming a 40% chance of an MMOD-caused hole in the ISS (over a 15-year lifespan of the station), with a 12.8% chance (1 in 8) of the resultant hole necessitating a crew evacuation.⁴⁰

The longer a spacecraft is in orbit, the greater the risk of being struck by some form of space matter. The number of micrometeoroids is fairly constant in interplanetary space (e.g., during flights between Earth and Mars), but the gravitational forces near a planet alter density patterns and trajectories. In addition, orbital debris is naturally more plentiful in frequently used orbits, because there have been more opportunities for debris to detach from spacecraft in those areas. As a result, orbital debris varies with altitude and inclination, and NASA is currently able to estimate the probability of an impact with an object for a given orbit.

Impact Emergency Procedures

An example of an MMOD impact occurred on STS-94, when an impact crater in one of Columbia’s windows was found after landing.⁶⁷ The crater was approximately 1 mm diameter, and the MMOD object was estimated to be approximately 100 µm (0.01 cm) in size. Tests indicated that it was aluminum oxide residue from a solid rocket motor. Figure 114-11 shows an MMOD impact from STS-7.

FIGURE 114-11 An MMOD impact on the orbiter window during mission STS-7.

(From NASA Orbital Debris Program Office. http://www.orbitaldebris.jsc.nasa.gov/photogallery/photogallery.html.)

Another example occurred in 1996, when a briefcase-sized piece of an Ariane 1 rocket body collided with a Cerise communication satellite at 14 km/sec (31,500 mph), severing the satellite’s 6-m stabilization boom.^73,83

Following the Mir/Progress collision in 1997, when a Spektr module was punctured and lost its pressure, the crew was able to evacuate safely in a controlled fashion because the leak was relatively slow. The Spektr module was sealed off, and although it eventually dropped its internal pressure to vacuum, the rest of the station never dropped below 675 mm Hg (normal is 750 mm Hg).⁵²

Later in 2006, the space shuttle Atlantis sustained one of the largest debris hits in the history of NASA’s shuttle program. Following the craft’s safe landing on September 21, a hole 12 mm deep and 2.7 mm wide was discovered in a radiator on the inside surface of the shuttle’s right payload bay door. The location means the strike must have occurred while the doors were open, and accounts for why the hole was not spotted during in-orbit inspections prior to deorbit. The object—possibly a meteoroid or a fragment of space junk—must have hit the orbiter at a relatively high speed. Indeed, the windows in the crew cabin have to be replaced regularly due to damage from small impacts, or “dings.”

Radiation

Radiation exposure remains one of the most challenging obstacles in executing a prolonged manned mission in space. On Earth, humans are protected from ionizing radiation by the magnetosphere, the magnetic field that filters out high-energy solar and cosmic particles from deep space.

There are three main sources of radiation that pose risk to manned spaceflight: galactic cosmic rays (GCRs), solar radiation (solar particle events [SPEs]), and the Van Allen radiation belts. The Space Shuttle and ISS lie in LEO, where most exposure comes from protons and electrons trapped in the Van Allen belt and from SPEs.

In 1958, Dr. James Van Allen first described a “belt” of particles, predominantly protons and electrons, trapped in Earth’s magnetic field (Figure 114-12). The proton belts extend to an altitude of approximately 20,000 km (12,428 miles) with peak intensities at 5000 km (3107 miles), and the electron belts extend to 30,000 km (18,642 miles) with peaks at 3000 km (1864 miles) and 15,000 km (9321 miles). Although the magnetic field traps potential sources of radiation exposure, it also acts as a filter for the much more energetic heavy ion radiation of deep space. Galactic cosmic rays are extremely energetic ions ranging from protons to iron nuclei and have the most potential for radiation exposure on deep space missions, such as landing on Mars.

FIGURE 114-12 Artist’s rendition of the Van Allen Belts.

(From What Is Space Radiation? http://srag-nt.jsc.nasa.gov/AboutSRAG/What/What.htm.)

Solar particle events emit very high doses of radiation over a short period of time. They mostly consist of protons and are strongly associated with solar flares. Although solar flares are known to vary in 11-year cycles, there are currently no reliable models that can accurately predict the onset and intensity of SPEs. Crewmembers on EVAs or inadequately shielded spacecraft remain vulnerable to SPEs and could exceed career limits for radiation exposure in as little as 12 hours.

Radiation exposure in LEO comes primarily from the Van Allen belts and from GCRs. Crewmember exposure is measured in Sieverts (Sv), which is a product of radiation energy absorbed (Gray [Gy]) and a dose-equivalent quality factor that takes into account different types of radiation (alpha, beta, gamma rays). The amount of exposure varies with time, altitude, orbital inclination, and radiation shielding. A 1-week Shuttle mission has had exposures ranging from 0.005 to 0.05 Sv, and a 1-year Mir mission resulted in 0.584 Sv dose-equivalent exposure. NASA guidelines have established that the radiation dose limit for blood-forming organs (the most sensitive system to radiation effects) is 0.5 Sv/year. This is estimated to confer a 3% excess cancer mortality over a 10-year career.⁷⁴

A round-trip exploration-class mission to Mars has been estimated to last 2.5 years, with approximately 1 year interplanetary travel and concomitant exposure to deep space radiation. This remains one of the key obstacles for the success of a Mars mission and is a current area of active research. Previous concern regarding astronaut radiation exposure has focused on cancer risk, cataract development, and reproductive health (particularly important for any eventual plans for space colonies). One area that may affect performance during the mission is radiation damage to the central nervous system, specifically early onset of neurodegenerative disorders or acute effects that affect crew performance.

Countermeasures for prolonged radiation exposures are mostly unproven. Genetic screening has been proposed as a strategy for identifying crewmembers with vulnerability for oncogenic mutations.⁷⁶ Other proposals include prophylactically dosing the crew with radioprotective chemotherapy. Side-effect profiles have limited this as a viable option but may hold promise for the future. Less toxic pharmaceutical interventions may incorporate antioxidants (vitamins A, C, and E; trace amounts of iron, zinc), or tumor growth disruptors, such as dimethylsulfoxide (DMSO) or protease inhibitors, as a way to minimize cell damage.²⁰ Certainly for exploration-class missions, preflight cryogenic gamete preservation would be recommended for all crewmembers of childbearing age, and along the same lines, bone marrow preservation is recommended in case an autologous transplant should be necessary on return.

Shielding is an obvious solution to ambient radiation, but it requires inventive engineering solutions—such as water tanks built into the outer shell or the use of high-density polyethylene. Although the idea of a “storm shelter” (e.g., a location on the vehicle where crew could weather the relatively short-lasting SPEs) has been proposed, GCR exposure is constant, and the entire vessel would need to be shielded. Radiation exposure has an exponentially inverse relationship with protective shielding, meaning multiple layers of shielding confer small decreases in radiation exposure. Several meters of shielding would be required to eliminate GCR exposure—an impractical engineering demand given current technology. One creative solution for a Mars-bound vessel suggests using superconductors to create a synthetic magnetic field²⁹; obviously, the physiologic effects of such a field would require extensive study before implementation. Until a combination of solutions emerge to mitigate the effects of GCR, manned spaceflight will likely be confined to LEO and the relatively short trip to the Moon.

The Moon has no atmosphere and is also exposed to GCR, so lunar soil has long been studied as an abundant source of radiation shielding for prolonged stays (Figure 114-13). Although Mars has an atmosphere that consists mainly of CO₂, it is less than 1% as thick as Earth’s, and Mars has no global magnetic field. The Mars Radiation Environment Experiment (MARIE), part of the Mars Odyssey Orbiter, entered Martian orbit in 2001 for the purpose of determining GCR effects on Mars.⁵⁶ Previous estimates of dose-equivalent GCR exposure on the Martian surface are 0.12 Sv/year (to blood-forming organs).^23,91

FIGURE 114-13 As in this artist’s rendition, some plans for a lunar base envision subterranean habitats, so as to use lunar soil for shielding.

(From NASA Human Space Flight Gallery. http://spaceflight.nasa.gov/gallery.)

