323

18Space physiology and medicine

MICHAEL BARRATT

The field of space medicine practically began before the launch of Russian cosmonaut Yuri Gagarin into orbit in April of 1961. Aviation was progressing rapidly in the few decades prior to this, and space physiology and medicine represent a natural evolution from this world. Higher altitudes and the increasing acceleration loads of high-performance aircraft required a concerted effort to understand the effects on the human occupant and provide protection to optimize per-formance. High-altitude balloon flights had already demon-strated the efficacy of completely enclosed cabins in rarefied atmospheres, and most of the acceleration forces and toler-ances that might be encountered during launch and landing were understood from ground studies and aviation. What had to await this first launch was the understanding of the profound effects of weightlessness on the human body.

Since Gagarin’s first two-orbit flight, over 500 people have flown into Earth orbit and a few beyond, and the ability of humans to fly long durations and perform productive work on these missions is now a working assumption. Missions of six months’ duration have been routinely flown for decades in the Russian Salyut, Mir, and now International Space Station (ISS) programmes. Along with this we have devel-oped an incomplete but functional understanding of the predictable changes that occur as the human adapts to the spaceflight environment. Similar to the aviation, undersea and high-altitude arenas, we explore and operate in the space environment because of the benefits it offers. Like these others, spaceflight carries special hazards and risks, and a deliberate and systematic effort has been made to under-stand these and develop countermeasures and protections to the deleterious effects to optimize human performance.

The fundamental elements influencing space medicine may be distilled into altered acceleration loads – from the large loads associated with launch and landing to their complete absence in weightlessness – and altered atmospheres, with variable pressures and oxygen mixes that meet the opera-tional requirements of a mission scenario. Add to this the unique occupational exposures associated with human spaceflight, such as ionizing radiation and characteristics associated with the vehicles that take us there, and the dis-cipline of space medicine is defined.

At this point in time, the ISS as a permanently staffed laboratory has been manned for a decade and a half with long-duration crews, planning is underway for explora-tion missions beyond Earth’s orbit, and a commercial space effort is emerging to serve the research and tourist sec-tors in low Earth orbit (LEO). In addition, the commercial world is poised to provide suborbital spaceflight experi-ences on a scale unprecedented from previous programmes. Increasingly, space medical expertise will be called upon to support these efforts. This chapter overviews the physi-cal factors associated with spaceflight, the physiological changes that the human undergoes during dynamic flight phases and prolonged weightlessness, and common medical problems encountered.

VEHICLES/PROGRAMMES

Because many of the physiological implications of spaceflight are directly related to vehicle performance and characteris-tics, a brief review of these is in order. Sufficient experience has now been accrued to categorize these by vehicle type.

Vehicles/programmes 323Physical factors of spaceflight 325Physiological adaptation to weightlessness 335Clinical problems during spaceflight 345

Earth return and readaptation 348references 352Further reading 354

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MICHAEL BARRATT

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MICHAEL BARRATT

Copyrighted Physiological adaptation to weightlessness

Copyrighted Physiological adaptation to weightlessness Material The field of space medicine practically began before the

Material The field of space medicine practically began before the launch of Russian cosmonaut Yuri Gagarin into orbit in April

Material launch of Russian cosmonaut Yuri Gagarin into orbit in April of 1961. Aviation was progressing rapidly in the few decades

Material of 1961. Aviation was progressing rapidly in the few decades loads associated with launch and landing to their complete

Material loads associated with launch and landing to their complete absence in weightlessness – and altered atmospheres, with

Material absence in weightlessness – and altered atmospheres, with variable pressures and oxygen mixes that meet the opera

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tional requirements of a mission scenario. Add to this the unique occupational exposures associated with human Taylor

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Taylor spaceflight, such as ionizing radiation and characteristics associated with the vehicles that take us there, and the dis

Taylor associated with the vehicles that take us there, and the discipline of space medicine is defined.

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324 Space physiology and medicine

Capsules

The first vehicles that carried humans (and animals for that matter) to sustainable orbits were the classic capsules, simple vehicles that could easily attach to the top of a ver-tical rocket booster stack beneath an aerodynamic fairing and undergo atmospheric entry and parachute landing on return to Earth. These were a natural evolution from bal-listic missiles where the payload also rides atop a booster stack beneath a fairing. Arguably these remain the domi-nant design in human-rated spacecraft. They are compact and efficient, orienting crewmembers on their backs with respect to the spacecraft vertical axis for both launch and landing. The implications for dynamic loads are that the major forces associated with launch and landing are in the +Gx (chest to back) direction, the most favourable from the standpoint of physiological challenge. Capsules may land on water or land. By offsetting the centre of mass from the aerodynamic centre, a capsule may be steered during entry, affording a limited cross-range capability to fly towards a targeted landing site. Thus far all have landed with the aid of parachutes. In spite of some degree of cross-range steerage, capsules are fairly imprecise in their landing site. Once atmospheric entry is complete and orbital velocity has been shed, they are subject to winds and weather. As such they require a large unpop-ulated territory for landing and recovery, either water or land. Table 18.1 summarizes the attributes of human-rated space capsules used thus far.

Another advantage of a capsule involves abort modes during hazardous launch and ascent events. Being situated atop a vertical stack, a capsule has a clear path away from a mishap by means of an escape rocket system. The efficacy of this concept was demonstrated with the Russian Soyuz T-10A mission in 1983, when the occupants were safely cata-pulted clear of an explosion which engulfed the launch pad. Gemini capsules were unique in that they employed ejection seats for the two occupants, which would have catapulted

them laterally from atop the rocket stack in event of mishap on the pad or during early ascent.

Winged spacecraft

From a strict definition of the term ‘space’, being 50 miles for the US Air Force and 62 miles for the international com-munity, winged vehicles carried humans into space very early. The X-15 rocket plane performed the third suborbital flight after the first two Mercury flights. The X-15 and early lifting bodies represent a natural evolution of high-perfor-mance aircraft, and may be considered ancestral to the US Space Shuttle, Russian Buran and Spaceship 1. They offer the advantage of full aerodynamic control upon entry to the atmosphere, wide cross-range capability, and landing on a conventional (albeit long) runway. This does necessi-tate wings and landing gear, which are only useful during atmospheric flight and landing and otherwise represent a weight penalty during launch, ascent, orbital flight and initial atmospheric entry. Like the X-15, Spaceship 1 uses wings for orientation and lift as the rocket ascent is initi-ated. Until final atmospheric flight and landing, however, wings may also constitute a liability and safety risk. The US Space Shuttle Columbia was lost owing to fatal damage of the left wing during launch that went undetected until the catastrophic breakup during entry.

From the standpoint of the human occupant, the impli-cations are return to a horizontal reference frame and an upright seated posture similar to a standard aviation environment during the landing phase. This positions the human such that the aerodynamic loads of entry are taken in the body +Gz axis, which is less favourable for both sus-tained and impact acceleration tolerance. Entry acceleration levels for winged vehicles are of lesser magnitude but expo-sure is prolonged compared with capsules. In addition, the control inputs are basically akin to those of aircraft, and are more complex and demanding than for a capsule. As will be discussed, the transition from the weightless adapted state

Table 18.1 Space capsules – vehicles and attributes

Vehicle Years of operation Crew Pressurized volume (m3)

Cabin atmosphere Maximum flight duration

Vostok (r) 1961–1963 1 1.6 SL 4 day 23 hrMercury (US) 1961–1963 1 1.7 5 psi/100% O2 1 day 10 hrVoskhod (r) 1964–1965 2 1.6 SL 1 day 2 hrGemini (US) 1965–1966 2 2.6 5 psi/100% O2 13 day 18 hrapollo (US) 1968–1975 3 6.2 5 psi/100% O2 12 day 13 hrSoyuz (r) 1967–present 3 4.0 SL 5 day endurance*Shenzhou (PrC) 2003–present 3 6 SL 15 day†Shuttle (US) 1981–2011 7 65.8 SL 17 day 15 hrThe Soyuz, Apollo and Shenzhou spacecraft flew coupled to orbital modules with additional habitable volume. Crewmembers were con-strained to the capsules for launch and landing. The US Space Shuttle is added for comparison. PRC, People’s Republic of China; R, Russia; SL, sea level, 14.7 psi with roughly 20% O2/80% N2; US, United States.* Soyuz mission to transport crew to and return from orbiting space station for up to 210-day mission, remain docked during mission;

5 day maximum free flight endurance. † Included docked phase with Tiangong-1 station.

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stack beneath a fairing. Arguably these remain the domiCopyrighted

stack beneath a fairing. Arguably these remain the dominant design in human-rated spacecraft. They are compact Copyrighted

nant design in human-rated spacecraft. They are compact and efficient, orienting crewmembers on their backs with

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respect to the spacecraft vertical axis for both launch and landing. The implications for dynamic loads are that the

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landing. The implications for dynamic loads are that the major forces associated with launch and landing are in the

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major forces associated with launch and landing are in the (chest to back) direction, the most favourable from

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(chest to back) direction, the most favourable from the standpoint of physiological challenge. Capsules may

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the standpoint of physiological challenge. Capsules may land on water or land. By offsetting the centre of mass

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Copyrighted during entry, affording a limited cross-range capability to fly towards a targeted landing site. Thus far all have

Copyrighted to fly towards a targeted landing site. Thus far all have landed with the aid of parachutes. In spite of some degree

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landed with the aid of parachutes. In spite of some degree Material

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of cross-range steerage, capsules are fairly imprecise in their landing site. Once atmospheric entry is complete

Material

their landing site. Once atmospheric entry is complete and orbital velocity has been shed, they are subject to

Material

and orbital velocity has been shed, they are subject to winds and weather. As such they require a large unpop

Material winds and weather. As such they require a large unpopulated territory for landing and recovery, either water or

Material ulated territory for landing and recovery, either water or land. Table 18.1 summarizes the attributes of human-rated

Material land. Table 18.1 summarizes the attributes of human-rated

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Material an upright seated posture similar to a standard aviation environment during the landing phase. This positions the

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Taylor sure is prolonged compared with capsules. In addition, the control inputs are basically akin to those of aircraft, and are

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1 day 10 hr

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Francis1 day 2 hr

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Francis13 day 18 hr

Physical factors of spaceflight 325

to a standard aviation environment demanding of critical control inputs offers some unique challenges.

Orbital stations

Orbiting platforms that afford long stays in space were an early goal of both US and Russian programs. A large plat-form can support extended stays and research into the mechanistic understanding of spaceflight phenomena. As will be described, complete human adaptation to weight-lessness requires several weeks, which is beyond the inde-pendent duration limits of the vehicles noted above. Long stays on such platforms imply power, complex life support systems, physical countermeasures equipment and a supply chain to replenish food, water and other consumables. These platforms allow the study of effects of microgravity on basic physical and biological processes, as well as critical testing and development of advanced life support systems. In addi-tion, the study of basic physiological processes in this novel environment allows a complete understanding of many of these phenomena when away from the dominating influence of gravity. The general evolution of stations has been in the direction of increasing size and complexity. Table 18.2 lists attributes of the manned orbital stations thus far.

