Space Environments
James Perry Locke
The two problems we face in the space program are gravity and paperwork. We can lick gravity, but sometimes the paperwork is overwhelming.
—Wernher von Braun (1912-1977)
During the last four decades, humans have ventured beyond Earth’s biosphere. Many short-duration missions to low Earth orbit (LEO) have been conducted. Humans have walked on the surface of the Moon. In space stations circling the Earth, humans have been able to live and work for months at a time. Unoccupied spacecraft have also landed on distant planets, such as Mars. This next decade will see the completion of the international space station (ISS), which will be used to gain further expertise in long-term survival in space. Knowledge gained from ISS operations will facilitate the development of the more sophisticated spacecraft life support systems needed for even bolder human spaceflight missions. The coming decades could see human missions to Mars or a return to the Moon to establish human research facilities. In the distant future, life support technology may even facilitate the creation of permanent extraterrestrial bases or colonies.
In general, the conditions found on Earth are quite conducive to human survival. With a few exceptions, humans inhabit nearly every region of the planet’s landmasses. Humans modify their surrounding environment to provide more habitable conditions, but by and large, relatively little environmental modification is necessary on Earth. For the most part, humans take the intrinsic habitability of Earth’s surface for granted. However, in space, the environmental conditions found are not compatible with human survival. Defining the differences in environmental conditions between those found on Earth and in space shows why this is so.
It may appear that a suitably habitable environment can be created simply by attempting to reproduce terrestrial environmental conditions within the cabin. In actuality, it is first necessary to define the detailed environmental characteristics that humans require and then match these requirements with the other design constraints. Implementing these environmental characteristics within a spacecraft can be quite challenging. Balancing these various requirements and constraints can test the limits of available technology.
Using the life support systems of actual spacecraft as examples can illustrate the concepts and challenges associated with designing the systems used to support human survival in space. Finally, discussing cutting-edge life support technology points to how these design challenges might eventually be surmounted, enabling future human missions to Mars and beyond.
THE SPACE ENVIRONMENT
The space environment is markedly different from that found on the Earth’s surface and is not habitable, primarily because it lacks an appropriate gaseous atmosphere. However, there are other environmental characteristics that define the space environment that can also significantly affect human survival.
Lack of Atmosphere
Earth’s atmosphere consists of a combination of temperature, pressure, and gas composition necessary for human survival on the planet’s surface. The vast majority of the gas molecules that comprise the atmosphere are within 10,000 m of the surface. As distance from the surface increases, the number of atmospheric gas molecules per unit volume decreases and the number of collisions between individual gas particles decreases, causing atmospheric pressure to fall. Figure 10-1
illustrates the thinning of the atmosphere away from the Earth’s surface.
illustrates the thinning of the atmosphere away from the Earth’s surface.
 FIGURE 10-1 Tangential view of Earth’s atmosphere. |
There are other key physiologic and engineering constraints that occur at specific altitudes within the atmosphere. Pressurized oxygen delivery masks are ineffective above altitudes of 13,000 m (43,000 ft), making cabin atmospheric pressurization necessary (see Chapter 3). Above 18,900 m (63,000 ft), the total atmospheric pressure equals the vapor pressure of water at body temperature. It is above this altitude, known as Armstrong’s line, that the bodily fluids of an unprotected individual would spontaneously boil in a process known as ebullism. Above 27,000 m (90,000 ft), the atmosphere becomes too thin to support current air-breathing jet engines requiring the use of rocket motors. At altitudes above approximately 90 km (47 mi), the atmosphere is too thin to allow the use of aerodynamic control surfaces, necessitating the use of reaction motors to control the orientation of a vehicle. This is known as the Von Karmann line. Above altitudes of 180 to 200 km, air resistance becomes negligible, marking the true engineering boundary between the atmosphere and space. The upper limit of the atmosphere is defined as the point at which collisions between molecules become immeasurably infrequent (˜700 km). Above this is the hard vacuum of true space, known as the exosphere, where the number of air molecules thins to a density of approximately 1 to 20 mol/cm3.
Altered Gravity Environments
The force of gravity is a fact of life on Earth. Humans traveling to space from Earth, traveling in space, and returning to Earth can expect to experience significant alterations in gravitational effects. At the Earth’s surface, the mass of the planet exerts gravitational force equal to 9.81 m/s2 (32 ft/s2). This gravitational field extends for millions of kilometers beyond the surface (the Moon is trapped within this gravitational field). However, placing a spacecraft in orbit can counterbalance this gravity field, allowing the vehicle to remain in space (Figure 10-2). The speed of the vehicle must be sufficient to generate centrifugal force equal in magnitude to the planet’s gravitational force at that altitude. When this is done, the vehicle will not fall back toward the surface. In other words, an orbiting object is continuously falling around the planet, although its tangential velocity prevents it from falling closer to the surface. This balancing of centrifugal and gravitational force vectors simulates the lack of true gravity, and is more properly referred to as free fall or microgravity.
 FIGURE 10-2 Representation of the balance of forces that produces weightlessness (OG) in Earth’s orbit. OG, oxygen generator. |
The magnitude of the planet’s gravitational field determines how fast an object must travel to achieve orbit. Spacecraft in LEO at altitudes of 250 to 600 km (150-350 mi) must maintain tangential velocities of 28,000 to 42,000 kph (17,000-25,000 mph). Conceivably, objects could orbit at lower altitudes, but aerodynamic friction from the Earth’s atmosphere precludes this. Smaller planets exert less gravitational force, so the velocity required to reach orbit is less. Orbital velocities around Mars (where the surface gravity is one third as strong as Earth’s) and the Moon (where the surface gravity is one fifth of Earth’s) are much lower at comparable altitudes.