Microgravity

All tasks take longer in space. Experienced astronauts who have flown in space repeatedly and have spent extended periods on space stations acknowledge that, even after they have become familiar with the microgravity environment, “everything just takes longer to do.” In addition, some things that we take for granted in Earth’s 1G field (movement of carbon dioxide away from the face, air–fluid separation, the ability to put something down and find it still there when one looks again) do not exist in microgravity.

The way in which a crewmember uses his or her body is also affected, with the hands doing double duty of also having to hold the crewmember in place. Calluses are not uncommon and damage or injury to a hand becomes more significant. Interestingly, the Skylab module had seats placed at the science stations, which of course have no value in microgravity, and required the crewmember to be strapped in place. Again, this demonstrates the need to have physicians and human factors specialists fully integrated into program and mission design from the earliest stages.

Starting Out With “The Right Stuff:” Planning The Expedition

Crew Selection—Medical Criteria

The initial medical qualification standards for astronauts are significantly higher than the recurrent qualification criteria, as the latter reflect the substantial training investment made in the astronauts over the intervening time frame.⁶² A 48-year-old active astronaut with medical problems may thus be retained, whereas the same person as an astronaut applicant would be rejected.

It is important to understand the value placed on training and experience when reviewing the medical standards for space flight, as it reflects the stress placed on mission success. If the value added to the mission by a particular crewmember outweighs the medical risk (to both crewmember and mission) associated with his or her health, then he or she is likely to fly (Figure 114-14). In commercial space travel, where participants are passengers and not crewmembers, and mission success may be defined as a safe journey and satisfied customers, the need for training and screening drops dramatically.

FIGURE 114-14 Astronaut John Glenn on his 1998 mission at age 77.

(From NASA Human Space Flight Gallery. http://spaceflight.nasa.gov/gallery/images/shuttle/sts-95/html/s95e5055.html.)

Selection criteria are not always easily developed. For example, current society rightly considers it abhorrent to bar people from the astronaut corps because they belong to a certain racial, ethnic, or religious group, and there are laws to ensure that this does not occur. Gender, however, remains a somewhat thorny issue, as attested by the relatively low number of women astronauts. What will be the composition of the first Mars mission or the first lunar base? Will it differ from the composition of the Apollo 11 mission (July 1969) or the crew of the first ISS mission (November 2, 2000 to March 18, 2001)? Both of these consisted of three white male professional astronauts with five of the six having military backgrounds, despite occurring more than three decades apart.

Review and revision of medical standards are supposed to occur regularly, usually on a 5-year cycle for major changes (lesser changes every 2 to 3 years), but operational demands have been known to delay this review.^73,87 Since the days of the Apollo program, standards have been significantly relaxed, and more modern technology, such as endoscopy, ultrasonography, magnetic resonance imaging (MRI), and other imaging modalities, are used extensively in evaluating crew health.

The criteria by which space travelers (and potential space travelers) are currently scrutinized differ depending on their mission role. Pilots and commanders are screened by more stringent criteria than are mission or payload specialists.

The first space travelers were selected from terrestrial pilots, whose physical condition (health, reflexes, strength) was considered critically important. Subsequently, as the purpose of the space program moved from military to scientific objectives, the emphasis in crew selection has shifted to favor academic qualifications more strongly.

In each selection cycle, approximately 25% of NASA astronaut candidate finalists are disqualified on medical grounds. By far the most common reason for medical disqualification of astronaut candidates is inadequate distance vision. Applicants who pass the initial medical screening and are subsequently selected by the NASA administrator to be astronauts undergo annual medical examinations during their career as astronauts. With an aging crew population, wearing of glasses is commonplace among established crewmembers.

For exploration-class missions, the challenge for Space Medicine is to determine the requirements/policy for selection and certification of crewmembers. Examples of some conditions considered disqualifications for a Mars mission include:

1 Chronic conditions that require regular dosing of life-sustaining medication for the purposes of secondary or tertiary prevention (e.g., tumor suppression therapy or anticoagulation).

2 Conditions that will be worsened by spaceflight exposure, placing persons at additional personal health risk, though not necessarily increasing mission risk (e.g., previous radiation exposure or preflight osteopenia/osteoporosis)

3 Those that require medications to sustain life or that require medical procedures or diagnostics to monitor and treat same.

4 Those that require specialized medical devices for monitoring or treatment of a medical condition, including conditions that require special provisions to conduct routine required countermeasure activities (e.g., blood glucose meter or heart rate monitor).

5 Those which would prevent participation in many scientific and medical operational research studies, because the underlying condition would confound the collected data (given the small subject pool, broad participation in research projects by crew is critical in order to obtain scientifically valid data).

The commercial world is a very different situation, where participants are self-selected based on their ability and willingness to pay. The FAA established criteria for commercial passengers, and the initial suborbital passengers were screened and assessed to both evaluate their health and establish a database of their health status. However, it is clear that they will not be undergoing the rigorous evaluations of professional crew, nor arguably should they. Commercial passengers to the ISS have to meet stricter criteria than those of the FAA and also must be accepted by the RSA and NASA.

Preflight/Mission Planning

After the rigorous selection process, it may be several years before an astronaut is actually assigned to a space mission. Crews are encouraged to maintain physical fitness throughout preflight training and are subjected to routine wellness screening. More formal annual exams are conducted not only to ensure flight readiness but also to collect longitudinal biomedical data to identify any long-term health effects of spaceflight. The data thus far have shown (only) one concerning trend: astronauts have a higher number of (non-occupational) trauma-related deaths than do matched controls. Of course, this may be due to an adrenaline-loving lifestyle that led them to space in the first place.

Communication and team-building exercises are also staples of preflight training, with crewmembers trained in conflict resolution and stress management. Classroom activities are interspersed with mission simulation in the mock-ups, in the NBL, and during C9B parabolic flights. Land and water survival training courses are also part of the training regimen (Figure 114-15, online). Medical training for all crewmembers consists of the general principles of space medicine and physiology, as well as first aid and cardiopulmonary resuscitation (CPR) courses.

FIGURE 114-15 Susan Helms, Yuri Usachev, and Jim Voss, the ISS Expedition Two crew, participate in preflight Soyuz winter survival training.

(From http://spaceflight.nasa.gov/gallery/images/station/crew-2/lores/s98_04119.jpg.)

Predicting Likely Illnesses And Injuries

An organized approach to risk management follows the principles of primary, secondary and tertiary prevention, with the aim of preventing (or at least minimizing the risk of) a medical situation or, should such an event occur, supporting full recovery of the affected crewmember as quickly as possible.³¹

In the years ahead, commercial space ventures may make space travel available to ever-increasing numbers of people. The new millennium witnessed the advent of “space tourism” with the voyage of Dennis Tito to the ISS and the successful flight of the world’s first privately funded and built spacecraft (SpaceShipOne in July 2004). Until this point, space travelers had been a highly trained and carefully screened group of people, who could almost be considered “pathologically healthy” when compared with the general public. However, it is doubtful that such rigid standards will be maintained in the future, and so increasing accessibility is expected to lead to less fit people entering space, much as has occurred in “adventure tourism.”

In earlier times, adventurous expeditions to Mt Everest or the Antarctic were accessible only to the very fit. Now they are more widely enjoyed, and the space environment will likely follow suit. NASA has in recent years attempted to develop a system of care based on predicting the likelihood of events using a software modeling system with longitudinal astronaut and general population health data. However, although it is possible to predict generalized population-based outcomes (e.g., we know roughly how many people older than age 65 will have a myocardial infarction in a given population in a given period of time), it is almost impossible to predict for one person or a small crew.

Deciding What To Bring (Benefit Vs. Burden)

What do we know of illnesses during spaceflight and what can we expect as we increase the number of human-hours in space? NASA data on the first 89 Shuttle flights from 1981 to 1998 demonstrated that 98% of astronauts reported a medical event or symptom other than SMS during their mission.¹¹

As with any other human endeavor into an extreme environment, the medical system for space travel strives to minimize medical events among the crew and to maximize the ability to treat any problems during the mission with the most efficient use of resources. This cost-versus-benefit design is grounded in an analysis of known medical problems in space and analog environments. The health care system must be founded in evidence-based science, yet remain flexible and innovative. Its design must promote long-term health and optimize the performance of crewmembers throughout the mission.