Deep space vehicles and landers

Here ‘deep space’ is defined as away from the Earth’s vicinity, beyond the practical influences of atmosphere and geomag-netic fields. Thus far only the US Apollo lunar programme has carried human explorers beyond these limits, flying a total of nine missions to lunar vicinity, six of which involved landings of two individuals on the lunar surface. The trip from the Earth to the Moon required about three days and involved achieving a stable lunar orbit from which a lander could descend from a parent vehicle. Total mission dura-tions were typically eight to twelve days. Although a lim-ited programme, Apollo demonstrated that humans could perform precision piloting duties of the manually controlled landers and operate in a fractional gravity field of 1/6 G.

For destinations beyond the Moon, such as to Mars or near Earth asteroids, greater time periods in transit are expected. As such, the transit vehicle will be larger and more closely resemble some of the early orbital stations in size and capabilities.

PHYSICAL FACTORS OF SPACEFLIGHT

Spaceflight implies a basic removal of human and hardware from Earth’s surface, with vehicles operating well above the more familiar aviation environment. There is no definitive boundary of space, since the threshold of removal can be seen in the context of different factors. Table  18.3  shows some of these milestones and their corresponding altitudes that all spacecraft will traverse in leaving the planet. The most familiar aspects of spaceflight involve weightlessness and the absence of atmosphere; however, this is realized only after a highly dynamic rocket ascent. Simply put, leav-ing Earth implies overcoming substantial physical forces, a process that in turn involves counter-forces and substan-tial energies. These impart physical loads on the human body with physiological implications. Once in space, the environment is far from benign with the combined factors of radiation, chemical moieties in near Earth space, and weightlessness itself.

An understanding of the forces associated with flight dynamics and the characteristics of various altitudes is germane to piloted space operations just as for aviation, relevant to the level of protective methods and capabilities required for human occupants as well as mishaps that might occur at any of these altitudes.

The external spaceflight environment

It is natural to think of the spaceflight environment as devoid of the basic elements that sustain human life. However as we expand our thinking as an emerging spacefaring population, it is increasingly appropriate to consider more of what Earth is endowed with rather than what space is devoid of. The three dominant elements that have shaped and influenced

Table 18.2 Orbiting stations

Station Years of operation

Crew Mass(metric tons)

Pressurized volume (m3)

Cabin atmosphere

Orbit

Salyut series (r) 1971–1991 2–3 18.2–19.9 90–100 Sea level 51.6º; 200–222 km (1)219-270(4); 219–278 (7)

Skylab (US) 1973–1974 3 77.1 kg 320 5 psi 70% O/30% N2

50º; 434–442 km

almaz series (r) 1973–1977 2 19 47.5 Sea level 51.6º, same range as SalyutMir (r) 1986–2001 3 129.7 350 Sea level 51.6º; 354–374 kmISS* 2000–present 6 450 916 Sea level 51.6º; 409–416 kmtiangon-1 (PrC) 2011–present 3 8.5 15 Sea level 42.8º; 363–381 kmPRC, Peoples Republic of China; R, Russia; US, United States. * International Space Station partner agencies: Canadian Space agency, European Space agency, Japanese Space Development

agency, National aeronautics and Space administration (US), roscosmos (russia).

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will be described, complete human adaptation to weightCopyrighted

will be described, complete human adaptation to weightlessness requires several weeks, which is beyond the indeCopyrighted

lessness requires several weeks, which is beyond the independent duration limits of the vehicles noted above. Long

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stays on such platforms imply power, complex life support systems, physical countermeasures equipment and a supply

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systems, physical countermeasures equipment and a supply chain to replenish food, water and other consumables. These

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chain to replenish food, water and other consumables. These platforms allow the study of effects of microgravity on basic

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physical and biological processes, as well as critical testing and development of advanced life support systems. In addi

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attributes of the manned orbital stations thus far.

Here ‘deep space’ is defined as away from the Earth’s vicinity,

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Material germane to piloted space operations just as for aviation, relevant to the level of protective methods and capabilities

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Francis219-270(4); 219–278 (7)

326 Space physiology and medicine

our physiology and function are gravity, pressure atmo-sphere and background radiation level, all provided by our home planet, the latter largely as a function of shielding. We can vary any or all of these and still function, albeit with consequences. Melding these physical conditions with adap-tive physiology is a necessary discipline of space exploration.

GRAVITY

The standard gravitational acceleration of 9.8 m/s2 realized at Earth’s surface represents a static and uniform load on the body, with the gravitational force being the mass of an object multiplied by this acceleration. However, we inter-act with this force in a dynamic fashion by moving against it in the process of locomotion and mass handling. We constantly reorient our body position with respect to this downward pull, which ties the force of gravity prominently to such physiological processes as maintaining cerebral perfusion, positional sense and musculoskeletal loading. By convention, the specific force of gravity at Earth’s sur-face is denoted as ‘g’, with multiples of this denoted by the dimensionless quantity ‘G’. The force of gravity varies with distance between two masses, and for an object orbiting another is described by the equation:

F = G(m1m2)/r2

where G is the universal gravitational constant, m1 and m2 are the two masses, such as the Earth and a spacecraft, and r is the radius of the orbit (distance between the centres of masses). It is evident that in a LEO of 240 miles, the force of Earth’s gravity on an object is still prominent at about 90 per cent of its surface value. The combination of forward velocity around the Earth and resultant outward centrifugal force counterbalances the inward force of gravity to produce the state of weightlessness.

Human spaceflight involves short-duration accelera-tion loads greater than 1 G during launch and landing as described below, similar to high-performance aviation. More importantly, prolonged exposures to gravity levels between near zero and 1 G will be dominant in space medi-cine for many decades to come. Zero G implies the absence of a mechanical force on an object, such as is realized in the freefall environment in orbiting spacecraft and during the coast phases of trans-lunar and transplanetary flight, and constitutes the vast majority of human spaceflight experi-ence. Extra-terrestrial bodies of greatest interest to us, nota-bly the Moon and Mars, will imply fractions of G (roughly

Table 18.3 Physiologically and operationally relevant milestones with increasing altitude

Altitude (m, km) Milestone or limit1000 km Lower border of inner Van allen radiation belt at equator (lower at poles). Higher radiation

threat because of trapped ionized particles.700 km Collisions between atmospheric gas molecules become undetectable. Particle concentration

diminishes over several thousand km to free space density of 1–10 per cm3.410 km average altitude of International Space Station.200 km Essentially no aerodynamic support and minimal drag such that sustainable orbit possible.140 km atmosphere ceases to provide significant protection from micrometeoroids.120 km atmospheric entry interface for returning spacecraft; initial onset of perceptible acceleration

forces, control surface resistance. Dysacoustic zone – insufficient atmospheric density to facilitate the effective transmission of sound.

100 km Karman line, threshold of effectiveness of aerodynamic surfaces. Minimal active light scattering, ‘blackness of space’.

40 km atmosphere ceases to protect objects from high-energy radiation particles.37.65 km Highest altitude aircraft flight, conventional air breathing engines, pressure suit (non-sustained

altitude).25–30 km Practical limit of ram-pressurized cabin; above this altitude, fully enclosed pressurized cabins

are required.19,200 ‘armstrong’s line’; ambient pressure equivalent to tension of water vapour at body temperature

(47 mmHg).16,000 Practical limit of atmospheric weather processes and phenomena at equator (lower near the

poles).12,192 Limit for breathing unpressurized 100% oxygen. Pressure environment (suit or cabin) needed

above this. 5000 approximate limit of human acclimation for long-term dwelling.3048 USaF requires supplemental oxygen for aircrew.1525–2440 Cabin pressure of commercial airlines, typically operating up to 12 km in altitude.Sea level Pressure 760 mmHg.Convenient numbers are given, but there is a range associated with each milestone based on physiological variability and atmospheric pres-sure dynamics. USAF, United States Air Force.

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& of masses). It is evident that in a LEO of 240 miles, the force Francis

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of Earth’s gravity on an object is still prominent at about 90 per cent of its surface value. The combination of forward

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Physical factors of spaceflight 327

one-sixth for the Moon, one-third for Mars). Since the body undergoes physical deconditioning associated with reduc-ing the normal force of gravity, prolonged periods in frac-tional G will inherently involve active countermeasures to ensure normal function with the assumed endpoint of Earth return.

ATMOSPHERE

At near vacuum, the external spaceflight environment is not survivable. So far as we know, the standard sea level atmosphere with 14.7 psi pressure and roughly 80 per cent nitrogen/20  per cent oxygen mix is unique to Earth, and certainly exists nowhere else in our solar system. For the foreseeable future, a pressurized breathable atmosphere will be a take-along requirement and imply sealed and care-fully controlled habitats. These two aspects of the human atmosphere each have specific thresholds and must be independently controlled.

Pressure must be at a minimum to hold gases in solution in body tissues and blood vessels and support respiratory gas exchange. A well-known milestone of atmospheric pressure as one ascends is Armstrong’s Line, encountered at roughly 19 200  m (63 000  feet). Climbing through this altitude, the ambient pressure transitions through 47  mmHg, the vapour pressure of water at a normal body temperature of 37°C. Body fluids directly exposed to this pressure or lower will ‘boil’ away, such as from the tongue or exposed mucus membranes. As the ambient pressure decreases, elastic tis-sue forces are overcome and dissolved gases throughout the body will evolve into bubbles, including oxygen, nitrogen and water vapour, a phenomenon known as ebullism.

Oxygen is actually the determinant of overall pressure required for a habitable atmosphere, with a normoxic envi-ronment containing an oxygen pressure of 160  mmHg, or roughly 3  psi to support respiration and metabolism. This is 21 per cent of the normal sea level atmosphere, but a pressure of 3 psi and 100 per cent oxygen can support a human for prolonged periods. The pressure suits used in the Mercury and Gemini programs, including that used for the first US spacewalk, were pressurized to 3.7 psi. Spacecraft themselves have utilized a range of cabin atmospheres as seen in Tables 18.1 and 18.2. It follows that at or above an altitude with a corresponding ambient pressure of about 3 psi, roughly 15 250 m (50 000 feet), full pressure protection is required either by pressurized cabins or suits.

RADIATION: SOURCES

The radiation milieu of the space environment is complex, and a complete description is well beyond the scope of this chapter. However, radiation is recognized as the major lim-iter of human spaceflight because of the potential health implications of exposure. Space medicine practitioners must be aware of the basic factors of space radiation includ-ing major sources, health consequences, protective mea-sures, and impact on mission planning and risk prediction. Unlike gravity and atmosphere, there are limited technical solutions for the resulting health hazards.

Radiation involves the transfer of energy in the form of charged and neutral subatomic particles or electromagnetic energy propagated in waves. Space is a radiation environ-ment, continually bathed in a wide spectrum of radiation largely from stellar sources and processes involving high energies and great distances. Although there are low level emissions from naturally occurring radionuclides associ-ated with Earth’s geology, the relatively small steady back-ground radiation provided by Earth is largely a function of shielding from the planet’s magnetic fields, atmosphere, and mass. Even with this, high-energy radiation continually encounters and traverses surface organisms. Radiation is functionally partitioned into two categories: Non-ionizing, in which electromagnetic waves such as light or radiofre-quency energy impart thermal effects, and ionizing, in which interactions with particles or waves displace electrons from impacted atoms and give rise to charged ion entities. Both may cause damage to biological systems.