Microgravity significantly affects the habitability of space because humans are naturally adapted to the gravity of the terrestrial environment. Before the first human spaceflights proved them wrong, some scientists were worried that humans could not live without gravity (1). Microgravity does affect human health, from the immediate effects of motion sickness and possible alterations to gastrointestinal functioning to the long-term effects of bone demineralization. Further discussion of the medical risks of microgravity is addressed in Chapter 28.
As noted earlier, traveling to space using current rocket technology requires reaching orbital velocities. To return from space, spacecraft must lose velocity by using the Earth’s atmosphere as a brake. These large changes in velocity impose significant acceleration and deceleration forces on spacecraft and their inhabitants. Mercury astronauts experienced acceleration forces of up to 8 Gx (chest to back) for short periods of time (2). In contrast, shuttle passengers now experience a maximum of 3 Gx during launch. During reentry, Soyuz passengers can experience up to 10 Gx. Shuttle passengers experience lower forces (1.3 Gy), but for longer duration, and in a different orientation (head to foot).
Changing a spacecraft’s linear or rotational velocity while in space also subjects passengers to acceleration forces.
Changing a spacecraft’s linear or rotational velocity while in space also subjects passengers to acceleration forces.
Gravitational forces can also be simulated by centrifugal force caused by rotating a spacecraft about its axis. In the future, rotating spacecraft might have the pseudogravity of centrifugal force to mitigate the adverse effects of weightlessness.
Orbital Mechanics
Although a detailed discussion of orbital mechanics is beyond the scope of this text, an explanation of basic orbital nomenclature illustrates the impact of orbital mechanics on spacecraft operations. Orbital mechanics play an important role in the capability of launching payloads into orbit. As shall be seen, orbital mechanics also play a central role in determining the radiation exposure a spacecraft receives.
While launching a rocket into Earth orbit, the angle of the launch vector with regard to the equator defines how much acceleration is needed to reach the orbit and hence how much payload can be lifted into it. This is because the Earth’s rotation profoundly influences what orbits are achievable. The tangential velocity of the Earth’s surface is greatest at the equator (1,040 mph or 1,734 km/hr), and drops off to zero at the Earth’s poles. When launching a rocket in an eastward direction, the rotational velocity of the Earth can be applied to the amount of velocity needed to reach orbit. The lower the latitude, or more southerly the launch site is, the greater the available rotational velocity is and the less rocket propellant will be required. Attaining orbit is most efficient at the equator and most difficult at the poles. Changing the angle of the launch vector changes the angle of inclination of the resultant orbit. Orbits are typically referred to by this angle of inclination. Launching a vehicle due eastward from the equator would give an orbital inclination of 0 degrees. From any given launch site, the most efficient possible orbital inclination is equal to the latitude of the launch site. Therefore, a rocket launched due eastward from Kennedy Space Center in Florida will enter a 28-degree orbit, and a vehicle launched eastward from the Baikonur launch site in Kazakhstan will have an orbital inclination of 51.6 degrees. Any rocket launched from the Earth’s pole will have a 90-degree inclination. Orbits of different inclination can be achieved from a given launch site by shifting the launch vector northward or southward from due east, but this comes at the expense of significant increases in the amount of propellant required to reach orbit. Therefore, a space shuttle launched from Kennedy Space Center to the ISS (which is in a 51.6-degree orbit) can carry much less payload than if it were launched into a 28-degree orbit. Launching a rocket westward requires extrapropellant to counteract the Earth’s rotation, making such orbits prohibitively inefficient.
Radiation
The space environment is relatively devoid of matter, but it can be full of energy, especially in the vicinity of a sun such as ours. This energy comes in the form of electromagnetic radiation and high-energy particles. The electromagnetic radiation found in space is of many wavelengths, from low-frequency microwave radiation and below to infrared (heat), visible light, and ultraviolet wavelengths and all the way to high-frequency x-ray radiation and γ radiation and beyond. Particles such as protons, electrons, neutrons, α particles, and heavy ions may be of inconsequential mass but can be highly energetic. The Earth’s atmosphere and surrounding geomagnetic field partially shield the planet’s surface from space radiation. During spaceflight outside of LEO, spacecraft can leave this protective envelope, increasing the risk of exposure of the passengers to potentially harmful radiation.
Ionizing versus Nonionizing Radiation
Particle and electromagnetic radiation can be grouped and classified as either ionizing or nonionizing radiation. Ionizing radiation has sufficient energy to knock material from atomic structures during a collision, which can release further electromagnetic or particle radiation. Astronauts have reported that, when their eyes are closed, they occasionally see small flashes of light, evidence that the energetic products of nuclear collisions occurring within the eye can activate retinal visual receptor cells (3). If ionizing radiation particles collide with atomic nuclei inside human cells, the resultant energy release can damage cellular DNA. This genetic damage can lead to cell death or the cellular mutation that underlies carcinogenesis.
Nonionizing electromagnetic radiation may not be energetic enough to directly damage genetic material, but it can still be quite harmful. Solar ultraviolet electromagnetic radiation, unattenuated by the atmosphere, can cause severe sunburn and retinal burns after only seconds of exposure. Solar infrared radiations that can cause significant thermal loading, are discussed later.
Ionizing radiation comes from several sources: galactic cosmic radiation (GCR), solar radiation, and geomagnetically trapped radiation. Because the altitude and trajectory of a spacecraft’s flight affect the amount of radiation received from these sources, this information is used to calculate projected radiation dose profiles that are used during mission planning.