To date, NASA health care systems have relied heavily on ground-based support led by the flight surgeon. This physician is primarily responsible for the health and well-being of the crew and has the authority to dictate care and, in extreme cases, theoretically to terminate the mission. He or she plays a role in the health monitoring and training of the crew over the 2 or more years preparing for the mission. During the mission, the flight surgeon can guide the CMO in giving appropriate care to other crewmembers for LEO missions. The CMO is usually not a health care practitioner and receives limited training in medical care prior to the flight.

One of the challenges of a long-term mission (such as to Mars) is this historical reliance on ground-based support. During such an extended mission, there may be delayed communication with Earth (depending on orbital positions), so a more independent system of health care delivery (likely involving a physician crewmember, the specifics of whose training remains the source of great debate) will be required. This reflects the challenges of wilderness and military medicine in remote locations on Earth, where equipment is minimal, medical expertise is not always available, and evacuation may not be available for considerable periods.

Designing The Vehicle

Particularly in the case of medical facilities, good design features can make a critical difference. Unfortunately, none of the aerospace medical systems to date has shown particularly good architectural integration. On both the Shuttle and ISS, the galley and toilet are geographically close to each other. In the event of an illness, hygiene could be compromised and food consumption areas contaminated. In the Shuttle’s cramped environs, there is no dedicated location where a sick crewmember can be placed to avoid contamination of the rest of the crew—or merely to decrease what is often (particularly for nonclinicians) the provocative stimulus of someone vomiting nearby. These close quarters can pose hazards to both general health and performance.

The ISS, with its larger internal volume, has a dedicated location for casualties, complete with “bed” (a foldout, electrically isolated crew medical restraint system). However, the ISS medical system is based in the U.S. Lab module, at the other end of the station from the original toilet facility. This suggested that if someone were seriously ill, they might have to choose between the two—having access either to medical supplies (including medical oxygen) or to a toilet. Not only could this increase the amount of care other crewmembers would have to provide to a casualty, but it also creates additional challenges for managing medical wastes. The addition of the “Tranquility” module in February 2010 provided the ISS with a second toilet (or more precisely, a “Waste and Hygiene Compartment”).

The flip side of this debate is the undesirability of sequestering highly valuable in-flight space for exclusively medical purposes. Medical issues are not among the primary mission objectives, so many people feel that the space currently assigned to medical tasks and storage could be better utilized by payloads or other systems. It can be difficult to strike the proper balance between contingency-based systems (such as clinical assets or fire suppression systems) and those that are used during nominal operations (such as those for propulsion or food preparation). Similarly on wilderness expeditions, there is always a balance between a “fully stocked” medical kit bag and using the limited availability of space and weight for other essentials of living and survival.

The equipment packaging itself is more driven by engineering requirements than by usability. The ISS medical checklist is a telephone book–sized document that was initially compiled in alphabetical order to suit engineering protocols, although studies showed that this type of organization was the least successful in guiding medical intervention and successful resuscitation.⁵⁷

The needs of LEO missions differ from those of exploration missions, but the overarching principles remain the same. Should space be dedicated to certain functions or remain flexible? Will there be a designated user with specialized training and knowledge (as is the case in terrestrial hospitals, where only certain personnel know how to operate the CT scanner, for example), or will the system be sufficiently user-friendly that any member of the team will be able to use it even without prior familiarization? Costs for equipment meeting the latter conditions likely will be significantly higher, because there are few commercially available, off-the-shelf devices that meet those criteria. On the other hand, if only a single team member has the necessary skills, what will the team do if that person is incapacitated and cannot provide or guide care?

Even if a trained caregiver is available, decision-support software and a robust informatics infrastructure can provide a great deal of assistance. In the event of a mass casualty—such as a widespread epidemic of food poisoning or multiple cases of hypoxia—even minor time savings can be important.

Medical systems must accommodate the number of patients they are expected to treat. Will there be two crewmembers, as on some past ISS missions, or 200, as might be expected in a permanently manned lunar base? In the latter case, a large, dedicated space with associated supplies and equipment might be needed, but such a resource would be a waste for the former crew complement. When evacuation and terrestrial-based care are impractical, as on a Mars mission, it may make more sense to invest in resources that will enhance wellness and embrace a lower threshold for nonsurvivable conditions.

Currently, there is a professional, highly trained, and extremely motivated astronaut corps. However, the current astronauts are, in many cases, less fit than the first Mercury Project test pilots, and it is likely that as more people venture into space, be they tourists or maintenance workers on a moon base (Figure 114-17, online), fitness levels will decrease further. Accordingly, the medical system must be prepared to handle a wider variety of preexisting conditions. This is similar to trends seen terrestrially, as a wider range of people embark on “adventure tourism” and as more contractor personnel are used in military operations overseas.

FIGURE 114-17 Artist’s rendition of a moon base.

(From NP-119 Science in Orbit: The Shuttle & Spacelab Experience, 1981-1986. http://history.nasa.gov/NP-119/ch2.htm.)

Training The Crew

During a LEO mission, there is often no physician available, and the astronauts must rely on a fellow crewmember to be their primary caregiver. This system of “buddy care” training is formalized in the role of the crew medical officer (CMO). ISS CMOs receive approximately 40 hours of training on topics such as diagnostics and therapeutics, phlebotomy, suturing, splinting, and endotracheal intubation (Figure 114-18, online). Part of their role is to provide information to the ground-based Flight Surgeon in Mission Control, who has the ultimate responsibility for the health of the crew. Studies using the NEEMO underwater research facility and crews simulating medical intervention for emergencies have shown the limitations of the CMO system.^35,36 Medical instructions and protocols using diagrammatic checklists and decision support technologies have also been proposed and/or developed for the ISS, using similar approaches to those used in Earth-based prehospital care.

FIGURE 114-18 Astronaut Sergei Krikalev receives training in suturing from Tyler Carruth, PA-C, as part of his preflight CMO (crew medical officer) training.

(From http://spaceflight.nasa.gov/gallery/images/station/crew-11/lores/jsc2004e26778.jpg.)

Treating The Casualties

Keeping Them Healthy: Creating A Home

The nominal atmospheres of the Space Shuttle, the Soyuz, and the ISS are dual-gas oxygen–nitrogen (21% oxygen with the balance nitrogen) maintained at sea-level pressures. However, pure oxygen is breathed during extravehicular activities or during emergencies when the crewmembers are directed to don portable breathing apparatuses (PBAs). PBAs are used to protect the wearer from atmospheric contamination (as in cases of fire) or to maximize the alveolar partial pressure of oxygen and delay development of hypoxia (during depressurization).

The ISS ventilator (AutoVent 2000) also uses 100% oxygen exclusively.⁴² As a result, medically compromised patients aboard the ISS can receive either 100% oxygen or room air, but there is currently no means of providing supplemental oxygen between 21% and 100%. This may create difficulties in seriously ill or injured crewmembers who require mechanical ventilation or supplemental oxygen for prolonged periods, as studies have shown that deleterious effects from breathing 100% oxygen at sea-level pressures occur in as little as a day. In addition, there are limited oxygen supplies and therefore thought needs to be given as to how best to use a ventilator in space and how to wean a crewmember off the ventilator, assuming of course the CMO is competent to intubate in the first place. The issue here is that it is easier to start care than to sustain it in environments where there are limited supplies and limited options for definitive care. For example, what would happen if a crewmember on the way to Mars required ventilation? At some stage, the supply would expire and a return to definitive care may not be an option.

A single-gas environment has several benefits. It permits a lower habitable atmospheric pressure, thus decreasing the risk of decompression sickness during EVA; it minimizes the required strength of the spacecraft hull because it will not need to withstand a higher internal pressure; and it requires smaller stores of gas. However, it is a highly artificial environment in which certain experiments cannot be performed at all, and other products would be of limited usefulness because they were not performed in the standard sea-level, dual-gas atmosphere. In addition, the higher oxygen concentration (albeit at a lower pressure) could pose a higher fire risk.