Above the atmosphere, the dominant form of non-ion-izing radiation relevant to humans is solar ultraviolet (UV) light. Intensity relates directly to our proximity to the Sun, and lessens with the square of the solar distance; in Mars orbit, the intensity is less than half of near Earth space. UV radiation exists in wavelengths between 100 and 400 nm in the electromagnetic spectrum with the lower wavelengths being the most biologically damaging. Wavelengths below 180  nm, nearly completely filtered out by Earth’s atmo-sphere, are particularly hazardous in near Earth space. Interestingly on the surface of Mars, the UV flux between 200  and 400  nm is similar to Earth’s, with the distance being offset by the highly rarefied atmosphere. Compared to Earth though, the shorter more damaging wavelengths contribute relatively more to this flux, rendering the Mars surface a more hostile UV environment.

Energy from UV radiation transfers to a physical object as heat, and for the human space flyer, implications include direct UV tissue damage and thermal effects on the habitat. Eyes are particularly vulnerable to UV radiation, necessitat-ing heavily filtered windows or space suit visors. This has been inherent in hardware design, although some selected windows have used unfiltered glass to maintain the high-est optical purity to facilitate photography. Crewmembers utilizing these windows are at risk for both superficial skin burns and UV keratitis during orbital flight.

The more pervasive aspect of solar UV radiation is the ther-mal energy absorbed by spacecraft and any exposed object. In the relative vacuum of space, there is no conductive medium to facilitate dissipation of absorbed heat, and large radiators that are shielded by structure or feathered relative to solar radia-tion flux are required. Thus rejecting heat is a greater relative problem in near Earth space than generating it. Failure of cool-ing systems for manned stations in LEO is a potentially cata-strophic event. When shielded from direct sunlight, however, physical objects may become quite cold. The surface tempera-ture of an object in full sun may be 150°C, while in full shadow may fall to –100°C. This has obvious implications for extrave-hicular activity, which requires constant adjustment between

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328 Space physiology and medicine

heating and cooling depending on orbital phase (e.g. whether in full sunlight or in eclipse on the ‘night’ side of the orbit).

Ionizing radiation comes from multiple sources and may induce immediate and long-term tissue damage via several mechanisms. The three main origins of ionizing radiation are solar particles and wave radiation, geomagnetically trapped radiation and galactic cosmic radiation (GCR). Table  18.4  summarizes the sources and types of ionizing radiation of most interest to human spaceflight.

Solar ionizing radiation consists primarily of continu-ously streaming protons and electrons, known as the solar wind. These particles are of high flux but relatively low energy, are easily stopped by spacecraft structure, and pose little threat to humans in space. Although the dynamic pressure is extremely low, in the vacuum of space this con-stant radiation pressure must be taken into account to accu-rately guide transplanetary trajectories and even orbital management. At about 1 Newton for a 128 000 square metre area at the Earth’s distance from the Sun, it could actually be harnessed by solar sails for propulsion. Periodically, solar activity involves the episodic ejection of much more highly energetic protons and electrons, known as solar par-ticle events (SPEs). The Sun follows a roughly 11 year cycle of activity, with solar particle events being more frequent during the periods of maximal solar activity. Solar particle events can produce particle energies and fluxes hazardous to humans, particularly if outside the geomagnetic fields.

Geomagnetically trapped radiation consists primar-ily of protons and electrons that are captured by Earth’s magnetic fields, either from incoming solar and galactic

particles or from local decay of neutral particles into charged species. These particles bounce back and forth between the north and south magnetic poles, and follow the curved dipole tracks. As such, altitude above Earth’s surface where the lower inner belt begins varies, from roughly 1000 km at the equator to as low as 200 km at high latitudes. Trapped particles are of higher energies than solar wind, and contribute significantly to the health threat of humans. As Figure  18.1  shows, spacecraft operating at higher altitudes or at higher inclination orbits will incur greater exposure to trapped particles.

GCR particles likely emanate from violent stellar events throughout the galaxy that generate high-energy species that may attain relativistic speeds as they travel through space. These are isotropic in distribution and fairly stable, and consist primarily of protons (87 per cent), alpha particles (12 per cent), and other higher atomic number high-energy species (1  per cent) representing ions of all naturally occurring elements (Zeitlin et al. 2013). These particles are of much lower flux but much higher energies as compared with solar particles.

Because of their charged quality, ionizing radiation par-ticles are influenced by magnetic fields that emanate from the Sun and Earth, and of course other stars and planetary bodies with magnetic fields. There are a couple of practical implications. First, geomagnetically trapped particles are held at predictable altitudes corresponding to the Earth’s magnetic dipole. These fields, known as the Van Allen belts, form an effective shield from incoming charged particles, and define the practical upper limit of LEO for human spaceflight. There is one geographically defined region

Table 18.4 radiation sources and types of concern to human spaceflight

Origin Particle or wave type Energy range CommentsSolar particles Continually streaming

solar wind Solar particle events

(solar flares, coronal mass ejections), vary with solar cycle

ProtonsElectrons

ProtonsElectrons

100eV–3.5 KeV

10s to 100s GeV range

Moderate flux, low energy; effectively shielded by minimal structure.

Large flux, high energies; potentially very dangerous to crews in deep space.

Solar wave radiation X-raysGamma rays

KeV rangeMeV range

Energy varies inversely with wavelength.

Geomagnetically trapped particles ProtonsElectrons

600 MeV5–7 MeV

Flux increases with altitude and inclination for LEO flights.

Galactic cosmic rays (GCrs) 98% Baryons (mass > a proton)

Protons 85% alpha particles 14%

Heavy nuclei2% Electrons

Wide range of energies depending on mass; 10 GeV up to 10s of GeV for heavier particles

Smaller flux, larger energy; isotropic distribution, significant source of Ir to space crews.

Increased solar activity correlates with reduced GCr flux in inner solar system.

Secondaries from particle collision with spacecraft structure and atmosphere

Neutrons 1 to several 10s MeV Short lived, 11-min half-life; deep tissue penetrance.

This is a simplified summary of the sources of ionizing radiation (IR) and rough energy ranges associated with those that contribute most to human doses acquired during spaceflight. LEO, low Earth orbit.

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Solar wave radiationCopyrighted

Solar wave radiation

Geomagnetically trapped particles Protons

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Geomagnetically trapped particles Protons

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Galactic cosmic rays (GC

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Galactic cosmic rays (GC

2% Electrons

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Neutrons

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NeutronsMaterial

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Material heating and cooling depending on orbital phase (e.g. whether

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Material This is a simplified summary of the sources of ionizing radiation (IR) and rough energy ranges associated with those that contribute most to human doses acquired during spaceflight. LEO, low Earth orbit.

Material human doses acquired during spaceflight. LEO, low Earth orbit.

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Francisand other higher atomic number high-energy species (1  per cent) representing ions of all naturally occurring elements

Franciscent) representing ions of all naturally occurring elements . 2013). These particles are of much lower flux but

Francis. 2013). These particles are of much lower flux but much higher energies as compared with solar particles.

Francismuch higher energies as compared with solar particles.

Physical factors of spaceflight 329

where the inner belt extends to a lower altitude owing to a misalignment of the Earth’s geomagnetic dipole and the rotational axis about the gravitational centre. Passage through this region, known as the South Atlantic Anomaly, can account for 50 per cent of the radiation dose incurred during a given orbit depending on the shielding depth of the dose considered. At the 52° inclination of the ISS, the South Atlantic Anomaly passage is expected roughly ten times per day. Second, on a much larger scale, the stream of charged particles emanating from the Sun, known as the solar wind, also modulates the influx of GCR. During the typical 11 year solar cycle, GCR in the inner solar system is at its minimum during maximal solar activity; known in shorthand as solar max, it can be thought of as maximal solar protection from the more high-energy GCR.

RADIATION: HEALTH EFFECTS

Because ionizing radiation exists in different forms with varying energies, defining the standard units and mea-sures is helpful to understand exposures and scale human health risks. The transfer of one joule of radiation energy to one kilogram of matter defines the Gray (Gy), the SI (International System of Units) unit of radiation absorbed dose. The Gy describes a purely physical quantity of energy transfer regardless of resulting biological effect, and reflects what a physical radiation detector would register. Because different particles and waves of the same physical energies may lead to different effects on tissues, a scaling factor is needed to correlate radiation type with biological outcome.

The radiation weighting factor (WR), similar to the long-used quality (Q) factor, is a value that is applied to specific radia-tion types to allow direct comparison of resulting biological effects. X-rays and gamma rays as well as electrons have a WR of 1, whereas energetic protons may have values of 2 to 5, and alpha particles and heavier high-energy GCR spe-cies may be 20  or higher for the same radiation absorbed dose. The seivert (Sv) measures the actual biological effect resulting from absorbed dose of a specific radiation type, and thus is the product of the absorbed dose and the radia-tion specific WR.

Ionizing radiation can induce tissue damage via sev-eral different mechanisms and cause both acute and long-term health effects. Broadly these effects are categorized as deterministic and stochastic. Deterministic effects typi-cally involve high relative doses of ionizing radiation and describe direct damage or killing of multiple cells and tis-sues. Greater absorbed dose, usually measured directly in Gy, correlates with greater severity. The acute radiation syndromes are clinical manifestations of ionizing radiation, matched to dose in Table 18.5. These would be expected ter-restrially in nuclear power accidents with leakage of radia-tion or with nuclear weapon detonation, and in space from solar particle events with minimal shielding. Since space weather has been monitored since the 1950s or so, there have been several solar particle events that would have caused ionizing radiation exposures leading to acute clini-cal syndromes for astronauts outside of the geomagnetic field. Stochastic effects are associated with lower levels of

1

11.5°

Geomagnetic axisRotational axis

Magnetic�eld lines

Earth radii(6380 km)above surface

Geomagneticequator

Geometricequator

Outer belt

Innerbelt

Low earthorbit zone

SouthAtlanticanomaly

2 3 4

Figure 18.1 Distribution of trapped radiation associated with the inner and outer Van allen belts and influence of altitude and inclination on exposure. Piloted low Earth orbit (LEO) flights are below the inner belt. the South atlantic anomaly rep-resents a geographically centered area where the geomagnetic field is relatively weak due to the offset between Earth’s rotational axis and geomagnetic dipole, allowing a greater flux of trapped particles at a given altitude.

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330 Space physiology and medicine

ionizing radiation below a threshold for significant acute cell killing and may involve damage to a single cell, but pro-duce damage to DNA or other cellular elements that may lead to eventual induction of cancer or genomic instability. The probability rather than the severity of an adverse clini-cal outcome directly correlates with increasing exposure, the signature concern being development of cancer in later years. The risk probability for these stochastic outcomes is correlated to the dose equivalent expressed in Sv. There is recent evidence that long-term stochastic effects may also include degenerative vascular changes and central nervous system (CNS) effects.

The implications of stochastic radiation effects have a significant influence on human spaceflight. Radiation expo-sure for real time mission planning, from the standpoint of time in space and type of radiation dose received, must be weighed against the long-term health risks. As an agency, NASA has set an upper limit of three per cent excess risk of exposure-induced death (REID) due to cancer as a health standard for flight in LEO. The actual exposure associated with this risk level is not uniform between individuals. Risk of eventual fatal cancer for females and those exposed at an earlier age is relatively higher owing to age at exposure effects and differences in tissue types and susceptibilities; therefore exposure limits are age and gender weighted. Other factors that may affect lifelong cancer risk, such as smoking, also influence the allowable spaceflight acquired limit. Because there remains considerable uncertainty in translating radiation dose to clinical outcomes, particularly with high-energy GCR, further conservative step limits are added to ensure that these limits are met with a 95th percen-tile upper bound confidence.