Galactic Cosmic Radiation
GCR is radiation that originates outside of the solar system and is probably generated by the cataclysmic extrasolar events such as supernovae. GCR is predominantly particle radiation, consisting of α particles, β particles, and the heavier nuclei such as tin or lithium. These particles often travel at tremendous velocities, imparting them with very high energy usually in the range of 0.3 to 2 GeV (109 eV). Although there is a relatively constant flux of GCR into the solar system, the amount of GCR in the region of planetary bodies is influenced by solar activity. Because planetary magnetic field strength increases during periods of high solar activity, scattering of charged GCR charged particles away from the planetary environment is maximal during these periods, thereby decreasing the amount of GCR that penetrates the geomagnetic belts.
Although they can be diverted by electromagnetic fields, the high velocities of galactic charged particles allow them to penetrate through meters of solid matter, making passive shielding (e.g., the aluminum walls of the spacecraft) essentially useless. Fortunately, its high velocity and low flux density makes it likely that GCR will pass through an astronaut without striking any atomic nuclei, yielding minimal energy transfer to the cellular components. Increased amounts of passive shielding increases the likelihood that GCR particles will collide with nuclei within the shield. These nuclear collisions may release a shower of secondary electromagnetic radiation and high-energy particles (usually neutrons) that may have more adverse biologic effects than the original particles (4).
Solar Wind and Solar Cosmic Radiation
The solar wind consists of proton-electron plasma that is ejected from sun at velocities of 400 to 500 km/s (240-300 mi/s). This solar wind or solar cosmic radiation (SCR) is the most variable portion of the background space radiation and changes density considerably during the Sun’s 11-year cycle of activity. During periods of high solar activity, SCR can be the major source of the astronaut’s space radiation exposure, especially when the solar particles become trapped in the Earth’s geomagnetic belts. In periods of minimal solar activity, the trapped radiation belts are not as energized, and GCR becomes the predominant source of exposure. Solar flares can cause a 1,000-fold increase in the radiation flux of the SCR in the form of solar particle events (SPEs).
Solar Flares and Solar Particle Event Radiation
Magnetic disturbances on the Sun’s surface can lead to solar flares, which consist of electromagnetic radiation, as well as SPEs that consist of high-energy protons. The SPEs can contain particles of sufficient energy and flux density to result in a lethal radiation exposure to an unshielded space traveler unfortunate enough to be caught within it (5). Fortunately, the particle radiation released during such events is not uniformly distributed in all directions, so not every SPE will expose a spacecraft in a given location to the maximum flux of radiation emitted. The rate of onset and rate of dissipation of an SPE radiation flux density can also be quite variable and can vary in duration from minutes to days. SPEs typically occur during the active period of the solar cycle.
Unfortunately, it is very difficult to predict exactly when an SPE might occur, as well as the duration and flux of the radiation that the SPE will produce and how significant a spacecraft’s exposure to that radiation will be. Radiation detectors orbiting the Earth can measure the increases in solar electromagnetic radiation that accompany solar flares, and other devices can detect increases in proton flux density. Detection of significant increases in electromagnetic and particle radiation indicate that an SPE may be occurring. Spacecraft may then be alerted, so that crewmembers can conceivably take shelter in a shielded “safe haven” until the radiation flux returns to acceptably low levels (6).
The Earth’s magnetic field is an effective shield against even the most massive SPEs. The radiation contained by the largest SPE ever recorded (August 1972) would have been undetectably low to a space shuttle traveling in a typical orbit under the Earth’s magnetosphere; however, it could have posed a health hazard to an inadequately shielded spacecraft traveling in a higher orbit.
Magnetically Trapped Radiation
The size and nature of the Earth’s geomagnetosphere was examined by Dr. James Van Allen and his team in the 1950s. Generated by the rotation of the Earth’s molten ferromagnetic core, the Van Allen belts are toroidal structures that encircle the planet in an extremely powerful magnetic field. The thickness of these geomagnetic belts depends on the latitude, the thickest portion being near the equator. Note that the Earth’s magnetic pole is slightly offset from its rotational pole. Instead of striking the Earth’s surface, the high-energy particles of GCR and the solar wind become trapped within these magnetic belts, oscillating along the lines of magnetic force. The amount of trapped particles in these belts increases during periods of high solar activity.
The Van Allen belts are compromised of two layers, an inner belt (extending from altitudes of 300-1,200 km) that contains trapped protons and heavy ions and an electron-containing outer belt (extending from altitudes of ˜10,000 km to altitudes of more than 55,000 km, depending on the solar wind). Because of their toroidal shape, the magnetic fields of the Van Allen belts can lie close to the outer surface of the atmosphere at extreme northern or southern latitudes. (These low-lying regions are known as the auroral horns.) This is illustrated in Figure 10-3. At extreme latitudes, impact of trapped high-energy particles with the atmosphere can ionize the atmospheric gas molecules, causing the spectacular light shows of the Arctic Aurora Borealis and Antarctic Aurora Australis.
Most orbital flights occur at altitudes below the majority of the volume of these magnetic belts, providing some protection to spacecraft occupants from space radiation. However, because the rotational axis of the Earth does not coincide with its magnetic pole, this discrepancy causes the magnetic belts to dip to altitudes as low as 160 to 320 km (95-215 mi) in the region known as the South Atlantic anomaly (SAA). Orbiting spacecraft passing through this region are exposed to particle radiation trapped within the magnetic field that is equal in intensity to radiation found at altitudes of 1,300 km (750 mi). Most radiation received during shuttle missions with typical low-inclination orbits (28 degrees) occurs as a result of passing through this zone. The percentage of orbits that pass through the SAA depends on the angle of inclination. The higher inclination (51.6 degrees) orbit of ISS pass through the center of the anomaly less frequently than 28-degree low-inclination orbits. However, because the size of the anomaly increases at higher altitudes, a spacecraft orbiting at high altitude (e.g., ISS) will be exposed to more radiation than a spacecraft traveling along the same orbital track at a lower altitude.