Air supplies for spacecraft repressurization and crew respiration are usually stored in high-pressure tanks as liquids in cryogenic storage or in other forms.^14,16 Nitrogen, for example, can be stored as hydrazine (N₂H₂), which is also a spacecraft propellant. Four high-pressure gas tanks (two each for nitrogen and oxygen) are located on the exterior of the joint airlock on the ISS, and can be either refilled or replaced by the Shuttle (Figure 114-19).

FIGURE 114-19 An overhead crane at Kennedy Space Center lowers a gas tank onto the Spacelab Logistics Double Pallet while workers help guide it. Four of these storage tanks, two for gaseous oxygen and two for gaseous nitrogen, comprise the high pressure gas assembly that will be attached to the Joint Airlock Module during two spacewalks.

(From NASA Image Exchange. http://nix.larc.nasa.gov/info;jsessionid=1n21fjejgtner?id=KSC-01PP-0952&orgid=5.)

Ambient cabin pressure is maintained, and atmospheric leakage countered, by periodic injection of gas. Both ground control and on-board crew can control these valves and thus regulate ambient pressure within the spacecraft.

Hypoxia and hypercarbia are very real dangers during spaceflight, not only from malfunctions in the life support system, but also because in microgravity, gases do not diffuse away from the face as they do terrestrially. Carbon dioxide is heavier than sea-level atmospheric air, so on Earth it falls away from the face once it is exhaled. In addition, on Earth, there are breezes, local airflows, and weather- or gravity-related convective forces to remove carbon dioxide from around the nose and mouth. These forces are absent in microgravity, and, in the absence of adequate ventilation systems to move the air from in front of the face, pockets of carbon dioxide can accumulate, leading to hypercarbia. Similarly, when the crew must operate in small, contained spaces where the ventilation system is less effective, such as behind a rack, inside an unventilated sleep station, or when numerous items are out in the cabin, extra caution must be taken to prevent buildup of carbon dioxide.

During the 1985 recovery mission of the Salyut 7 space station, cosmonauts Dzhanibekov and Savinikh had to work in a module that lacked power and was thus both cold and unventilated. They reported headaches, lethargy, and sluggishness associated with the hypercarbia produced by their own exhalations. Providing active ventilation as soon as possible through portable fans and resumption of powered systems mitigated this effect.

Forced ventilation is thus a vital component of any atmospheric system for space travel. Without adequate blending of the cabin atmosphere, heterogeneous microenvironments can occur containing undesirable levels of carbon dioxide, particulates, water, or heat. Air movement also enables surveillance of the environment for toxins (such as smoke), permits extraction of undesirable components (such as carbon dioxide and trace contaminants), and allows control of both heat and moisture content of the atmosphere.

There is a great deal of ongoing research into onboard oxygen-generating systems (OBOGSs), including molecular sieves, electrolysis, various forms of air separation, membrane technologies, carbon dioxide reduction (e.g., a Sabatier reactor), chemical absorption, and biologic systems. However, only a few of these have actually been used in space up to this point.

Molecular sieves, which were first used during the Skylab missions, are also used in some military aircraft. Ambient air is forced over a zeolite crystal molecular sieve that separates the oxygen; the gas is then further concentrated and released back into the cabin atmosphere as needed.

Russian life support systems since the Mir space station have generated onboard oxygen through an Elektron oxygen system. In this system, stored water or even urine is used to produce oxygen via electrolysis (2H₂O + electrical current → 2H₂ + O₂). On the ISS, electrical current is provided from the station’s solar panels. The hydrogen is currently vented into space, though eventually it may be combined with carbon dioxide to create additional oxygen or water in the (planned) Russian Sabatier reactor. In this reactor, hydrogen from the Elektron oxygen system is mixed with carbon dioxide from the carbon dioxide scrubbing system at relatively high temperatures (482° to 649° C [900° to 1200° F]) to create first water, then oxygen.

Another OBOGS used since Mir is the solid fuel oxygen generator (SFOG) system, which employs a chemical reaction (heating lithium perchlorate) to create oxygen, the same technology used in the emergency oxygen mask systems of commercial aircraft. Each 2.2-kg perchlorate “candle” produces 600 L of oxygen, a supply sufficient for one person for 1 day.

For long-duration exploration missions, however, biologic systems are likely to be more heavily employed. In 1995, a closed bioregenerative system was successfully tested, using 11 square meters of wheat in the chamber to provide oxygen for and consume carbon dioxide from a single person for 15 days. However, current plant-based systems are larger, more labor-intensive, and (as any gardener knows) less reliable than the chemical-mechanical systems. Given the criticality of the atmospheric system, the current level of uncertainty associated with biologic systems is unacceptable, and additional research is needed.

Spacecraft life support systems must monitor several atmospheric parameters, including ambient pressure, partial pressure of each gas (including carbon dioxide), ventilation, temperature, and humidity.⁸² Whereas the oxygen monitoring system maintains the partial pressure of oxygen relatively close to that experienced on Earth, spacecraft carbon dioxide monitoring systems deal with levels significantly higher than what is seen terrestrially; 7.6 mm Hg of carbon dioxide is the upper limit of normal for both ISS and Shuttle life support (33 times Earth-normal). Current technology does not offer a method to reduce these levels further, and to date, no evidence exists that these elevated levels have long- or short-term effects on space traveler health.

Contaminants

Contaminants fall under two general categories: particulate and gaseous. The latter includes both biologic (such as those created during food preparation or produced by the crew) and chemical (as can be seen from off-gassing of cabin materials, cleaning supplies, or certain scientific experiments). Prevention is preferred to removal, however, and spacecraft designers plan accordingly. For example, judicious selection of spacecraft cabin materials can minimize sources of off-gassing toxins. Carbon dioxide is by far the most important contaminant to be removed from the environment.

However, not all contaminants can be avoided. Submarine and chamber studies showed as early as the 1970s that in a contained environment, even trace contaminants can build up to hazardous levels. Prior to the Skylab program, U.S. spacecraft had neither monitored nor analyzed trace contaminants (with the exception of carbon monoxide). From Skylab on, however, numerous methods have been used to monitor the cabin environment, including gas chromatography, mass spectrometry, ultraviolet and infrared spectroscopy, and sensors for individual contaminants (e.g., carbon monoxide or hydrogen cyanide). Exploration-class missions must consider additional potential contaminants such as Martian soil or lunar dust.

During emergencies (such as a fire), additional hazards from cyanide gas to molten metal particles can be introduced into the atmosphere. Removal of many of these substances can be achieved through use of filters (Figure 114-20). However, there are some contaminants (e.g., toxic by-products of the Halon used as a fire suppressant on the Shuttle) that current filters cannot remove.

FIGURE 114-20 Astronaut Jack Lousma inserts lithium hydroxide (LiOH) canisters into the air revitalization system servicing panel on the Shuttle middeck.

(From NASA Image Exchange. http://nix.larc.nasa.gov/info;jsessionid=1akxvdx2wwap4?id=S82-28921&orgid=8.)

Temperature

The life support system of a spacecraft must protect its occupants not only from the intense cold outside but also from buildup of heat within. The multiple sources of heat within the spacecraft that can affect thermal control are: electronics, lighting, solar heating, and metabolic heat (as produced by exercise). Equipment produces high thermal loads. In most cases, hyperthermia is more of a danger to space travelers than is hypothermia. The literature clearly documents decrements in task performance as crew comfort declines. Long-duration Soviet/Russian spaceflights suggest that ideal temperatures for a working environment are 22° to 24° C (72° to 75° F). When temperatures drop below 19° C (66° F), with a relative humidity around 70%, the crews complain of cold. An unusual example of contending with cold occurred on the damaged Apollo 13 spacecraft, which required strict conservation of energy on the return to Earth. The uncomfortable crew was very glad to get home (Figure 114-21).

FIGURE 114-21 The Apollo 13 crew, shortly after landing.

(From http://www.nasm.si.edu/collections/imagery/apollo/AS13/images/AS13_return.jpg.)

Humidity

Humidity is a key component for physical comfort aboard the spacecraft and it is integrally linked with temperature control. Although it is less noticeable at comfortable temperatures, as either extreme is approached, humidity levels become important. To maintain crew comfort, humidity must fall to promote evaporative cooling during increases in temperature, whereas higher humidity levels reduce evaporation and facilitate heat retention at cooler temperatures. The cabin environment is usually maintained at about 60% relative humidity, corresponding to about 0.2 psi (about 1.4 kPa) of water vapor pressure.