Table  18.6  shows representative exposure limits for a one year mission in LEO. On the ISS, daily dose rates range from about 0.4–0.6  mSv per day depending on the solar cycle, with about 80 per cent contributed by GCR. A six-month tour on the ISS at solar min, when GCR has its

greatest influence, may thus be associated with as much as 110  mSv exposure. A Mars mission using existing chemi-cal rocket technology presents a particular challenge. The cruise phase will occur almost entirely in deep space with-out the protection of the geomagnetic fields. Radiation instruments carried aboard the Mars Science Laboratory, which was launched towards Mars in 2011, provide direct measurements that inform the human exposure on such a voyage. Data suggest that an astronaut would receive a dose of 662  mSv during the 360  days of cumulative out-bound and inbound deep space cruise associated with a realistic reference mission to Mars (Zeitlin et  al. 2013). In addition, surface data collected from the Mars Science Laboratory suggests that for a human exploration crew, the cumulative equivalent dose for the cruise phase and sur-face stay of 500 days will be roughly 1.01 Sv for this current solar cycle (Hassler et al. 2014).

With the ubiquitous presence of radiation and the health risks noted, space travellers must be considered radiation workers and undergo strict monitoring for

Table 18.5 acute radiation syndromes

Dose (sieverts) Syndrome Tissue effects Clinical manifestations0–0.25 threshold for acute effects None0.25–1 Haematopoietic Bone marrow stem cell damage,

lymphatic and splenic tissue damageMinimal; mild nausea, loss of appetite

1–3 Haematopoietic Bone marrow stem cell depletion, mild to moderate leukopenia, eventual pancytopenia

Mild to severe nausea, infection, weakness, fatigue

3–6 Gastrointestinal Moderate to severe leukopenia, gastrointestinal mucosal cell damage

Severe nausea, moderate diarrhoea, haemorrhage, epilation; mortality near 100% without treatment

6–10 Gastrointestinal,cerebrovascular

Extensive gastrointestinal mucosal cell damage, direct CNS damage

Seizures, cognitive impairment, headache, severe diarrhoea, hypotension

>10 Cerebrovascular Extensive CNS damage; cerebral oedema

tremors, ataxia, incapacitation

These are broadly divided into haematopoietic, gastrointestinal and cerebrovascular syndromes in order of increasing ionizing radiation dose required to elicit. Low dose effects carry over and combine with higher dose effects. CNS, central nervous system.

Table 18.6 Ionizing radiation exposure limits weighted for age and gender for a 1-year mission in low Earth orbit, correlated with a 3% risk of exposure-induced death (rEID)

Ionizing radiation dose in sieverts, 3% REID

Age, years Males Females25 0.52 0.3730 0.62 0.4735 0.72 0.5540 0.8 0.6245 0.95 0.7550 1.15 0.9255 1.47 1.12

From Cucinotta and Durante (2009).

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Gastrointestinal,

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Gastrointestinal,cerebrovascular

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cerebrovascular

Cerebrovascular Extensive CNS damage; cerebral

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Cerebrovascular Extensive CNS damage; cerebral

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Physical factors of spaceflight 331

radiation dose received. This includes personal dosimeters that are worn by crewmembers at all times during mis-sions as well as various dosimeters and detectors deployed throughout a space vehicle. Human spaceflight neces-sitates a comprehensive radiation monitoring capability that tracks equivalent dose received by each individual and applies this against future missions. Long-term sur-veillance for defined clinical outcomes as for any industry radiation worker should be in place. Prevention is key, with mission planning accounting for crewmember’s accumu-lated dose, duration of flight, solar activity, and shielding characteristics of the vehicle or platform to keep the dose equivalent as low as possible.

MONATOMIC OXYGEN

In the Earth’s atmosphere, oxygen predominantly exists in its stable molecular form consisting of two bound oxygen atoms (O2). In the upper reaches of the atmosphere, exposed to harsh UV light in wavelengths less than 243 nm (UVB), O2 readily dissociates into monatomic oxygen. As altitude increases, the rarefied atmosphere precludes significant recombination into O2  or ozone (O3), such that mona-tomic oxygen becomes the most dominant species between 180 and 650 km (Banks et al. 2004). Monatomic oxygen is highly reactive and contributes heavily to erosion and dis-coloration of carbon-containing materials exposed to exter-nal space in LEO. It is one component, along with ionizing and direct UV radiation and orbital debris, that contributes to the observed phenomenon of degradation of external spacecraft materials. Special coatings, primarily silicate lay-ers, are resistant to monatomic oxygen and are frequently used to protect spacecraft exposed surfaces.

ORBITAL DEBRIS AND MICROMETEOROIDS

To add to the background flux of charged particles and chemical moieties, Earth’s orbit is populated with material from both natural and artificial sources. Meteoroids are nat-urally occurring fragments resulting from collisions among asteroids and from the evaporation of comets near the Sun, and range in size from the micron level to large and (for-tunately infrequent) metre- and decametre-sized objects. As a point of reference, the Chelyabinsk meteorite that exploded in the atmosphere over Russia in 2013 was about 18 m across. Orbital debris, as the name suggests, consists of unused pieces of spacecraft or whole derelict satellites, shed fragments from frangible bolts and spacecraft collisions, vapour particles from metallic rocket fuels, and other mate-rials associated with spaceflight activity. Collectively these are frequently referred to as micrometeoroid and orbital debris (MMOD). Orbital debris makes up the vast majority of the mass of material over several microns in size and has become increasingly problematic for spacecraft operations in LEO owing to risk of collision. Average relative velocities in LEO are roughly 10 km/s and may be as high as 15 km/s. Natural meteoroids although of much lesser influence carry great momentum, with greater average relative velocities from 15 to as much as about 70 km/s.

For convenience, orbital debris may be classified as small (less than 1 cm), medium (1–10 cm), and large (greater than 10  cm) based on methods used to detect and track items. Large objects can be seen and tracked by ground radars, which permits spacecraft with propulsive capability to per-form avoidance manoeuvres to prevent catastrophic colli-sions. The ISS periodically performs such debris avoidance manoeuvres, and on a few occasions ISS crews have taken precautionary refuge in escape spacecraft when objects have come particularly close. As of 2011, over 22 000 objects in this category were being tracked by the international Joint Space Operations Center (Committee for the Assessment of NASA’s Orbital Debris Programs 2011). Medium-sized objects may be seen and counted by ground radars but not tracked, which gives an assessment of risk without the abil-ity to predict and thus avoid collisions. It is estimated that over half a million objects greater than 1 cm in size populate LEO, which drives risk assessments and shielding require-ments for manned spacecraft. MMOD collision and sudden depressurization of the ISS and other spacecraft are possi-bilities that must be prepared for with pressure refuges and escape plans.

Smaller objects make up the vast majority of the mass of artificial material and present less of an acute threat, but contribute greatly to the degradation of materials continu-ously exposed to the external space environment. Over time the surface of structures becomes peppered with tiny, often sharp-edged craters of sub-millimetre, millimetre and cen-timetre sizes, with the preponderance being in the smaller range as evidenced by spacecraft that have been returned to Earth for analysis. An interesting hazard that has been observed on the ISS is the formation of sharp-edged defor-mations resulting from small objects colliding with metal surfaces. These constitute a perforation and subsequent decompression risk for spacesuits, particularly gloves, dur-ing space walks.

Flight dynamics of ascent and entry

Suborbital spaceflight implies a profile that ascends to an altitude above the threshold of aerodynamic surface con-trol but follows a trajectory that re-intersects the atmo-sphere rather than attaining a stable orbit. Implicit in this rarefied atmosphere is the need for rocket propulsion and a fully enclosed pressurized cabin. Alan Shepard’s 1961 flight aboard the Mercury mission Freedom 7  was suborbital, attaining an altitude of 187  km (116  miles) and lasting 15 minutes. There was a period of free fall experienced as zero gravity which lasted about five minutes before the onset of entry loads. This sortie was a deliberate test point en route to orbital flight capability and by and large these were not repeated.  Interestingly, the spaceflight tourist industry is expected to begin offering suborbital flights to paying cus-tomers on a large scale. The aim will be to afford tourists the sensation of rocket ascent, a brief period of weightlessness and a spectacular view of the Earth before re-entering. This implies new spacecraft and flight operations that will support

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equivalent as low as possible.

MONATOMIC OXYGEN

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332 Space physiology and medicine

such flights on a large commercial volume. Eventually this flight profile may expand to include rapid suborbital transit profiles between surface points.

Orbital spaceflight defines a condition whereby a space-craft has attained an altitude and forward velocity matched to that altitude sufficient to remain stable in that orbit for a substantial period of time. The tendency to fall to Earth is exactly balanced by the forward velocity so that the space-craft essentially ‘falls’ around the Earth; the physiological condition of weightlessness is realized by this condition of freefall. Figure 18.2 depicts a sustainable Earth orbit.  It implies rising above the major part of the atmosphere such that atmospheric drag is negligible over short periods of time. Minimum altitudes of typical spacecraft are around 160–200  km, where atmospheric drag is still enough to require reboost after a few days. The ISS orbits at an altitude of around 400 km. Even at this altitude, with the substantial cross sectional area afforded by structure and solar arrays, reboost requires several thousand kilograms of propellant each year.

Often the term ‘outer space’ is used to describe any vehicle in orbit or beyond, invoking impressions of great distances, but this is misguided with typical LEO altitudes of 200–400  km.  What really separates the occupants of an orbiting spacecraft from the ground is a velocity dif-ference.  Orbital velocity in LEO is roughly 28 160  kph at the altitude of the ISS; for launch this must be attained between lift-off and orbital insertion, and for return it must

be dissipated between deorbit burn and landing. Although the relative velocity change is roughly the same for launch and landing, the forces acting to realize this change differ greatly. Insertion of a spacecraft into LEO involves transit-ing the atmosphere to overcome drag and build sufficient velocity to maintain a stable orbit. A typical ascent profile will involve primarily vertical flight at first to loft to an alti-tude where atmospheric drag is sufficiently low, then a turn towards a predominantly downrange trajectory to build orbital velocity. For ascent, the velocity is provided by pow-erful rocket engines operating near the limit of materials and systems capabilities. For entry and landing, a combina-tion of rocket impulse, atmospheric drag, atmospheric lift, and parachute descent (for capsules) is required to bring the spacecraft safely to a desired point on the ground.

From launch pad lift-off to engine cut-off, the ascent to orbit for contemporary spacecraft typically requires eight to nine minutes, reflecting a balance between accelera-tion loads on the human occupant and optimum hardware performance. If the spacecraft is to eventually dock with another orbiting platform, the launch is precisely timed to occur as the launch site rotates through or near the orbit of the target platform. Crewmember work and sleep schedules are matched to this time well in advance to ensure optimal performance during this demanding flight phase and the associated physiological challenge. Figure  18.3  shows the typical acceleration versus time profiles for the US Space Shuttle and the Russian Soyuz. Tolerance to these loads is

A T M O S P H E R E

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Gravitationalforce

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Orbital altitude400 K International

space station

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Spacecraft

LAUNCH

Figure 18.2 Simplified depiction of a circular spacecraft orbit (not to scale). the gravitational force on the spacecraft is counterbalanced by the forward orbital velocity which is specific for a given altitude (and hence gravitational force), such that the spacecraft continues to ‘fall’ around the Earth.