Also, high-inclination orbits pass through the auroral horns, where the geomagnetic belts dip closer to the surface, thereby increasing exposure to trapped belt radiation. Because the geomagnetosphere does not cover the Earth in polar regions, spacecraft traveling in very high-inclination orbits are also exposed to a higher dose of GCR.
Also, high-inclination orbits pass through the auroral horns, where the geomagnetic belts dip closer to the surface, thereby increasing exposure to trapped belt radiation. Because the geomagnetosphere does not cover the Earth in polar regions, spacecraft traveling in very high-inclination orbits are also exposed to a higher dose of GCR.
 FIGURE 10-3 Van Allen radiation belts. |
Radiation on the Moon and Mars
Because it lacks both an atmosphere and a geomagnetic field, the Moon’s surface is not shielded from radiation as Earth’s surface is. The lunar surface, composed of rock and powdered debris from meteor impacts (regolith), is therefore subjected to steady bombardment by GCR and solar radiation, including the occasional SPE. Mars also lacks a significant geomagnetic field, but its thin atmosphere of carbon dioxide does provide some radiation shielding. Because a planet’s spherical mass shields a given point on its surface from a geometrical proportion of cosmic radiation (this is known as 2π shielding), the radiation experienced on the surface of these bodies is roughly half of what would be experienced in similar regions of free space. Planetary materials can also be used as radiation shielding. If an inhabited lunar station were established, covering it with a thick coating of regolith could serve as an effective radiation shield for everything except high-energy protons and neutrons (7). However, if the regolith coating were insufficiently thick, secondary radiation scattering from nuclear collisions within the layer could paradoxically worsen the radiation exposure to the crewmembers beneath it.
Thermal Instability
The Earth’s atmosphere not only provides pressurization and oxygenation but also serves as an insulating blanket, maintaining remarkably stable temperatures on the surface. The terrestrial atmosphere regulates the temperature changes caused by the day/night cycle. This temperature-regulating ability is strongest at sea level, where the mass density of the atmosphere is greatest, and degrades with increasing altitude. Anyone who frequents Alpine climates is familiar with the intense solar radiation during the day, followed by rapid temperature drops after sunset as the daylight’s heat radiates away.
In space, there is no atmosphere present to regulate temperature. Radiation is the sole method of energy transfer. At the Earth’s distance from the Sun, solar energy radiation is sufficient to heat the exposed surfaces of objects to temperatures that can cause significant thermal damage to materials and equipment. The intensity of solar radiation decreases with increasing distance from the Sun. In the farther reaches of the solar system, the Sun provides little heat. Also, because there is no atmosphere in space to retain heat, heat dissipates very rapidly. Even at the Earth’s distance from the Sun, surfaces not exposed to sunlight typically have temperatures far below the freezing point of water. Marked temperature differences between the sun-exposed and nonexposed surfaces of any given object can impose significant thermal stress.
Humans cannot tolerate these thermal stresses. Astronauts working outside of their spacecraft in spacesuits during extravehicular activities (EVAs) can simultaneously experience intolerably hot and intolerably cold temperatures. They must be protected from these massive fluctuations in temperature to survive. The environmental control systems necessary to maintain thermal stability is discussed later.
Space Debris
Earth’s gravitational field attracts solid objects, most of which enter the atmosphere and burn up as shooting stars. These objects are called meteorites or micrometeorites, depending on their size. It is estimated that as much of 10,000 metric tons of micrometeorite material reaches the Earth’s surface daily. However, it is estimated that only approximately 200 kg of this material is suspended in orbit within 2,000 km of the surface at any given time. Of this micrometeoroid material, the vast majority is extremely small (much less than 0.1 mm in diameter). Although small, their extreme velocities give these particles tremendous kinetic energy. A millimeter-sized object moving at orbital velocities would
easily penetrate the thin aluminum skin of a standard spacecraft.
easily penetrate the thin aluminum skin of a standard spacecraft.
Of greater concern is the large amount of orbital debris created by human activity. It is thought that there are millions of kilograms of human-related space debris suspended within LEO. This human-created orbital debris is not only primarily composed of very small objects such as paint flakes and aluminum oxide particles from solid rocket fuel but also consists of fragments of launch vehicles, old satellites, and even tools dropped by astronauts. The North American Air Defense Command (NORAD) tracks orbital debris larger than 10 to 20 cm in diameter, of which there are approximately 6,000 in orbit. Objects smaller than this are not routinely tracked.
Examination of the surfaces of spacecraft exposed to the orbital environment for extended durations indicate that collisions with orbital space debris occur frequently, but these collisions usually involve extremely small particles. The construction of typical spacecraft is sufficient to protect them from collisions with micrometeoroids of this small size. The spacesuits of astronauts performing EVAs offer some protection from such impacts. To further decrease the risk of micrometeoroid impacts, mission planners orient the shuttle so that the bulk of the vehicle protects the spacewalking astronauts from most potential micrometeoroid impacts.
Isolation
Although essentially unrelated to habitability, physical isolation is a characteristic of the space environment that significantly affects human survival. Physical resources of one form or another are always on hand on the Earth’s surface. The sophistication of modern transportation technology ensures that a journey between any two points on the Earth’s surface requires only a matter of hours. Supplies, replacement equipment, and additional personnel are seldom very far away. In contrast, orbiting spacecraft are separated from their terrestrial points of origin by both distance and velocity.