Circulation of cabin air is the mainstay of temperature and humidity control. As air is recirculated, moisture can be removed from it either by a desiccant (as is usually the case in spacesuits) or through condensation of the vapor and removal of water from the gaseous air. This latter method is used more often in spacecraft, because it allows reuse of the water thus produced. In general terms, condensation occurs when cabin air is cooled on heat exchangers, and the resultant water is then removed by separators (either via capillary action or hydrophobic–hydrophilic surfaces). A description of the life support system for the ISS is shown in Figure 114-22.

FIGURE 114-22 Description of the life support system for the ISS.

(From http://mix.msfc.nasa.gov/IMAGES/MEDIUM/0102167.jpg.)

Odors in the cabin atmosphere are generally removed using charcoal filters. As with other contaminants, prevention is preferred to removal, prompting the need for an effective waste control system. As occurs lamentably often in history, previous lessons were forgotten when the ISS was designed and launched; experiences in trash handling gained aboard the Mir and Skylab space stations were not utilized. For example, there was no flight rule to prevent stowage of trash near food. The problem of waste accumulation, handling, and disposal quickly became so acute that NASA established a team tasked solely with developing an ISS Trash Plan.

Waste Management

Generally speaking, waste collection is designed to be as simple as possible for the crew. In some cases, crewmembers merely place the refuse in a container and stow it. In other cases, the waste may be automatically collected and stored, as is the case for the toilet appliance. In all cases, however, limited stowage space remains an issue that drives waste disposal policy. For waste material that the crew should not handle for reasons of health or safety, the appropriate systems usually collect and package the waste automatically. Virtually all forms of waste disposal currently make use of Earth, either by returning trash to the ground for final disposal or by utilizing atmospheric reentry to destroy trash in the Progress rockets. Unfortunately, these options are not ideal for long-duration exploration missions, when Earth will not be available. In these situations, waste will need to be reused or destroyed in more efficient ways, so as not to take up valuable storage space.^60,93

Biologic solid wastes, such as those from food, generally contain 40% to 90% moisture and soluble organic compounds.⁴⁴ As a result, these wastes cannot be stored for extended periods, because they decompose (leading to growth of undesirable anaerobic microorganisms), produce noxious gases (including N₂O, NH₃, H₂S), and create foul odors.

In addition, wastes may be in mixed form (such as semisolid), and they may be handled by more than one system, as when water condensed from ambient air by the atmosphere-management system is sent to the water-processing system for final disposition. Life support designers must also take into account the need to conserve resources wherever possible. Particularly on long-duration and exploration missions, wastes must be recycled or reused, which imposes requirements on how wastes are categorized and handled. For example, liquid waste from food preparation might be able to be reused for hydroponics projects and should therefore be captured and stowed separately from liquids contaminated by toxic chemicals, as might result from certain payload experiments. Composting, the process of accelerating decomposition in an aerobic environment, is also being explored as an option for safely handling biologic wastes.

For exploration-class missions, another category of waste must be considered: contaminants from the external environment, such as Moon dust. The habitats used by crew on lunar and Martian missions will need to have systems, such as forced air blowers, sticky strips, or other devices, to remove any such pollutants from an astronaut’s suit or tools and to dispose of them in a safe manner.

In addition, there is a desire not to contaminate external environments with terrestrial compounds. For example, studies into the possibility of life (past or present) on Mars could be confounded if terrestrial bacteria were carried from a habitat module into the larger Martian environment. Accordingly, the waste collection and disposal system will likely be used by astronauts as they leave the base, as well as when they reenter it, creating (potentially) twice as much waste material of two very different kinds, one endogenous to the habitat and the other exogenous.

Even missions to orbit risk exposure to outside contaminants, as was seen during STS-98, when an EVA crewmember became covered in ammonia crystals during connection of the U.S. Lab module to the rest of the ISS³⁰ (Figure 114-23).

FIGURE 114-23 During an extravehicular activity on STS-98, astronaut Bob Curbeam was exposed to ammonia.

(From http://spaceflight.nasa.gov/gallery/images/shuttle/sts-98/lores/s98e5190.jpg.)

Medical Waste

Medical waste falls into a special category, as it can be more hazardous to the crew than are other wastes. To date, there have been very few cases of illness (other than rare cases of SMS) in which crewmembers have generated large amounts of medical waste. Eventually, however, someone will become seriously ill or injured and produce a large amount of medical waste in the form of vomit, sputum, blood, diarrhea, or any other potentially contaminated body tissue or fluid. Microgravity significantly magnifies the problems associated with medical waste and caregiver protection.

There is also the question of what to do with medical samples. The current ISS blood analysis device, the i-STAT Portable Clinical Blood Analyzer,⁴² only uses a few drops of blood, which are wicked into a self-contained cartridge. However, if longer-duration missions have expanded laboratory facilities, additional attention to the question of discarded samples will be needed.

Handling of human urine and feces also poses health hazards. If the waste collection system is compromised, poor hygiene could lead to disease outbreaks among the crew. Most current toilet systems, both Russian and American, work the same way: as wastes exit the body, they are drawn away by a stream of air and captured by the toilet.

Crews also use urine collection devices during EVAs. Male astronauts wear a pouch-like device similar to a condom catheter, and women wear “disposable absorption and containment trunks,” a pair of multilayered shorts that contain an absorptive powder.^1,51,61 Each device has a storage capacity of roughly a liter of fluid. Crew also wear urine collection garments (like adult diapers) underneath their launch and entry suits, because the Shuttle toilet is unavailable for use during those phases of flight (and even if it were available, the crew cannot easily doff the ACES suits).

To handle feces, the toilet has a cylindric container in which a plastic bag is placed before use. Crewmembers use foot restraints and bars positioned over the thighs to hold themselves on the toilet seat, ensuring a good seal with the contoured, soft seat. The good seal is necessary to ensure that the toilet’s airflow will be able to draw the waste away.³³ The hole for solid wastes is only 10 cm (4 inches) in diameter, and crewmembers actually practice positioning themselves on a toilet trainer before flight. A camera placed within the toilet trainer, directly beneath the hole for solid waste, allows them to see if they are positioning themselves correctly. A video showing proper use of the Shuttle toilet can be found at http://edspace.nasa.gov/text/livespace/gottago.html.

Feces enter the commode through the seat opening and are drawn in by air flowing through holes under the seat. This downrushing airflow (850 L/min) substitutes for gravity in collecting and keeping the waste material inside the commode.³³ The wastes are then broken up by rotating vanes and deposited along the walls in a thin layer. A hydrophobic liner inside the commode prevents free liquid and bacteria from leaving the collector. The plastic bag is then sealed, and a plunger attached to a lever forces it to the bottom of the cylinder. A new bag is then placed in the toilet for the next astronaut. When the cylinder is filled, it is replaced by a new cylinder. Figure 114-24 shows the design.

FIGURE 114-24 Shuttle toilet schematic.

(From http://lsda.jsc.nasa.gov/kids/L&W/hygiene.htm.)

On the ISS, the first toilet was in the Russian-built Service Module. This looks more like an Earthbound toilet and uses metal containers for storage. Prior to use, the containers are stored like empty buckets; however, once full they present considerable stowage and hygiene issues. The consequences of failure of the waste management system in a closed environment cannot be understated. This has occurred repeatedly on the ISS and has resulted in the crew having to use “Apollo Bags” to contain waste matter, which is a messy and unhygienic solution. A similar problem to this on a mission beyond Earth’s orbit could have significant negative consequences and shows the value of integrating the medical team into all aspects of a vehicle or mission design.

When other plant and animal life exist in the spacecraft, there are additional wastes produced, both in terms of quantity and types, with associated new microbial flora as well. This further increases the hazards to crew health in the event of improper waste management.