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condition of weightlessness is realized by this condition Copyrighted

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Physical factors of spaceflight 333

dependent on crew positioning with respect to the veloc-ity vector. Thus far all crewmembers travelling to LEO have been oriented during launch in a recumbent position so that the ascent loads are taken in the +Gx (chest to back) direction, and these loads are well within human tolerances. Vibration forces are vehicle dependent, but are designed to be within the tolerances of both spacecraft structure and its human occupants. These come into play largely with read-ing displays and tasks requiring fine motor control, such as switch throws. The ascent phase ends as the engines shut down and the spacecraft transitions abruptly from the final acceleration loads to weightlessness, typically 3 Gs to zero.

For atmospheric entry and return, the spacecraft fires an engine impulse into the orbital velocity vector, not so much to slow down as to lower the orbital trajectory so that it intersects the atmosphere. The deorbit burn is precisely con-trolled for magnitude and direction of thrust, start time and duration, and relative ground position to arrive at a specific atmospheric entry point. The drag of the atmosphere then does the work of dissipating the orbital velocity, inducing a

great deal of frictional heat on the spacecraft structure in the process. The atmospheric entry phase ends when a space-craft is predominantly in a vertical descent for a capsule, or has transitioned to predominantly aerodynamic control for a winged spacecraft. Actual landing may involve a fairly gentle runway touchdown as for the Shuttle, or a water or land landing following parachute descent as for a capsule that may involve a significant impact event.

Entry following prolonged periods in weightlessness involves spaceflight-deconditioned crew, which implies a reduced tolerance for acceleration forces.  As will be dis-cussed, the +Gz axis is particularly vulnerable owing to this deconditioning, and most spacecraft have positioned returning crewmembers such that entry forces are taken in the +Gx axis. The exception has been the US Space Shuttle, which returned crew in an upright seated position after flights up to 18  days in duration. Figure  18.4  shows entry acceleration loads of the Soyuz and Space Shuttle. Even with the relatively low loads involved, deliberate protection for crewmembers in the form of anti-G garments, saline fluid loading, active cooling, and occasionally volitional straining manoeuvres has been required. Several crewmembers were returned on the Space Shuttle from long-duration flight, arbitrarily defined as greater than 30 days in duration, from the Mir and International Space Stations. This necessitated devising a recumbent seat system to fit on the Shuttle mid deck to orient long-duration flight crewmembers in the +Gx direction with respect to the dominant acceleration vector.

As with aviation, there have been unfortunate mishaps in the history of human spaceflight that have influenced the crew protective equipment and procedures now routinely used. All of the mortality associated with actual spaceflight is attributed to accidents occurring during the dynamic phases of ascent and entry. Fatal accidents have occurred during launch, with the breakup of the Space Shuttle Challenger (1986), and landing with the loss of Soyuz 1  (1967) due to parachute failure, the Soyuz 11  crew due to high-alti-tude cabin depressurization (1971), and the Space Shuttle

500400300Time (seconds)

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2.0

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tion

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Francis

334 Space physiology and medicine

Columbia (2003) breakup during entry due to structural damage incurred on launch. In addition, acceleration forces much greater than the nominal loads shown above have been realized during launch aborts and ballistic entries.

Crewmembers wear pressure suits with safe breathing sources as a refuge against cabin depressurization or con-tamination during launch and entry phases. These may have the unwanted effects of thermal loading, decreased reach and visibility, and general discomfort, but are a necessary safeguard in what remains a dangerous activity. Launch and entry seating is designed to position and support crewmem-bers optimally for standard and off-nominal acceleration loads, typically as form fitting as possible with multi-point restraints. Active cooling is provided in the form of suit ventilation or liquid cooling garments. Various spacecraft have utilized a variety of escape mechanisms, including personal harnesses and parachutes (Space Shuttle), ejec-tion seats (Vostok, Gemini and the first four Space Shuttle flights), and escape towers to pull a capsule away from a rocket stack (Mercury, Soyuz, Apollo). For the possibility of off-target landing, crews are equipped with a minimum of survival equipment.

Orbital flight

THE ORBIT ITSELF

The orbit attained is determined by the ascent trajectory and is characterized by altitude, inclination, orbital period, and other factors beyond the scope of this discussion. However, medical support personnel should be familiar with basic elements that might affect crew operations.

The altitude is typically an average; few orbits are truly circular, instead consisting of an ellipse with a lowest point (perigee) and highest point (apogee). The ISS orbit is nearly circular, with typical perigee and apogee altitudes of 420 and 423 km, respectively (260 and 263 miles). Inclination refers to the angle of the orbital plane with respect to the plane of Earth’s equator. A spacecraft launching straight eastward from a launch site on the equator would have an inclina-tion of 0°, with a ground path that always traces the equator. Launching from the same site on a trajectory angled 45° to the north (or south) results in a 45° inclination orbit, with a ground path that oscillates relative to the equator between 45° north and 45° south latitude. (The same orbit results from launch straight eastward from a site located at 45° north or south latitude.) It is easily understood by imaging a hula hoop around a world globe tilted with respect to the equator, with the hoop representing the plane of the orbit. If the globe and the hoop were flattened out into a Mercator projection, the familiar sine wave pattern seen on mission control maps where spacecraft positions are tracked is eas-ily visualized (Figure  18.5). The orbital period is the time required to make one complete revolution.

The ISS orbits at an average altitude of 400 km, with an inclination of 51.6° and an orbital period of roughly 90 min-utes. The practical implications are that crewmembers can directly see Earth features between 51.6° north and south

latitude, and could possibly land anywhere in this broad band for a normal landing or emergency station evacuation. Crew survival training and equipment are provided accord-ingly for both land and water landings. A 90-minute orbit translates into 16 sunrise/sunset cycles per 24-hour period, which requires artificial regulation of day and night cycles for crew duty day and sleep considerations.

TRANSITION TO WEIGHTLESSNESS

The onset of weightlessness after the acceleration forces of ascent is one of abrupt and stark contrast. There is an imme-diate equalization of fluid pressures throughout the body, sensed as a rush of fluids to the torso and head. The body floats away from the surface of the launch seat and is held lightly in place only by the restraining straps. Arm move-ment for display activation, switch throws, checklist use, etc. is suddenly different as motor control inputs must now subtract out the absent weight of the limb. Anything not restrained will float free, and management of equipment and flight support items quickly takes on a new dimension.

Along with this, there are inevitable generic crew duties that make this a labour and cognition intensive time. A  combination of manual control inputs and monitoring of automated processes require crewmembers’ attention, and task focus is essential despite the novel physiological changes occurring. Whatever structure carried the bulk of the final ascent fuel, such as the third stage booster of the Soyuz or the external tank of the Shuttle, will be jet-tisoned. Navigation systems and thrusters will be enabled to perform spacecraft attitude control, thermal control

51.6° inclinationorbit

Equator

51.6° North

22.5°2

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Figure 18.5 Ground track for a circular orbit of 51.6° incli-nation, similar to that of the International Space Station. the centre of Earth’s mass coincides with the centre of the orbit, which is tilted relative to the equator so that the orbital track ‘ascends’ to 51.6° north latitude, then ‘descends’ to 51.6° south latitude. Each successive orbit moves 22.5 degrees westward due to the Earth’s rotation beneath the orbit. there are other more subtle influences on the orbit such as the non-spherical nature and mass distribution of Earth and the effects of lunar gravity.

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Francisand flight support items quickly takes on a new dimension.Along with this, there are inevitable generic crew duties

FrancisAlong with this, there are inevitable generic crew duties

Physiological adaptation to weightlessness 335

systems will come online to handle solar heat loads, and life support systems will be thoroughly checked for proper function. Significant crew attention is focused on verifying cabin pressure integrity after the structural challenge of ascent; only then can suits be doffed and a somewhat more leisurely pace of spacecraft operations can ensue.

STEADY-STATE WEIGHTLESSNESS

In the absence of the dominating effect of gravity, some pro-cesses which influence human habitability are diminished and some enhanced. There is no buoyancy-driven separa-tion of gases or liquids, so that the concept of heavier gases accumulating at one’s floor has no meaning. Passive diffu-sion remains, although this is a very slow mixing process compared with buoyancy; gas concentrations will largely accumulate wherever they are introduced or fall where they are consumed. This necessitates forced convection to mix atmospheric gases, preventing carbon dioxide accumulation and replenishing oxygen around a human occupant. Active fans, which require power and produce nuisance noise, are a necessary component of life in weightlessness. There is no gas-fluid separation, so a distinct air fluid level in a vessel or a visceral organ will not exist, replaced instead by a suspen-sion of gas distributed uniformly in a liquid. Sedimentation, in which solids might fall to the bottom of a liquid column, is also absent so that particulates in a liquid will also exist in suspension. Capillary action and surface tension remain and become more prominent. In addition, there is no ‘sump effect’, in which fluids accumulate at a lower point in a con-duit. This drives management of plumbing conduits moving coolants and other fluids, but happily does not affect food ingestion and digestion, which become totally dependent on peristalsis.

From the standpoint of human activity, one can obvi-ously move once-heavy objects with ease, though larger mass items must be managed with care owing to their inertia. The relatively small volume inherent in space-craft becomes more habitable owing to the availability of movement and object placement in three dimensions. Locomotion shifts from a familiar upright posture powered by legs to a variable orientation with respect to the internal volume, determined by convenience of the task at hand and largely powered by arms and hands. The ability to use all dimensions is somewhat offset by the propensity for objects to float away if not restrained, so that management and memory of multiple items become critical skills for efficient operation. A more problematic aspect of weightlessness is that since small particles do not settle out onto surfaces, chance encounters may lead to foreign bodies impacting the eye or being inadvertently aspirated.

PHYSIOLOGICAL ADAPTATION TO WEIGHTLESSNESS

In the normal terrestrial condition, we interact with gravity in a highly dynamic fashion. Our upright posture involves a re-orientation to the gravity vector and its resultant

hydrostatic gradient many times daily in the activities of lying down, sitting, standing, and locomotion. As such our systems are designed to accommodate these variable chal-lenges with protective mechanisms to maintain perfusion pressure to the central nervous system, regulate fluid vol-ume, and move in a mechanically efficient manner. The complete effective removal of this dominant force places the body into a relatively static, unchallenged state, ren-dering many of these processes less necessary and in some instances a hindrance. Adaptation to microgravity can be thought of as a down-moding of many of these protec-tive mechanisms, minimizing processes that are no longer needed and reinterpreting mechanical and visual inputs to process in a manner more suited to weightlessness.

With the 50 plus years of human spaceflight experience accumulated, there is now an awareness of a constellation of predictable physiological changes with timelines and endpoints associated with adaptation to weightlessness. By and large these can be thought of as physiological efficien-cies that allow the body to function optimally in this novel environment. With few exceptions, these are enabling and only become problematic during return to unit gravity. This section will review the most significant changes along with their clinical implications.

Acute adaptation

Although all processes involved in adaptation to weight-lessness begin upon engine cut-off following ascent, there are three noteworthy phenomena with almost immedi-ate clinical manifestations. These are sensorimotor effects, fluid shifts, and anthropomorphic changes. Although each of these will continue to change in a more benign fashion towards new set points, it is important to appreciate the acute changes for their implications to crew operations and functional performance. There are overlaps in symptomo-logy, and as with long-term adaptation each system must be viewed in the context of the holistic changes occurring. In fact these can be considered primary effects from which many of the further changes seen cascade as secondary responses to weightlessness.