Because spacecraft in space are so completely separated from the resources of the terrestrial surface, the spacecraft must carry everything that the astronauts need to complete their mission or until they are resupplied. Apart from solar energy, space contains no resources that can feasibly be collected to replace used materials.
Rendering physical assistance to a spacecraft in need would require tremendous expenditures of resources, if such assistance were even possible. Even for a spacecraft to abort its mission and return to the protective confines of Earth can be a monumental task, as attested to by the experiences of Apollo XIII crew, who successfully struggled to guide their damaged spacecraft back from the Moon.
This physical isolation also affects communication. Large swaths of orbital tracks remain out of range of current radio communication equipment, cutting spacecraft off from contact with ground mission control for minutes at a time. Component failure in space-ground communications systems could leave orbiting astronauts to face dangers without the assistance of experts on the ground.
REQUIREMENTS FOR HUMAN SURVIVAL IN SPACE
The differences between the space and terrestrial environments discussed previously show why humans cannot survive in space without the provision of a suitably habitable environment, either within a spacecraft, space station, or spacesuit. There are many factors that must be considered when creating the appropriate environmental conditions. First and foremost are the human habitability requirements. In the spacecraft environment, how these habitability requirements are achieved depends on other external factors relating to the mission objectives and the constraints of available technology.
Human Habitability Considerations
Although habitability could be defined as being conducive to survival, an exact definition of the concept is elusive. Like the concept of health, environmental habitability is easier to define by its absence than by its presence. Humans live in an environment that is innately habitable to them, which gives them an implicit understanding of the environmental conditions that they can tolerate. Moreover, when given a choice, they demonstrate a high degree of specificity in selecting environmental conditions that they prefer. Yet when asked to define the characteristics that define an optimally habitable environment, most people would focus on nonessential elements, taking for granted the conditions that they truly need to survive. Because it is difficult to define, habitability is perhaps best considered in terms of how environmental conditions affect humans.
The primary habitability consideration for a given environment is that humans must be able to survive. This implies that the most basic physiologic parameters are met. The longer the human must survive, the more environmental components must be supplied.
Survival is not the only consideration that defines a habitable environment. The environmental conditions must also be of sufficient quality to permit the occupants to perform their required tasks. A wildly tumbling space capsule may keep its inhabitants alive, but the tumbling may keep them from reaching a critical control panel. Environmental conditions may also cause human performance to degrade with time. How quickly this degradation occurs depends on how poor the environmental conditions are.
The final consideration that defines environmental habitability relates to the health and safety of its inhabitants. Humans may be able to survive in a given environment and may be able to perform their required duties there, but their health could be affected by being there. Exposure to radiation may produce no obvious symptoms or decrements to performance, yet can cause significant health problems in the future.
Maintenance of a Breathable Atmosphere
The key to atmospheric habitability is to maintain the oxygen concentration at a partial pressure of oxygen (PO2)
near the sea-level partial pressure of 21 kPa (3.1 psi or 160 mm Hg) to avoid adverse physiologic effects such as decreased night vision, impaired memory and cognitive performance, unconsciousness, and death. These effects can begin within a few seconds of oxygen deprivation, depending on the degree of hypoxia, so it is extremely vital that the oxygen supply be uninterrupted (2). On the other hand, higher PO2 starting at 32 kPa (4.7 psi or 240 mm Hg) can also lead to physiologic problems, such as lung irritation and damage and central nervous system impairment. Figure 10-4 illustrates the ranges of atmospheric pressure and oxygen concentration that are acceptable for use in spacecraft. For safety reasons, oxygen concentration and partial pressure must also be kept as low as possible to minimize the risk of fire. Current guidelines require that American spacecraft atmospheres nominally contain less than 23.5% oxygen at sea-level atmospheric pressure. Russian spacecraft may exceed these concentrations.
near the sea-level partial pressure of 21 kPa (3.1 psi or 160 mm Hg) to avoid adverse physiologic effects such as decreased night vision, impaired memory and cognitive performance, unconsciousness, and death. These effects can begin within a few seconds of oxygen deprivation, depending on the degree of hypoxia, so it is extremely vital that the oxygen supply be uninterrupted (2). On the other hand, higher PO2 starting at 32 kPa (4.7 psi or 240 mm Hg) can also lead to physiologic problems, such as lung irritation and damage and central nervous system impairment. Figure 10-4 illustrates the ranges of atmospheric pressure and oxygen concentration that are acceptable for use in spacecraft. For safety reasons, oxygen concentration and partial pressure must also be kept as low as possible to minimize the risk of fire. Current guidelines require that American spacecraft atmospheres nominally contain less than 23.5% oxygen at sea-level atmospheric pressure. Russian spacecraft may exceed these concentrations.
 FIGURE 10-4 Human physiologic limits of atmospheric pressure and oxygen concentration. |
The terrestrial atmospheric level of carbon dioxide (CO2) is 0.032 kPa (0.0046 psi or 0.24 mm Hg). Higher concentrations of carbon dioxide can have adverse physiologic effects, including hearing loss, headache, decreased cognitive performance, and eventually unconsciousness and death. To avoid these adverse effects, atmospheric CO2 partial pressures should be maintained below 1.0 kPa (0.15 psi or 7.6 mmHg) (2). However, levels up to 1.6 kPa (0.20 psi or 15 mmHg) can be tolerated for short periods during emergencies. Because CO2 is continually generated during normal metabolic respiration, it must be constantly “scrubbed” from the atmosphere of a spacecraft habitat so that acceptably low concentrations are maintained.