Another question (that has largely been avoided to date) is what to do in the event of a death on orbit. Leaving aside the questions of how death would be ascertained and declared, what effects such an event would have on the other crewmembers, the impact on the mission, and other such matters, there still exists the issue of what to do with the remains. Although returning the remains to Earth might be feasible, albeit unpleasant, on the Shuttle, the tight quarters of the Soyuz make it much more difficult to transport a body. And what about exploration missions? Even if a crewmember’s death were to force abandonment of the mission, the time required to return to Earth might be too long to permit transport of an unpreserved cadaver. Procedures to handle a death on an exploration-class mission remain under discussion.^8,9

What To Wear?

In the early days of the space program, astronauts wore pressure suits derived from X-15 suits during virtually the entire (short) mission. The Mercury “silver foil” spacesuit was a custom-fitted, modified version of the Goodrich U.S. Navy Mark IV high-altitude jet aircraft pressure suit (Figure 114-25, online, shows Mercury program boots, helmet, and gloves). By contrast, current spacecraft have a “shirtsleeve” cabin environment; in other words, no special gear or equipment is needed to live comfortably in the environment.⁷¹ However, protective suits are worn during launch and reentry phases of flight. Although the cabin environment at these times remains unchanged from the “shirtsleeve” parameters, protective suits are worn because of the dangers inherent in these periods of flight. The suits differ somewhat between the Shuttle and Soyuz spacecraft, but both are designed to protect the crew in the event of a problem during take-off or landing.

FIGURE 114-25 Mercury program boots, helmet, and gloves.

(From http://web.mit.edu/16.00/www/aec/spacesuit.html.)

One unique item of apparel used to counter physiologic adaptation to space is the Russian “penguin suit.”⁷² Designed as a countermeasure for muscle atrophy associated with prolonged exposure to microgravity, the penguin suit is a snug-fitting, full-length, long-sleeved jumpsuit that zips vertically to the crotch, with horizontal zip pockets on either side of the chest and just below the waist. The suit is made of a synthetic fabric with elastic inserts at the collar, waist, wrists, and ankles and along the vertical sides of the suit. The inside of the suit contains a system of elastic straps and buckles that are used to adjust fit and tension of the suit. When astronauts and cosmonauts tighten tension on these bungee-like bands, the suit can exert resistance, opposing the astronauts’ movements and simulating some of the effects of gravity (see Figure 114-7, online).

Another Russian countermeasure suit makes use of lower body negative pressure and resultant redistribution of blood volume to combat postflight orthostatic hypotension.²⁵ This suit was employed on Soviet and Russian space stations, but is not in use on the ISS. The ISS is of course a shirtsleeve environment, and any long-duration exploration-class mission is likely to be the same.

The clothing that is worn during an EVA is quite different from that worn by astronauts inside the spacecraft (known as intravehicular activities [IVA]). The American EMU (extravehicular mobility unit) or Russian Orlan is like a tiny spaceship, one that is large enough only for a single inhabitant. Just like the Shuttle or ISS, it has its own life support and communication systems, provides a pressurized environment, and protects its wearer from the vacuum of space.

While they are in the EMU, crewmembers wear a liquid cooling (and ventilation) garment (LCG). The LCG contains a network of vinyl tubing that circulates cool water and air in order to maintain core body temperature at normal levels.^1,61 In conjunction with a thermal undergarment, it keeps an astronaut relatively comfortable while he or she is outside the vehicle and exposed to wildly varying temperatures (about −100° C to +120° C [about −150° F to +250° F]), depending on whether he or she is in sunlight or in shadow). The LCG that is worn under the launch and entry ACES suit only circulates water for cooling purposes. The LCG that is worn under the EMU also circulates air in order to scrub out the CO₂ (Figure 114-26, online) (http://www.nasa.gov/audience/forkids/home/CS_Astronaut_Fashion_Show.html).

FIGURE 114-26 Shuttle crewmembers doff their extravehicular mobility units after a spacewalk, showing the liquid cooling garments worn underneath. The bottom half of the extravehicular mobility unit is visible on the crewmember at left.

(From NASA Human Space Flight. http://spaceflight.nasa.gov/gallery.)

The current EMU is designed for use in orbit only. The spacesuits used by Apollo astronauts who walked on the Moon were different, in part because of the different technology in those days, but also because the suits were subject to different strains, from Moon dust to lunar gravity. Crews on planetary exploration missions in the future will need new spacesuits, some of which are currently being designed and tested (Figure 114-27).

FIGURE 114-27 A, Neil Armstrong’s Apollo spacesuit. B, A new model spacesuit for use on Mars is tested in Arizona.

(A from http://www.fi.edu/pieces/hiley/images/jigsaws/Apollo%20Spacesuit.jpg; B from http://www.nasa.gov/vision/space/preparingtravel/desertrats.html.)

Personal Hygiene

With water a potentially mission-limiting consumable, personal hygiene in space is different from that on Earth. Water pressure on the ISS, for example, is about half that in a U.S. household. However, for morale purposes as well as basic hygiene concerns, washing, shaving, and toothcleaning must be available for the crews, particularly on long-duration missions. Although both Mir and Skylab had showers on board (Figure 114-28), none of the current spacecraft has one; instead, sponge baths using wet washcloths are the norm. This reduces the quantity of water required from 50 L for the average terrestrial shower to 4 L for the average ISS “bath.”³⁷ Handwashing in space uses only about 10% of the water used for handwashing on Earth.^48,55

FIGURE 114-28 Astronaut Pete Conrad makes use of the Skylab shower.

(From http://aerospacescholars.jsc.nasa.gov/HAS/cirr/em/11/11.cfm.)

A rinseless body bath solution is stored in standard drink pouches. Crewmembers add warm water to the pouches before use, then perform a modified sponge bath with washcloths. Waste water from bathing is either captured and sent to the wastewater tank or blotted up by towels, which can then be allowed to air-dry in the cabin. Because of difficulties associated with hair washing, as well as the problems posed by stray hairs floating about the cabin, many crewmembers, particularly those on long-duration missions, prefer to keep their hair short. Nail clippings can also pose a problem. They can be inhalation hazards as well as having the potential to cause corneal abrasions.

Astronauts wash their hair with a rinseless shampoo originally designed for bedridden hospital patients. Crewmembers first wet their hair with water, then apply the rinseless shampoo (Figure 115-29, online). The hair is scrubbed carefully to avoid flinging water drops throughout the cabin, then dried with a towel. Blow drying is not an option on orbit—crewmembers must wait for their hair to air dry. Generally, the cabin humidity is held around 60%, which allows relatively rapid drying, although it does put additional strain on the humidity control system. A video of Pam Melroy washing her hair in space is available at http://spaceflight.nasa.gov/living/spacehygiene/hygiene_1.html.

FIGURE 114-29 Astronauts Bob Curbeam, Sandra Magnus, and Charlie Precourt wash their hair on the Shuttle with the rinseless shampoo.

(From NASA Human Space Flight Gallery. http://spaceflight.nasa.gov/gallery.)

For dental hygiene, astronauts use commercial toothpaste and toothbrushes. In weightlessness, saliva becomes more concentrated, which can lead to more tartar formation on the teeth. To compensate, many astronauts chew gum. The major difference between brushing teeth on Earth and in space is that astronauts must either swallow their toothpaste or spit it into a tissue.

Most male astronauts prefer to shave rather than allow their beards to grow. Generally, electric razors are preferred over safety razors and shaving cream, simply for ease of use when running water is not available.

Premenopausal female crewmembers often use pharmacotherapy to suppress menstruation on orbit. For short-duration Shuttle crewmembers, it can be as simple as juggling the placebo week of oral contraceptives, delaying their periods until the mission is over. For ISS missions, however, female astronauts often turn to longer-acting medications, such as Depo-Provera, in order to avoid menses altogether. The alternative, using pads or tampons, creates additional burdens of ensuring adequate supplies and dealing with contaminated waste.