NEUROSENSORY EFFECTS

There are many cues that guide positional awareness and motion control of body and limb on Earth. The neuroves-tibular system utilizes specialized organs that sense both motion and motion rates. The inner ear otolith organs and semicircular canals making up the vestibular system function as tiny accelerometers for linear (translation) and rotational motion, respectively. Proprioceptors in the mus-culoskeletal system also provide positional input for motion based on mechanical stresses, and tactile (haptic) sensors provide inputs whether standing, sitting or recumbent. These normally work in a steady-state gravity field and are tightly tied to this background input that serves as a ref-erence condition. Not directly related to gravity is visual input, which provides powerful cues as to positional sense

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336 Space physiology and medicine

that are integrated in the CNS with the familiar correspond-ing inputs of the previously mentioned systems.

Humans adapt to and function in several novel motion environments, such as sea surface, underwater and flight. All of these involve some degree of disruption of normal motion control cues, and all are associated with a set of neg-ative reactions and illusions as well as adaptation responses and strategies that enhance function. Weightlessness induces a more global change by nullifying many of these inputs. Simply put, the abrupt unloading of the body’s grav-iceptors disrupts the integrated sum of these inputs, giving rise to sensory conflicts. The tonic force of gravity no longer pulls on the otoconia of the otolith organs, which in a pure form emulates a state of falling (without the excitement). An illusion of ‘inversion’ is commonly experienced at the abrupt transition to weightlessness, tied to this sudden unloading of the otoconia. Although visual cues will show motion compared to a reference background, tilting one’s head no longer involves a downward pull to one side, so that the vestibular and visual inputs are mismatched. Two immediate manifestations are space adaptation syndrome (SAS) and motion control disturbances.

SAS, also known as space motion sickness, is a clini-cally significant result of the mismatch of these sensory cues and may be seen within minutes to hours after arriv-ing in weightlessness. The incidence among first-time fly-ers may be as high as 50–70  per cent. Nausea and emesis may occur, often with very little warning. Lethargy, fatigue and diminished appetite are common, and headache may be present though this may overlap with symptoms caused by the acute fluid shift. Rapid and radical motions are pro-vocative, particularly rotational head motions that take the body out of the sensed visual vertical plane based on the spacecraft structural reference. As such, slower motions are encouraged along with maintaining a visual vertical reference frame for the first several hours of adaptation. Interestingly, the incidence of SAS seems to be lower in smaller vehicles, possibly owing to the diminished freedom of movement compared to larger volumes such as the Space Shuttle afforded and hence less provocative motion. Veteran flyers show a lesser incidence of SAS, although whether this is due to some degree of retained conditioning or learned protective strategies is unclear.

Medications are frequently used in the clinical manage-ment of SAS. Oral antiemetics may be taken prophylacti-cally just prior to launch, though their use is discouraged among critical flight crew members owing to possible seda-tive effects. Once inflight, parenteral promethazine has been found to be highly effective for symptomatic SAS. Readily available emesis containers are a must, and often present the first zero G fluid containment challenge to a new flyer. Oral hydration should be encouraged. Fortunately the clinical course is self-limited; most symptoms gradually improve and abate within 48  hours. As frequent as SAS is seen, it rarely has a significant impact on crew operations.

Motion control disturbances are seen with both limb and whole-body movement, although practically no formal

investigations have been done in the acute phase of adapta-tion owing to mission constraints. Arm motions for switch throws and other control inputs must be made slowly and deliberately as crewmembers become accustomed to the lower muscular force required to move a limb that is now weightless. For whole-body locomotion, there is a strong ten-dency to maintain a vertical reference and ‘walk’ from point-to-point utilizing foot restraints. An amusing observation is the subconscious lofting correction for gravitation when an object is thrown or passed, and when crewmembers launch themselves a short distance from one surface to another. The strong tendency to ‘aim high’ is eventually replaced by the experience of all objects moving along the precise trajectory imparted; usually this requires a few hours to a few days to fully transition. As for SAS, repeat flyers show a much more rapid adaptation. Significantly, this operationally inten-sive period that may involve mechanical systems actuation, rendezvous and docking, and other dynamic flight tasks is typically executed without problems. Motion control dis-turbances are apparently overcome by such factors as rapid adaptation, task familiarity, greater reliance on visual inputs and feedback, and in some cases displayed data.

ANTHROPOMORPHIC RESPONSE

In weightlessness, the human undergoes changes in body shape as well as relative position of individual organs. Those that are most closely related to gravity occur immediately, while others that are tied to tissue composition, such as hydration of intervertebral discs and muscle mass, change over several hours to days. The most dramatic effect is the neutral body posture, seen in Figure  18.6. This posture is very definitive; if a crewmember forces himself into a rigid position that mimics standing in 1 G then suddenly relaxes, the body will snap into this position within a few seconds. This is the position a body will naturally assume dur-ing sleep, and restraints for both work and leisure should be designed mindful of this position. Along with the pos-tural change, the abdominal girth decreases as the viscera are no longer suspended from the mesenteric connections to the diaphragm; all shift cephalad. Chest circumference increases, with a slightly greater anteroposterior diam-eter resulting in a more circularly shaped rib cage. Other anthropometric changes can be seen over time, but these acute effects have direct bearing on physiological responses to weightlessness as discussed below.

FLUID SHIFTS

The terrestrial conditions in which humans have developed and thrive involve a hydrostatic gradient, as gravity con-stantly pulls down on the water column created by the vas-cular system and tissue fluids. This is primarily expressed in the body Z axis when upright, shifting to the X axis when recumbent. Once in weightlessness, this gradient is abruptly abolished as venous and arterial pressures roughly equalize throughout the body in their respective tracts. Figure  18.7  shows this as a comparison with the upright posture that represents the most extreme gradient as well as

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Physiological adaptation to weightlessness 337

Body-centredhorizontal reference

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From NASA Human Integration Design Handbook, NASA/SP-2010-3407/Rev1, June 2014.

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Figure 18.7 the hydrostatic gradient of the vascular column on the human during standing in 1G and immediately upon arrival into weightlessness. Mean arterial pressures are expressed in mmHg. the shift of fluid volume and vascular pres-sure to the head may cause nasal stuffiness and mild facial oedema, which eventually improves as the plasma volume down-regulates to a new setpoint appropriate for weightlessness. these pressure changes may be seen in brief 20 second periods of weightlessness during parabolic flight. CVP, central venous pressure.

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338 Space physiology and medicine

the terrestrial recumbent position. Along with this, vascu-lar volume that normally resides as capacitance in the lower extremity circulation and probably splanchnic vascular beds (Petersen et  al. 2011) shifts quickly towards the tho-rax and head, followed by a volume of mobilized interstitial fluid from lower extremity musculature that is no longer held in place by hydrostatic pressure. There is a sensation of a fluid rush to the head, often described as similar to hanging upside down – this most certainly contributes to the inversion illusion noted above. In the first few hours in weightlessness, fluid shift may be manifested by congestion, stuffiness, mild headache, and facial oedema.

One of the more interesting and unexpected aspects of adaptation to weightlessness becomes evident with these fluid shifts. In ground analogues of weightlessness, such as water immersion and head down tilt, the rapid thoracic shift of one to two litres of vascular volume leads to an increase in central venous pressure and triggering of atrial, aortic and carotid stretch receptors in a sensed volume overload condition. These in turn induce a parasympathetically mediated vasodilatation to maintain a normal perfusion pressure. The cardio-renal baroreceptor response, which involves salt and water diuresis in response to this sensed volume overload, is seen in both of these scenarios. During spaceflight, echocardiographic results obtained on launch days show an increase in cardiac filling, with atrial disten-sion and increased cardiac output. However, compared with the preflight supine position, central venous pressure is seen to decrease immediately upon insertion into weight-lessness as determined by direct central venous catheters

with pressure transducers worn during launch and entry. The apparent resolution is that the increased thoracic diam-eter noted above effectively decreases thoracic pressure to a greater degree than the decrease in central venous pressure, effectively increasing transmural pressure and augmenting venous return and cardiac filling. The resulting lower inter-pleural pressure compared with 1G supine simultaneous with the central blood volume expansion represents a con-dition unique to weightlessness. Figure 18.8 shows a central venous pressure trace in an astronaut during launch and insertion into weightlessness. This effect is rapid and can also be seen during the 20–25  seconds of parabolic flight (Videbaek & Norsk 1997). Other key physiological param-eters may begin to change during this acute period, notably intracranial pressure, but these await investigation.

The head fullness and facial oedema resulting from fluid shift is usually little more than an unpleasant sensa-tion. Infrequently decongestants and analgesics are used for symptomatic relief. As for SAS these symptoms are self-limited, reducing in magnitude over a period of a few days as circulating vascular volume diminishes during further adaptation of fluid regulatory mechanisms.

General adaptation

While our understanding of adaptation to weightlessness is functionally useful, it is most certainly incomplete and largely based on the purposeful study of systems involved with the more detrimental clinical entities associated with spaceflight. Further investigations are giving a methodical

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Figure 18.8 Central venous pressure (CVP) as measured by indwelling catheter during launch into weightlessness on the Space Shuttle. CVP decreases slightly compared with preflight supine with the abrupt transition from launch acceleration loads to weightlessness as the ascent engines cut off. a, prompt decrease in central venous pressure 2 minutes before launch was due to closing of visor of helmet. B, Lift-off. C, Solid rocket booster separation. D, main engine cut-off and onset of microgravity. Note that the suit room measurements were in the supine position, whereas the launch pad mea-surements reflect a recumbent/legs elevated position of the ascent seats.

From Foldager et al. Central venous pressure in humans during microgravity. Journal of Applied Physiology 1996; 81: 408–12.

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the inversion illusion noted above. In the first few hours in Copyrighted

the inversion illusion noted above. In the first few hours in weightlessness, fluid shift may be manifested by congestion, Copyrighted

weightlessness, fluid shift may be manifested by congestion, stuffiness, mild headache, and facial oedema.

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Physiological adaptation to weightlessness 339

and more holistic picture of the physiology of weightless-ness. In the meantime, it is convenient for understanding to classify the responses to weightlessness as primary and sec-ondary. Along with the primary conditions of sensorimo-tor effects, anthropometry changes and fluid shifts resulting from abrupt unloading noted above, the unloading of the musculoskeletal system serves as a fourth primary response to weightlessness from which other significant changes cas-cade. While the atrophic changes of bone and muscle also begin immediately, these are not sensed as clinical entities and become more of a long-term concern. Most of the pro-cesses below can be considered secondary as all of these con-tinue towards new set points for the weightless condition.

An exhaustive review of the physiology of weightlessness is beyond the scope of this discussion; the significant effects that should be understood by the space medical practitio-ner are presented. There are some special considerations in interpreting these findings. The final adapted state of an individual system or the body itself reflects the sum total of influences including atmosphere, countermeasure per-formance, nutrition, activity, and other factors, and so may differ between vehicles and programmes. Also, results of inflight physiological investigations occasionally contradict one another. This may be accounted for by the small subject numbers available for study, differing methodologies which themselves may have unforeseen nuances in this novel envi-ronment, and influence of other physiological factors whose relevance we have not yet determined. Finally, for effects that are not clearly mapped to weightlessness, it may be dif-ficult to untangle aspects related to gravitational loading from influences of the enclosed cabin, novel diet, and other factors related to the expeditionary environment.