Trace atmospheric contaminants also pose a significant potential problem for spacecraft atmospheres and can cause both short- and long-term health problems. There are many potential sources of atmospheric contamination. Metabolic by-products (such as methane and hydrogen sulfide) and spilled cleaning and experimental chemicals are potential sources of contamination, as are toxic propellants (such as hydrazine) or coolants (ammonia) that leak into the cabin or are brought in from the outside on the surfaces of spacesuits. Some organic polymer materials can slowly release toxic volatile organic compounds (such as toluene), and the thermal breakdown of other materials release other chemicals. Fires can release toxic products of combustion. It is essential to monitor the spacecraft environment for these trace contaminants and to have an effective method for removing them. For each of these atmospheric contaminants, spacecraft maximum allowable concentrations (SMACs) can be defined to ensure that atmospheric concentrations of a given contaminant are within safe levels (8). SMACs for many potential contaminants can be based on terrestrial regulatory standards for individual compounds, defined by such organizations such as the American Council of Governmental Industrial Hygienists (ACGIH), the Occupational Safety and Health Administration (OSHA), and the Environmental Protection Agency (EPA). When terrestrial exposure guidelines for a given compound do not exist (e.g., hydrazine), available research data is used to determine appropriate SMACs. SMACs have been defined for approximately 200 potential atmospheric contaminants.
Temperature Regulation
Although humans can survive in a relatively wide range of temperature and humidity conditions, the range that is comfortable for working and living is fairly narrow and depends on the level of activity. Ideal temperatures range from 18°C to 27°C (65°F-80°F) and ideal humidities range from dew points of 4°C to 16°C (40°F-60°F) [relative humidities (RH) from 25%-70%]. Optimal human performance requires staying within this comfort zone of temperature and humidity to provide a shirtsleeve working environment. Humans also prefer some degree of control of their environmental temperature and humidity to match their comfort with their level of activity (2).
Potable Water Supply
For humans to survive, they require a steady supply of potable water for drinking, cooking, and personal hygiene. Potable water can be procured from stored supplies, recycled from wastewater, recovered from atmospheric condensation, or generated from hydrogen/oxygen fuel cells. Potable water supplies must contain acceptably low concentrations of organic, inorganic, and microbial contaminants. Water quality must also be monitored to ensure that quality standards are maintained throughout the duration of the mission. Spacecraft water exposure guidelines (SWEGs) have been established for potential contaminants of potable water to ensure that water of sufficient purity is available (9).
Minimize Radiation Exposure
As mentioned previously, the space environment confers radiation exposure significantly greater than is found in typical terrestrial environments. This radiation exposure may cause a variety of adverse physical effects, leading to short- and long-term medical consequences. The impact of radiation on human health in the aerospace environment is discussed in greater detail in Chapter 11. By minimizing exposure to radiation, the risk of associated adverse sequelae can be minimized; however, because this radiation exposure in the space environment cannot be eliminated, the principle of maintaining exposure levels as low as reasonably achievable (ALARA) is used. Vehicle shielding and careful planning of mission profiles can decrease spacecraft crewmembers’ exposure to radiation. However, these methods cannot prevent crewmembers’ exposure to significant levels of radiation during spaceflight.
Ensuring that crewmembers’ career radiation exposure (both short-term and lifetime cumulative) remains within acceptably safe levels minimizes the associated risks. This requires that crewmembers’ radiation exposure during spaceflight be carefully measured. Acceptable radiation exposure limits are then used to ensure that crewmembers are placed at unacceptable risk by their radiation exposure history. If crewmembers’ cumulative or interval radiation exposures approach these limits, their spaceflight activities can be limited to ensure that accepted exposure guidelines are not exceeded. Astronauts’ career radiation exposure limits are based on the exposure limits set for terrestrial radiation workers by the National Council of Radiation Protection (NCRP), which allows a maximum annual radiation exposure of 0.05 Sv (5 rem). However, because of the unique work environment encountered during spaceflight, NCRP recommendations drafted in 1989 set radiation exposure standards for astronauts in LEO that were considerably higher than allowed for terrestrial workers [e.g., they permit annual radiation exposures of the blood forming organs of up to 0.5 Sv (50 rem)] (10). At present, the NCRP is revising the radiation limits recommended for LEO, taking into account new data and new concepts of acceptability of risk. The new space radiation exposure standards will probably accept a 1% increase in lifetime fatal cancer risk attributed to the radiation exposure. They will account for astronauts’ gender and age at time of exposure and will allow less radiation exposure than the prior standards (11). Different standards may be required for future missions beyond LEO.
Nutritional Support
Adequate daily food intake is necessary to replace metabolic energy stores and to provide the biologic substrate for repairing and maintaining bodily tissues. Micronutrients, vitamins, and minerals must also be supplied to maintain health. Humans prefer that food be palatable, with attention given to ensure acceptable food temperature, appearance, texture, taste, and aroma. Food supplies that meet these metabolic, nutritional, and palatability requirements must be available for the duration of the spacecraft’s mission, which requires adequate food preservation, storage, and preparation capabilities (12).
Waste Management
Wastes generated in space habitats consist of several general types: solid wastes (such as used food containers), moist solid wastes including feces and vomitus, liquid wastes including urine and wastewater, and gaseous wastes. Other waste subcategories include biologic and sharp hazard wastes. Such wastes must be removed or isolated from the habitable cabin to limit unhygienic and potentially hazardous environmental contamination.
Human Factors Requirements
Humans must maintain adequate health and fitness, both physically and psychologically, if they are to maintain their ability to perform tasks. If they are unable to do this, their performance ability degrades with time. Their surrounding environment plays an integral role in their ability to maintain adequate psychologic and physical health. Human factors and human performance are discussed further in Chapter 24.