“Marching On Their Stomachs”

Food is a key part of crew morale and performance. Napoleon recognized that “an Army travels on its stomach.” The U.S. Navy has a great deal of experience showing how lack of variety in a menu or poorly prepared food can negatively impact a mission. Indeed, in addition to decrements in job performance, increased interpersonal conflicts have been reported when palatable foods are in short supply.^24,64,70,85

As a result, there are concerted efforts to offer space travelers a variety of nutritionally balanced foods with menus individually tailored to each. Current U.S. menus include a variety of foods, such as nuts, peanut butter, chicken, beef, seafood, tortillas (preferred over bread because of the latter’s crumbs), candy, rice, and brownies. When international astronauts fly on the Shuttle, they often bring foods from their own countries. The current ISS food system is made up from contributions of the individual space agencies as their astronauts join the ISS crew. Typical ISS foods include jellied perch, grits with butter, honey cake, beef with barbecue sauce, Russian cheese, cinnamon rolls, lasagna, apple-black currant juice with pulp, granola bars, stewed cabbage, lemonade, barley kasha, chicken noodle soup, and chocolate pudding.

Unfortunately, despite these efforts, both the Russian and American space programs have consistently documented weight loss in crews during spaceflight. During the Mir program, up to 15% body mass losses were seen over the course of a 3- to 4-month mission. Approximately 1% of the loss can be attributed to water loss, due to the cephalad fluid shifts and redistribution associated with microgravity; the rest was due to bone, muscle, and adipose tissue losses. Loss of lean body mass is particularly concerning because it is associated with increased risk of skeletal fractures and thus compounds the microgravity-related effects on bone.

When adipose and muscle tissue losses are seen, particularly in the setting of lower caloric intake, inadequate nutrition is the default diagnosis. The stressful, isolated environment of spacecraft, early SMS, limited menu choices, and altered taste sensation are some of the factors quoted to explain crews’ weight loss in space. Microgravity-associated muscular atrophy (particularly in leg muscles, where there are significant decreases in muscle mass and performance) and bone loss (especially in weight-bearing bones) have also been implicated.

Refrigeration for crew food has recently been added to the ISS to allow cool drinks and fresh produce, thereby decreasing reliance on processed foods. Many of the physiologic adaptations to weightlessness may affect (or be affected by) nutritional intake. Although it is unlikely that diet will prevent all of these changes, it is distinctly possible that consuming too few nutrients can worsen the adaptations or at least delay an astronaut’s postflight recovery.

Protein balance during spaceflight is particularly problematic. Decreased body proteins are associated with impaired performance, decreased immune function (which is also a documented effect of space flight), and clinical depression. Both caloric and protein intakes are traditionally lower in space; however, loss of body mass has occurred even in the presence of adequate nutritional intake. This suggests that protein balance may be altered significantly in the microgravity environment. Studies have shown increased whole-body protein turnover during space flight, but the etiology and relevance of this finding remain unclear.

One major problem with consumption of processed foods on orbit is that they tend to be high in sodium. One study on the Shuttle documented dietary sodium intakes higher than 4000 mg per day.^64,70 Terrestrial research has demonstrated that excessive sodium increases bone turnover and urinary calcium excretion, which are already abnormal in space because of the microgravity environment. There is thus concern that the processed foods may contribute to (or worsen) bone demineralization and increase renal stone risk during flight.

Systems for water storage in microgravity environments are significantly more complex than are terrestrial systems. Normally, gravity ensures that water stays in one place, separates from air, and moves down an incline. None of these things occurs in microgravity, necessitating enclosed containers, pressurized lines, and other methods for air–fluid separation. In microgravity, it is much easier to force water out of flexible-walled containers, because they can be squeezed or rolled, much like toothpaste tubes on Earth.¹³ Unfortunately, these containers tend to have shorter life spans than do their rigid counterparts. Some systems make use of both; in the Soyuz, for example, a rigid-walled tank holds a flexible bladder containing the water, and mouth suction is used to draw water out as required.

For many years, the color, smell, and taste of water in space left much to be desired. Early U.S. spacecraft (Mercury and Gemini) used water from municipal water supplies. Tang, the drink mix forever linked to the early American space program, was actually created to mask the metallic taste of the Apollo spacecraft’s water, although additional minerals and trace nutrients were also added to the mix. The Shuttle’s use of iodine as a bactericidal agent led not only to complaints about the taste and color of the water but also to elevated thyroid-stimulating hormone levels. An iodine removal system has since been added to the galley water supply to remove iodine from the water just prior to consumption. The addition of refrigeration to the ISS for crew use is likely to increase consumption of plain water by the crew who have previously had to drink lukewarm water in addition to using it in drink mixes (e.g., tea, coffee) or soups (bouillon, broths).

Water quality can be compromised through biologic contamination (“brown sludgy water”), chemical contamination (“green glowy water”), or both. On short-duration missions, preflight verification of water quality is considered sufficient, but on longer missions, in-flight testing is also required, as contaminants could enter the storage containers during the course of the mission.

Water is routinely delivered to the ISS via the U.S. Shuttle (often in 90-lb, duffle bag–like water containers) and Russian “Progress” rockets (in large tanks), but doing so is expensive and creates dependency on ground supplies. With the addition of Node 3, and the new Water Recovery System, the ISS now has a degree of autonomy¹³ (Figure 114-30, online). It was in part this dependence on Shuttle-delivered water that forced a decrease in ISS crew size following the grounding of the Shuttle fleet in February 2003 after the Columbia disaster. In the absence of regular visits from the Shuttle to replenish water supplies, a three-person crew could not be sustained aboard the ISS, and the crew complement dropped temporarily to two during the 2003-2005 timeframe in ISS Expeditions 7 through 11.

FIGURE 114-30 Astronaut Don Pettit with ISS water containers.

(From NASA Human Space Flight. http://spaceflight.nasa.gov/gallery.)

In-Flight Exercise Countermeasures Programs

As early as the 1960s, Busby identified “periodic physical exercise” as a means for “maintaining an optimum level of physical ‘fitness’ during space missions.”¹⁷ In the early days of the space program, vehicle dimensions did not permit much physical exercise. Since the Skylab program, in-flight exercise has been used as a way not only to have fun (as numerous crew photos attest), but also to monitor physical condition and to prepare for various tasks, such as an EVA.

One of the systems most affected by prolonged exposure to microgravity is the musculoskeletal, because lack of a gravitational field causes muscle atrophy, bone demineralization, diminished exercise tolerance, loss of strength, increased risk of kidney stones, osteoporosis, and increased fracture risk. Even during a mission to LEO, musculoskeletal changes can prove dangerous, such as excessive fatigue leading to premature end of an EVA. Exploration-class missions could be seriously compromised if crewmembers become so weakened by their time in microgravity that they are unable to carry out physically challenging tasks on arrival. An expert panel voiced concern that “an abrupt return to gravity whether back on Earth or at the destination of an exploration mission imposes high workloads on weakened muscles and may cause post-space travel pathologies. Adaptation to the lower workload in microgravity may render muscle tissue more prone to structural failure when it is reloaded.”¹¹

One way to avoid such outcomes is through regular in-flight physical exercise, because the majority of muscular alterations during space flight are believed to be caused by gravitational off-loading. As such, the changes should be responsive to exercise-based countermeasures. Some of the standard terrestrial exercises require adaptation for use in the microgravity environment. Weightlifting is meaningless in the microgravity environment—there is no “weight” to “lift.” Instead, strength training occurs through resistive devices, ranging from very simple (e.g., bungee cords, which were used as far back as the Gemini program) to highly complex equipment, such as the ISS RED (resistive exercise device). For aerobic training, crewmembers have used everything from rowing devices to exercise bicycles.

The goals of the physical exercise countermeasures program include maintaining lean muscle mass and strength (thereby enhancing EVA performance and reducing risk of injury), preserving aerobic capacity, maintaining bone mineral content and structure (to reduce risk of injury during and after the mission, including the risk of long-term osteoporosis), and lowering excretion of urinary calcium (in order to reduce risk of renal stones).

The value of strength training versus aerobic conditioning for astronaut health, performance, and postflight recuperation has been much debated, but there is little solid evidence to determine which (if either) is the best countermeasure. Similarly, there are few data to identify the preflight training regimen that will best prepare astronauts for long-duration space flight and their eventual return to Earth’s gravitational field. As a result, both strength and aerobic exercises are currently performed by long-duration crewmembers before and during their sojourn on the ISS. Usually 30 minutes of each 90-minute training session are devoted to aerobic conditioning, with the remaining 60 minutes devoted to resistive exercise.⁹⁴ The RED allows numerous muscle groups to be exercised, including the back, quadriceps, and arms (Figure 114-31, online).