NEUROSENSORY

Once clear of the first few days in weightlessness and the frequent occurrence of SAS, humans continue down a path of more generalized adaptation, in particular with improve-ment in three-dimensional spatial orientation and motion control. Overall CNS adaptation to weightlessness has been reviewed by Clement and Ngo-Anh (2013). It is difficult to completely differentiate specific mechanisms of neurosen-sory system adaptation from cognitive strategies and grow-ing environmental familiarity. However some predictable findings and timelines can be seen.

From the beginning there is a shift to greater reliance on vision for position and motion sense, which remains throughout the time period in weightlessness. Body loco-motion gradually shifts to primarily arm-powered transla-tion, often with the Z axis in line with the direction of travel, with feet being used more for restraint and optimizing the upper body’s position at a worksite. Mass handling becomes more fluid, often with larger objects being carried for lon-ger distances between the feet. Occasionally crewmembers may experience mild transient spatial disorientation when moving between modules that may have different clocking orientations; this resolves over time. An interesting marker of neurosensory adaptation is being able to transition a

perceived work volume from an allocentric reference frame, based on the vehicle orientation, to an egocentric frame based on the self. Complete neurosensory adaptation for a first-time flyer typically requires several weeks based on many of these observations, and contributes greatly to human task performance. As might be imagined, this level of adaptation to weightlessness becomes particularly prob-lematic upon return to gravity.

FLUID REGULATION

Because fluid regulation and plasma volume changes were noted very early in human spaceflight, several sophisticated studies were conducted during the Skylab and Shuttle pro-grammes to determine underlying physiological mecha-nisms. These have included isotopically labelled body water investigations (Leach et  al. 1996), chromium tagged red blood cell studies (Alfrey et  al. 1996), intravenous fluid challenges (Norsk et  al. 2006), and supporting serological and urine studies. Coupled with the central venous pres-sure observations, these studies have afforded a functional understanding of fluid regulation in weightlessness. This topic is nicely reviewed by Liakopoulos et al. (2012).

The distribution and regulation of body fluids are gov-erned by the new and novel conditions described in the acute adaptation phase. Following the increase in venous return and sensed volume overload condition, the body responds by beginning to decrease plasma volume, a pro-cess which completes over about a one-week period and results in a 12–15 per cent reduction. Rather than involving salt and water diuresis, both sodium and total body water are maintained. Instead, body water is redistributed as vas-cular permeability increases to allow extravasation of both water and sodium into extravascular spaces. This is presum-ably facilitated by atrial natriuretic peptide, which is seen to increase on the first flight day in response to atrial stretch. In addition, antidiuretic hormone is elevated acutely upon entering weightlessness, further opposing a saline diuresis. Antidiuretic hormone is known to increase in response to motion sickness, and this may be a significant contributing factor to the acute response to fluid regulation in weight-lessness. As plasma volume settles to its new set point, atrial natriuretic peptide rebounds to a level lower than pre-flight. Body water and sodium remain in intracellular and interstitial storage spaces presumably throughout the stay in weightlessness.

As plasma volume decreases, a state of relative haemocon-centration results. This is apparently sensed and corrected fairly quickly by the process of neocytolysis, the selective splenic removal of newly released nucleated erythrocytes until concentration is appropriately matched to the reduced plasma volume. A similar phenomenon is seen in humans transiting acutely from prolonged periods at altitude to sea level. Though the actual trigger for this process is not known, within about a week both plasma volume and red blood cell mass are matched with a normal haematocrit and stabilized at a new norm that persists unchanged while in weight-lessness. The result is a euvolaemic state appropriate for

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begin immediately, these are not sensed as clinical entities Copyrighted

begin immediately, these are not sensed as clinical entities and become more of a long-term concern. Most of the proCopyrighted

and become more of a long-term concern. Most of the processes below can be considered secondary as all of these con

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cesses below can be considered secondary as all of these continue towards new set points for the weightless condition.

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tinue towards new set points for the weightless condition.An exhaustive review of the physiology of weightlessness

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An exhaustive review of the physiology of weightlessness is beyond the scope of this discussion; the significant effects

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is beyond the scope of this discussion; the significant effects that should be understood by the space medical practitio

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that should be understood by the space medical practitioner are presented. There are some special considerations

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ner are presented. There are some special considerations in interpreting these findings. The final adapted state of an

Copyrighted in interpreting these findings. The final adapted state of an individual system or the body itself reflects the sum total

Copyrighted individual system or the body itself reflects the sum total of influences including atmosphere, countermeasure per

Copyrighted of influences including atmosphere, countermeasure performance, nutrition, activity, and other factors, and so may

Copyrighted formance, nutrition, activity, and other factors, and so may differ between vehicles and programmes. Also, results of

Copyrighted differ between vehicles and programmes. Also, results of inflight physiological investigations occasionally contradict

Copyrighted inflight physiological investigations occasionally contradict Material

differ between vehicles and programmes. Also, results of Material

differ between vehicles and programmes. Also, results of inflight physiological investigations occasionally contradict Material

inflight physiological investigations occasionally contradict one another. This may be accounted for by the small subject

Material

one another. This may be accounted for by the small subject numbers available for study, differing methodologies which

Material

numbers available for study, differing methodologies which themselves may have unforeseen nuances in this novel envi

Material themselves may have unforeseen nuances in this novel envi-

Material -

ronment, and influence of other physiological factors whose

Material ronment, and influence of other physiological factors whose relevance we have not yet determined. Finally, for effects

Material relevance we have not yet determined. Finally, for effects that are not clearly mapped to weightlessness, it may be dif

Material that are not clearly mapped to weightlessness, it may be difcess which completes over about a one-week period and

Material cess which completes over about a one-week period and results in a 12–15 per cent reduction. Rather than involving

Material results in a 12–15 per cent reduction. Rather than involving salt and water diuresis, both sodium and total body water

Material salt and water diuresis, both sodium and total body water - are maintained. Instead, body water is redistributed as vas- are maintained. Instead, body water is redistributed as vascular permeability increases to allow extravasation of both - cular permeability increases to allow extravasation of both Taylor

are maintained. Instead, body water is redistributed as vasTaylor

are maintained. Instead, body water is redistributed as vascular permeability increases to allow extravasation of both Taylor

cular permeability increases to allow extravasation of both water and sodium into extravascular spaces. This is presumTaylor

water and sodium into extravascular spaces. This is presumably facilitated by atrial natriuretic peptide, which is seen to

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Taylor increase on the first flight day in response to atrial stretch. In addition, antidiuretic hormone is elevated acutely upon

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lessness. As plasma volume settles to its new set point, atrial natriuretic peptide rebounds to a level lower than pre

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Francisflight. Body water and sodium remain in intracellular and interstitial storage spaces presumably throughout the stay

Francisinterstitial storage spaces presumably throughout the stay

As plasma volume decreases, a state of relative haemocon

FrancisAs plasma volume decreases, a state of relative haemoconcentration results. This is apparently sensed and corrected

Franciscentration results. This is apparently sensed and corrected

340 Space physiology and medicine

weightlessness, which almost instantly transitions to a hypo-volemic and anaemic state upon return to Earth’s gravity.

CARDIOVASCULARThe interpretation of cardiovascular findings associated with spaceflight must be mindful of the preflight posture (supine, seated, standing) to which inflight metrics are com-pared. However the overall picture is consistent and pre-dictable. In parallel with the fluid dynamics noted above, echocardiographically measured atrial and ventricular vol-umes increase acutely. Cardiac output increases while mean arterial pressure and heart rate are maintained. In the brief 20-second period of weightlessness of parabolic flight, car-diac output was seen to increase by 29 per cent compared with the 1 G seated position. Systemic vascular resistance decreased by 24 per cent to maintain heart rate and mean arterial pressure unchanged. Corresponding studies per-formed one week into spaceflight show similar results, with cardiac output increased by 22 per cent and systemic vascu-lar resistance decreased by 14 per cent, this in concert with the plasma volume reduction (Norsk et al. 2006). Over the long term, cardiac chamber dimensions decrease to preflight values (Herault et al. 2000); heart rate and blood pressure tend to settle on values equal to preflight supine, while mean arterial pressure settles to a level between preflight supine and seated and systemic vascular resistance remains lower that preflight (Verheyden et al. 2010; Fraser et al. 2012).

There is imagery evidence of decreased left ventricular mass following one to two week spaceflights (Perhonen et  al. 2001); however, this may be at least partially attrib-uted to fluid redistribution and loss of cardiac interstitial fluid (Summers et al. 2005). Cardiac contractility and over-all function seem to be normal throughout long periods of weightlessness. In addition, arterial baroreflex sensitivity is seen to increase during the first few days in weightlessness, then return to a value unchanged from preflight supine (Hughson et al. 2012). The cardiovascular response to exer-cise is altered and is described below; however, in general the cardiovascular system adapts very well to long periods in weightlessness with no limitation on inflight crew per-formance. An interesting observation is that in spite of the decrease in vascular resistance, overall sympathetic nervous activity remains increased during spaceflight, as evidenced by elevated catecholamine levels and augmented response to lower body negative pressure (Norsk & Christensen 2009).

PULMONARYThe lungs, involving tissues of significantly different densi-ties, elastic recoil forces, and environmental gas exchange, are known to be influenced by gravity and acceleration loads. Lung structure and function is thus expected to be influ-enced by the mechanical effects of thoracic shape change and loss of the hydrostatic gradient. However, lung function undergoes limited and fairly rapid adaptation to weightless-ness, remaining constant and supporting oxygenation and ventilation unimpeded. Lung function in weightlessness is thoroughly reviewed by Prisk (2014).

The unloading of the abdominal viscera along with the circularization of the thoracic rib cage causes a cephalad shift of the diaphragm, which no longer bears the weight of the abdominal viscera during respiration. The abdomi-nal contribution to tidal volume increases significantly, as abdominal wall compliance increases while thoracic rib cage compliance decreases somewhat. A more even distribu-tion between ventilation and perfusion would be expected with the loss of the gravitational gradient on blood flow and this is indeed seen. With the effective decrease in physio-logical dead-space, tidal volume is reduced by about 15 per cent. Respiratory frequency actually increases somewhat to roughly 9 per cent, with a resulting decrease in ventilation of about 7  per cent. The subjective sensation of shallower breathing being adequate is perceptible to some crewmem-bers. However, some inhomogeneity in ventilation and per-fusion distribution does persist, suggesting that gravity may not be the overall determinate it has been thought to be.

Vital capacity and forced vital capacity are reduced slightly in the first days of weightlessness, possibly related to the acute increase in central circulating volume, but recover to preflight norms within a few days and remain constant thereafter. Membrane diffusion across the alveolar–capil-lary interface as measured by diffusing capacity for carbon monoxide undergoes an abrupt and sustained increase, by about 28  per cent compared with preflight standing, and remains elevated. This is likely due to a more uniform pul-monary capillary filling compared with the terrestrial con-dition of decreased capillary filling in the apical regions. This should be taken into account for physiological studies and toxicity considerations for airborne agents.