Adequate sleep is crucial for human performance. To maintain long-term human performance, an environment must be provided that is conducive to sleep or that at least does not actively interfere with sleep. Appropriate light/dark cycles facilitate proper circadian cycling. Noise, vibration, and other disturbances should be minimized to promote proper sleep, as well as to prevent their adverse affects on human health and task performance.
Habitat design itself can also significantly affect human performance. Illumination sufficient for the performance of tasks must be provided. The volume, spatial arrangement, and decor of the spacecraft habitat should ideally be designed to provide a comfortable environment. Windows may be included to provide connection with the outside environment.
On the other hand, isolation from the terrestrial environment imposes other psychologic stressors. Opportunities to communicate with family and friends on Earth help crewmembers to maintain a crucial link to their terrestrial lives. News of current events should be provided so crewmembers do not feel out of touch with their homes. To
avoid boredom during long missions, entertainment must be provided.
avoid boredom during long missions, entertainment must be provided.
Mission Requirements
The goals and objectives of the spacecraft’s mission strongly influence what type of life support systems are necessary to supply the required environmental parameters, as well as the type and amount of supplies that are needed. Crew size determines the amount of air, food, and water that will be needed per day during the mission, as well as how much CO2 and other metabolic waste products will need to be processed. The more the crewmembers, the more habitable volume the spacecraft must provide. Space station designers may also have to account for occasional increases in the number of inhabitants during crew transfer and supply missions. To a given extent there is an economy of scale, so a relatively modest increase in life support system capacity may result in the ability to have significantly more crewmembers.
Mission duration also determines the amount of supplies that will be necessary. For short-duration missions, all necessary supplies can be taken along. However, as mission duration increases, the weight of supplies quickly becomes prohibitive. Expendable supplies must be replaced with regenerable supplies. Waste products may also be used in lieu of fresh supplies (e.g., using the water by-product from fuel cells as the source of potable water instead of bringing a separate water source). The longer the mission, the more carefully supplies and resources must be conserved.
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The scope of reclamation and recovery of usable resources from waste streams of spacecraft life support systems is known as mass loop closure. Consumable resources, such as oxygen, potable water, and food are converted by metabolic activity into waste by-products, and their mass is potentially lost to reuse. Figure 10-5 shows the daily usage of consumable resources that would be expected during a space mission, as well as the amount of waste by-product that would be produced. Recycling and recovering usable resources from waste by-products in effect returns material to the stores of consumable resources, which decrease the total amount of supplies required by a mission. This process of recovery of usable mass from waste by-products is therefore known as mass loop closure. Systems that do not recycle any useful material for reuse have no mass loop closure and are referred to as open systems. In general, all short-duration spacecraft have open systems because resources are not regenerated from waste by-products. It must be noted that scrubbing metabolically derived CO2 from the cabin atmosphere does not constitute mass loop closure because the oxygen in the CO2 is not regenerated and reused. Total mass loop closure, in which all waste mass is recycled into usable material so that no extra supplies are needed, is possible in theory, but not in practice. The water recovery systems used in recent space stations represent the first practical efforts of mass
loop closure. However, for long-duration missions, such as to Mars, much greater mass loop closure will be necessary. Table 10-1 shows various levels of mass loop closure and the mechanisms used to achieve those levels, along with practical examples.
loop closure. However, for long-duration missions, such as to Mars, much greater mass loop closure will be necessary. Table 10-1 shows various levels of mass loop closure and the mechanisms used to achieve those levels, along with practical examples.
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The destination of the spacecraft not only determines the consumable resources required for the specified mission duration but also affects the design of the life support system, especially in terms of reliability and maintainability. For spacecraft that remain in LEO, faulty or damaged equipment may be replaced with relative ease. However, if a device fails during a mission to Mars replacing it may be impossible, so it must be extremely reliable. In such situations in which replacement is impossible, devices should be designed so that they can be repaired using available skills, equipment, and materials. Such robustness and resilience are essential design qualities of equipment used for exploration-class missions.
Technical Requirements
When designing spacecraft life support systems, the human and mission requirements can be used to generate the technical requirements, such as how large the cabin must be, how much oxygen and water must be supplied, and so on. Because spacecraft life support systems are composed of a number of complex subsystems, careful consideration must be given to how the various components are integrated.
First, life support subsystem components must efficiently perform their required tasks with minimum of wasted consumables. To ensure that this is done, the various life support system components must be strongly integrated with one another (Figure 10-6). As shall be seen, it is often difficult to separate the functioning of the thermal control system from the humidity control system, on which the atmospheric moisture recovery system is dependent. The capabilities and shortcomings of one subsystem strongly influence the functions of the other components. Life support systems may also be affected by other equipment and devices within the spacecraft. For example, thermal control systems must account for the heat generated by avionics equipment.
When spacecraft dock with each other, their life support systems must interact. Differences in the underlying operational parameters of the life support systems of the involved vehicles must be accounted for if such dockings are to be successful. As shall be seen, Russian and American life support systems have very different operating principles and philosophies. Integrating these systems in the two segments of the ISS has been quite challenging.
Life support systems must also integrate with their human occupants. The systems must meet human physiologic requirements. They must also be operable and maintainable by humans. Human-systems integration also incorporates the concepts of reliability and safety.
Technical Constraints
Several technical constraints limit what solutions are available to meet the requirements for spacecraft life support systems that were outlined briefly. First and foremost of these are the restrictions imposed by the spacecraft design itself. There are finite limits to the size and mass of objects that can be placed in orbit by current rocket technology. Such launch vehicle payload limitations restrict the size and weight of the spacecraft habitable volumes and configurations and
significantly affect the amount of resources with which the spacecraft can be supplied. Because of payload limitations, increasing the mission supply requirements without being able to increase the amount of available supplies necessitates a greater degree of mass loop closure.
significantly affect the amount of resources with which the spacecraft can be supplied. Because of payload limitations, increasing the mission supply requirements without being able to increase the amount of available supplies necessitates a greater degree of mass loop closure.