FIGURE 114-31 Astronaut Yuri Gidzenko working on the ISS interim RED (resistive exercise device).

(From http://spaceflight.nasa.gov/gallery/index.html.)

Cycle ergometry use has been associated with improvements in cardiovascular deconditioning, but it does little to affect skeletal muscle atrophy. Exercise on the treadmill (Figure 114-32), by contrast, may be able to improve cardiovascular parameters and prevent a common problem of foot-drop positioning with associated extensor range shortening. It can also minimize skeletal deconditioning through its associated axial load and heel strike. When used in conjunction with visual stimulation (e.g., watching a DVD while running on the treadmill), it may also enhance neurovestibular readaptation. However, additional controlled studies are required before any particular form of exercise can be clearly labeled “best.”

FIGURE 114-32 Astronaut Andy Thomas uses a treadmill aboard the Russian Mir Station.

(From http://spaceflight.nasa.gov/gallery/images/shuttle/sts-91/hires/91375011.jpg.)

Cycle ergometers (Figure 114-33, online) proved popular, not only because bicycling is a familiar terrestrial activity but also because with a simple substitution of hand grips for pedals, the same device can be used for upper-limb training as well as lower-limb. This is particularly useful for EVA crewmembers, as the majority of the work done during spacewalks is performed by the upper body.

FIGURE 114-33 Astronaut Frank Culbertson on the Shuttle middeck works out on the bicycle ergometer.

(From http://spaceflight.nasa.gov/gallery/images/shuttle/sts-105/hires/s105e5004.jpg.)

Various types of treadmill devices have been flown over the years, from unpowered ones with a Teflon-like surface that allowed easy sliding (Figure 114-34), to powered ones such as the current ISS model. In all cases, a restraint harness is required to hold the astronaut down against the treadmill surface.

FIGURE 114-34 The treadmill developed for Skylab 4 consisted of a Teflon-coated aluminum plate bolted to the “floor,” a restraint harness, and bungee cords.

(From http://images.jsc.nasa.gov/luceneweb/caption.jsp.)

On the ISS, devices recently added for strength and aerobic exercise include the new Combined Operational Load Bearing External Resistance Treadmill (COLBERT) which is placed inside Tranquility Node 3 (see http://www.nasa.gov/images/content/177228main_Mobility6.jpg).

Generally, heart rate and equipment settings are used to track efficacy of the exercise program. This information is downlinked to the flight surgeon and athletic trainers with whom the crewmember had worked to prepare for the mission, and in-flight exercise “prescriptions” are then modified as appropriate. This is a highly staff-intensive method of establishing proper exercise regimens; in the future, with Moon or Mars bases and increasing numbers of personnel, it is likely that some less intensively monitored or IT-based system will need to be developed.

To date, the vast majority of space flight experience has been with relatively short missions in a microgravity environment, but the next decades may well see a return to the Moon and even a journey to Mars. These missions present new questions, as for the first time in more than 30 years, space travelers will be exposed to gravitational forces (approximately  G on Mars,  G on the Moon), rather than microgravity. Although the impact of partial gravity on physiologic alterations remains unclear, it is prudent to assume the worst and plan for the same level of exercise as that required to counteract the effects of microgravity.

Psychological Support

On long-duration missions, psychological support becomes critical to mission success. This takes many forms, from support of the family that has been left behind to avoidance of overwork and fatigue (Figure 114-35).

FIGURE 114-35 A graphic explains the damage caused in the 1997 collision between the Mir station and a supply ship. Crew fatigue was cited as a causative factor in the collision.

(From http://www.cnn.com/TECH/9707/mir/repair/mir.collision.lg.jpg.)

It is well established that complex task performance decreases after a certain period of persistent work, and that cumulative fatigue and boredom play causative roles in the deterioration. Although drugs such as dextroamphetamine sulfate (Dexedrine) and caffeine can temporarily improve performance, the effect rapidly wanes, often to a level below that originally noted. By contrast, a period of restful sleep restores subjects to their original level of performance—demonstrating the value of healthy sleep–wake cycles.

As a result, the international space medical community has developed rules governing crew rest. NASA flight surgeons attempt to prevent fatigued crews from performing dangerous tasks. Although lack of normal circadian cues in space makes it more challenging to determine when crews are at their best (or worst), it is clear that crews require a few days to adjust to a new sleep schedule before they function at peak performance (Figure 114-36, online).

FIGURE 114-36 A sleep station in the ISS lab module.

(From NASA Human Space Flight. http://spaceflight.nasa.gov/gallery.)

Increasingly psychological and psychiatric support plays an important role in crewmembers’ lives, along with family support. This will be particularly important in exploration-class missions, which in many respects will parallel arctic and submarine crews, with the added complication of communication delays.

“Are We There Yet? I’m Bored!” Personal Recreation

For years, military leaders have been aware of the importance of providing adequate rest and recreation time to their subordinates, lest team morale and performance drastically decrease. In many wilderness expeditions, the surroundings and the activity involved in getting to the wilderness areas provide all the recreation one can desire. In space, however, although the views can be breathtaking (Figure 114-37), the sheer duration of the missions can lead to boredom.

FIGURE 114-37 Astronaut Susan Helms enjoys the view from the ISS.

(From NASA Human Space Flight (http://spaceflight.nasa.gov/gallery), Spacelab Experience, 1981-1986. http://history.nasa.gov/NP-119/ch2.htm.)

Recreation takes many forms in space, depending on spacecraft geography, mission duration, and crew composition. On Apollo 14, for example, astronauts played golf during lunar excursions, but that would not work well inside a habitat module. In Skylab, by contrast, where the three-man crews had a larger inhabitable volume than in any other platform before or since, they could engage in acrobatics and tumbling.

On the ISS, where space is more limited and daily exercise required, more sedentary pursuits are often enjoyed. Computers enable crews to enjoy music, games, books, or movies on a single device. Another popular source of entertainment is the ham radio. Many space station crewmembers have chatted with other radio enthusiasts during sojourns in space. True broadband access to the world wide web from the ISS remains unavailable as yet.

Bandwidth constrains communications to friends and family, but this has improved over the years. Astronauts on the ISS and Shuttle now enjoy a video teleconference with their families every week, and they also have e-mail and phone service, albeit somewhat intermittently.

Postflight Rehabilitation

After the crew has returned to Earth, the flight surgeon performs standard physical examinations on landing day and 3 days later. Flight surgeons meet the crewmembers at landing and accompany them to a crew rehabilitation site, usually at Johnson Space Center or Russia’s Star City. After a few days of minimal physical demands, the rehabilitation process begins in earnest and is considered the crewmembers’ top priority.

The goal is to return the astronaut to preflight condition (and thus to flying status) as quickly as possible. Following short-duration flights, this may happen within a few days or weeks, but recovery from long-duration missions may take months or longer. Major milestones along the rehabilitation pathway include return to the family home, unlimited driving privileges, return to terrestrial flight status, return to short-duration space flight status, and return to long-duration space flight status.

Particularly after long-duration space flight, rehabilitation is generally guided by the same athletic trainers and flight surgeon who supervised the crewmember’s in-flight exercise program. Continuity of care helps to improve the exercise regimen’s efficacy as well as the interpersonal dynamic between the crewmember and his or her health care team.

The “continuum of preventive, therapeutic, and rehabilitative care on the ground, during space travel, and upon the return from space travel” described by the Institute of Medicine as required by astronauts,¹¹ particularly those returning from long-duration missions, has worrying implications for exploration programs. If ISS astronauts, after flights of several months, require the close attention and rehabilitation currently provided to them, how will any Mars-bound astronauts be able to function on reaching their destination?

Perhaps travelers to the Moon’s surface will be less affected, since their trip is so much shorter, but if a moon base is established with mission assignments of months or even years, what in situ countermeasures might be needed by its personnel? On a base of any size, let alone a commercial venture such as a space-based hotel, support personnel (e.g., cleaners, cooks, mechanics) will be required. What will happen to their long-term health if they are less fit preflight than current astronauts or lack discipline for in-flight countermeasures?