 FIGURE 10-6 Schematic of the subsystem integration of a spacecraft habitable environment. |
Limitations to technology also limit the capabilities of spacecraft life support systems. As discussed later, it may not be technically feasible or possible to reach the degree of mass loop closure required for long-duration space missions, at least with the available payload volume. Very tight mass loop closure may be possible using currently available biologic technology, but such systems would be extremely large—too large to send into space. Nonbiologic physical-chemical methods may also be used for resource recovery and mass loop closure, but working systems have yet to be developed. Significant advances must be made in these areas of life support design before advanced resource recovery systems become feasible. It may also be possible to recover usable materials (e.g., water, oxygen) from planetary bodies, if these resources are present and available in adequate quantities.
The space environment itself imposes strict constraints on life support system design. Designing life support equipment that function in microgravity is challenging because solids, liquids, and even gases do not behave the same as in the terrestrial environment. For example, because separating liquids from gases is not easy without gravity, it is difficult to design an effective shower for use in space, and water storage tanks become complex devices. The vacuum of space also affects materials. Outgassing, or the loss of volatile components to the vacuum of space, can change the physical and chemical properties of a material. Solid materials can sublimate away at a faster rate in the vacuum of space than in the terrestrial environment (in an analogous process, sharp knives sitting unused for long periods become dull as molecules sublimate from the sharp edges of the knife). This process can lead to significant erosion of materials in space. Because there is no gas layer between them in space, closely adjacent materials may diffuse into one another a kind of cold welding process. These properties must be considered when selecting materials to be used in the vacuum of space (13).
Given these formidable requirements and constraints, designers of spacecraft life support systems have had to overcome many challenges. Examination of the evolution of the components that make up the spacecraft life support systems provides practical examples of the concepts involved in their design.
SPACECRAFT ENVIRONMENTAL CONTROL AND LIFE SUPPORT SYSTEMS
What environmental control and life support systems are chosen for a given spacecraft habitable environment depends on matching the human habitability requirements with the technical, mission, reliability, and safety constraints imposed by mission requirements. As was discussed previously, the nature of these various subsystems and components is often strongly influenced by how they are integrated. Historic examples illustrate the principles involved in the design of these subsystems, which gives an appreciation for the organization and functioning of the various subsystems and components. These subsystems and components can be organized by function, with some prioritization with regard to the criticality of the individual functions.
Atmospheric Control and Supply System
The life support cabins of the American Mercury, Gemini, and Apollo spacecraft contained 100% oxygen atmospheres at 34.5 kPa (5.0 psi or 260 mm Hg), yielding a PO2 slightly higher than that of standard air (the PO2 of terrestrial sea-level
air is 21.4 kPa (3.1 psi or 160 mm Hg). A low pressure, oxygen atmosphere was chosen because (a) it offered the lowest total atmospheric pressure, allowing for the lightest possible pressure vessel; (b) a single gas atmosphere minimized complexity of the atmospheric control system; and (c) lack of nitrogen eliminated the risk of decompression sickness (DCS) during EVAs (14). The slightly hyperoxic atmospheric composition was well tolerated during short-duration missions. A pure oxygen atmosphere was also used during ground testing, in which the cabins were overpressurized to 110.2 kPa (16.0 psi or 830 mm Hg) to avoid structural damage. Such hyperbaric oxygen conditions dramatically increase the risk of fire. This was tragically demonstrated during the Apollo I fire in January 1967, which took the lives of three astronauts during hyperbaric cabin testing on the launch pad (15). Because adequate atmospheric nitrogen concentrations decrease atmospheric flammability and lower the risk of fire, an atmosphere of 40% nitrogen and 60% oxygen was used during all subsequent ground operations. After launch, cabin pressure bled down to the operational level (34.5 kPa, 5.0 psi, or 260 mm Hg), while the oxygen content was enriched to 100% (16). Astronauts prebreathed pure oxygen in their spacesuits for 3 hours before launch to avoid DCS resulting from depressurization. (DCS is discussed later.) During reentry and descent to the Earth’s surface, the cabin was repressurized to avoid structural damage.
air is 21.4 kPa (3.1 psi or 160 mm Hg). A low pressure, oxygen atmosphere was chosen because (a) it offered the lowest total atmospheric pressure, allowing for the lightest possible pressure vessel; (b) a single gas atmosphere minimized complexity of the atmospheric control system; and (c) lack of nitrogen eliminated the risk of decompression sickness (DCS) during EVAs (14). The slightly hyperoxic atmospheric composition was well tolerated during short-duration missions. A pure oxygen atmosphere was also used during ground testing, in which the cabins were overpressurized to 110.2 kPa (16.0 psi or 830 mm Hg) to avoid structural damage. Such hyperbaric oxygen conditions dramatically increase the risk of fire. This was tragically demonstrated during the Apollo I fire in January 1967, which took the lives of three astronauts during hyperbaric cabin testing on the launch pad (15). Because adequate atmospheric nitrogen concentrations decrease atmospheric flammability and lower the risk of fire, an atmosphere of 40% nitrogen and 60% oxygen was used during all subsequent ground operations. After launch, cabin pressure bled down to the operational level (34.5 kPa, 5.0 psi, or 260 mm Hg), while the oxygen content was enriched to 100% (16). Astronauts prebreathed pure oxygen in their spacesuits for 3 hours before launch to avoid DCS resulting from depressurization. (DCS is discussed later.) During reentry and descent to the Earth’s surface, the cabin was repressurized to avoid structural damage.
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