MUSCULOSKELETAL SYSTEM AND AEROBIC FITNESS

Both bone and skeletal muscle are vital tissues capable of responding to sustained or repetitive physical loading demands by altering mass and strength. Increasing tissue mass in response to repeated or sustained loads ensures the most energy-efficient state, so that the unloaded condition does not match to excess capacity and its higher metabolic demands. As the primary role of the musculoskeletal sys-tem is moving the body against the background force of ter-restrial gravity, it is no surprise that the absence of this force has a profound influence on the mass and strength of both of these. The physical unloading of the musculoskeletal sys-tem that occurs in weightlessness is a primary effect from which others cascade, including atrophy of both bone and muscle, spilling of bone mineral constituents into serum and urine, potential impacts on physical performance, and risk of fractures and muscle injury. These are effects that are most problematic on return to Earth, causing minimal impact to operations during prolonged stays in space.

Bone tissue is constantly being remodelled, maintained in a dynamic balance between the opposing processes of for-mation and resorption. Even in the relatively short missions of the Apollo program, it was known that spilling of bone mineral constituents such as calcium and phosphate into

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echocardiographically measured atrial and ventricular volCopyrighted

echocardiographically measured atrial and ventricular volumes increase acutely. Cardiac output increases while mean Copyrighted

umes increase acutely. Cardiac output increases while mean arterial pressure and heart rate are maintained. In the brief

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arterial pressure and heart rate are maintained. In the brief 20-second period of weightlessness of parabolic flight, car

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20-second period of weightlessness of parabolic flight, cardiac output was seen to increase by 29 per cent compared

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G seated position. Systemic vascular resistance

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seated position. Systemic vascular resistance decreased by 24 per cent to maintain heart rate and mean

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arterial pressure unchanged. Corresponding studies performed one week into spaceflight show similar results, with

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MUSCULOSKELETAL SYSTEM AND AEROBIC Taylor

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Physiological adaptation to weightlessness 341

the urine was accelerated in weightlessness. Through sub-sequent long-duration missions on the Skylab, Salyut, and Mir space stations, several key characteristics of the bone loss of weightlessness have been determined. Chief among these is that bone loss is regional, with the greatest losses occurring in the prominent weight-bearing areas such as the calcaneus, hip, pelvis and lumbar spine. Losses in these areas show a greater degree of trabecular loss compared with that of cortical, with total decreases typically exceed-ing 1  per cent per month in weightlessness, significantly greater than the rate of loss associated with terrestrial dis-use osteoporosis or osteoporosis of the elderly (Shackelford et al. 2004). Interestingly, the wrist typically sees no loss and the skull may actually show an increase in density as deter-mined by dual X-ray absorptiometry. Chemical markers of bone resorption, such as increased urinary n-telopeptide, calcium and phosphate are seen to increase within a day of entering weightlessness and remain elevated until returning to Earth. The resulting excess urinary calcium adds to a risk of renal stone formation. The usual increase in bone forma-tion seen in response to bone resorption seems attenuated or absent in weightlessness, as these two processes appear to be uncoupled (Smith et al. 2012). Recovery following Earth return requires more time than that spent during the mis-sion in weightlessness (Orwoll et al. 2013), perhaps by a fac-tor of two or three. Ultimately, the overall concern is that of increased fracture risk, either in a postflight period that might involve rehabilitation on Earth or exploration activi-ties on the surface of Mars, or years following a spaceflight career owing to accelerated osteoporosis.

Skeletal muscle reacts in a similar fashion to bone, with those regions most involved in the support of upright pos-ture and locomotion most affected by the unloading of weightlessness. Calf and thigh musculature is lost prefer-entially, whereas thoracic muscle is less affected and upper extremity is minimally affected (Gopalakrishnan et  al. 2010). This can be appreciated by measuring diminished strength and muscle mass and volume in the lower extremi-ties. A noticeable decrease in calf circumference occurs, with more mass being lost in the soleus than the gastroc-nemius (Fitts et al. 2013), which coupled with the fluid shift from the lower extremities contributes to their thinning in the phenomenon known as ‘bird legs’. A transition towards fewer slow twitch and greater fast twitch fibres is seen on biopsy studies of the soleus and gastrocnemius similar to that seen in bed rest (Trappe et al. 2009), and diminished lower extremity strength is common.

The equilibrium of bone and muscle attained in weight-lessness is heavily influenced by the quality of physical countermeasures available to compensate for the absence of gravitational loading. There have never been crewmem-bers that performed no exercise during long-duration stays in space, so results must be interpreted along a con-tinuum with an unknown control point at one end (repre-senting total unloading) and Earth’s normal conditions at the other. Countermeasures equipment has been steadily improving since the beginning of human spaceflight, and

has recently undergone drastic and measurable improve-ment. Treadmill and cycle exercise devices have been used for several decades, primarily supporting aerobic fitness and the postural muscles and joints associated with loco-motion. Resistance exercise has been on a more evolution-ary path, with heavier loads emulating gravitational forces becoming available only in recent years. The advanced resis-tive exercised device (ARED) was deployed on the ISS in 2008, and represents an inflection point in maintenance of bone and muscle. The ARED is capable of axial loads up to 600 lb equivalent force with characteristics that emulate free weights, and supports a variety of exercises that can target postural sites. A typical day’s countermeasures activ-ity during long-duration flight includes about two hours of actual aerobic and resistive exercise.

Prescribed use of the ARED is associated with minimal losses of bone and muscle and increase in lean body mass during flight. There is also evidence that bone and muscle maintenance is related to adequate calorie intake and main-tenance of body mass inflight, which up until this point had been suboptimal as a rule (Smith et al. 2012). Interestingly, upgrades to the ISS galley system were implemented about the same time as the ARED was deployed that greatly facili-tated ad lib access to water for drinking and food rehy-dration as well as food storage and meal preparation. It is somewhat difficult to separate the relative effects of nutri-tional support and heavy resistive exercise on bone and muscle maintenance; however, it is clear that exercise is best supported by adequate nutrition and energy intake. The result has been significant to the point that investigations dealing with musculoskeletal fitness in spaceflight must be interpreted as prior to or after the deployment of the ARED.

In spite of the demonstrated effectiveness of heavy resis-tive exercise in preserving bone and muscle, bone resorption continues unabated during the non-exercise time. Recent studies of bisphosphonates, a class of drugs that inhibit bone resorption, have yielded very promising results as an adjunct for bone protection in weightlessness. The bisphos-phonate alendronate in combination with ARED exercise was demonstrated to globally attenuate losses in bone min-eral density in the postural areas of concern, as shown in Figure 18.9. In addition, urinary and serological markers of bone resorption along with calcium spilling were attenu-ated, so that the risk of nephrolithiasis should be reduced as well (Leblanc et al. 2013). Whether or not pharmaceuticals are used as a standard addition to bone health in weightless-ness, adjuncts are important as a backup means to preserve bone in event of crew injury, exercise equipment failure, or environmental control compromise that might limit perfor-mance of physical countermeasures; all of these have hap-pened during human spaceflight.

Aerobic fitness follows a characteristic profile during long tours in weightlessness, with an initial inflight decline followed by a gradual return back towards preflight baseline over time, then an immediate postflight decline followed by recovery. For several years inflight aerobic fitness measure-ments had been determined by deriving maximal oxygen

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ing 1  per cent per month in weightlessness, significantly Copyrighted

ing 1  per cent per month in weightlessness, significantly greater than the rate of loss associated with terrestrial disCopyrighted

greater than the rate of loss associated with terrestrial disuse osteoporosis or osteoporosis of the elderly (Shackelford

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use osteoporosis or osteoporosis of the elderly (Shackelford . 2004). Interestingly, the wrist typically sees no loss and

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. 2004). Interestingly, the wrist typically sees no loss and the skull may actually show an increase in density as deter

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bone resorption, such as increased urinary n-telopeptide, calcium and phosphate are seen to increase within a day of

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calcium and phosphate are seen to increase within a day of entering weightlessness and remain elevated until returning

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Copyrighted of renal stone formation. The usual increase in bone formation seen in response to bone resorption seems attenuated

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ties on the surface of Mars, or years following a spaceflight

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bisphosphonates

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Francisness, adjuncts are important as a backup means to preserve

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342 Space physiology and medicine

uptake (V. O2max) values from heart rate data and known exercise loads. Recently actual measurements of V. O2maxwere obtained on ISS crewmembers performing periodic maximal cycle exercise along with heart rate data and met-abolic gas analysis. Figure  18.10  shows the relationship of aerobic fitness by the measures of peak aerobic capacity and aerobic power from preflight, inflight and postflight phases (Moore et al. 2014). The initial decline is likely multifacto-rial, with the decrease in plasma volume and red blood cell mass, acute neurosensory adaptation and familiarity with motion and restraint in weightlessness all possibly contrib-uting. Although variability is high, inflight trends suggest gradual improvement over a six-month mission but not quite back to preflight baseline; however, individuals that undergo particularly intensive daily aerobic exercise inflight are most likely to approach and maintain a near preflight level. The postflight decline is more easily understood in the context of the relative anaemia, hypovolaemia, and general deconditioning associated with weightlessness. Postflight crew condition and performance are discussed below.

ANTHROPOMORPHIC ADAPTATION

Following the increased thoracic diameter and decreased abdominal girth, change in posture and visceral shifting that occur within minutes of entering weightlessness, the human body settles more gradually into a unique habitus characteristic of weightlessness. Spinal lengthening is seen owing to fluid imbibing of the intervertebral disks similar to that while recumbent terrestrially, but that is now unopposed

by the compression of upright posture. In addition there is a relative decrease in the natural thoracic kyphosis. As a result, seated height and standing height are increased; seated height may increase by as much as 6 per cent. Another measureable change is the typical biphasic decrease in lower extremity diameter. The fluid shift and redistribution of the first few days in weightlessness cause a loss of volume from diminished venous capacitance and interstitial fluid from leg musculature, which is complete within seven to ten days. Thereafter a further gradual decline may occur, dependent on exercise-supported maintenance of postural muscula-ture in the calf and thigh, and measurement of calf volume has long served as a surrogate inflight muscle mass assess-ment. This has been attenuated somewhat with the recent introduction of heavy resistive exercise.

Periodic assessment of body mass has proven to be a use-ful measure of health and nutrition, and is typically per-formed on a monthly basis during long-duration flight. Undue ‘weight’ loss is an indication of inadequate energy intake and may be used to guide food selection and quan-tity. Body mass is assessed using oscillating spring damper systems or linear acceleration devices that set the body in motion and derive the mass from the motion dynamics and known mechanical characteristics of the systems (Zwart et al. 2014).

The consequences of the inflight anthropometry changes are mainly related to fit into body restraint systems and highly customized critical flight crew equipment such as pressure suits and landing restraint systems. As example,

Lumbar SpinePelvisTotal HipTrochanterFemoral Neck–20

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Figure 18.9 Change in bone mineral density from selected sites as assessed by dual X-ray absorptiometry (DXa) after spaceflight with a mean of 5.5 months’ duration. Compared with once-standard countermeasures, bone is better pre-served with heavy resistive exercise, with further preservation when bisphosphonates are added. arED, advanced resis-tive exercise device; 1, p<0.05, pre versus post; 2, p<0.05 (bisphosphonate group significantly different from pre-arED); 3, p<0.05 (bisphosphonate group significantly different from arED). Pre-arED (n=18); arED (n=11); bisphosphonate (n=7).

From Leblanc A, Matsumoto T, Jones J, et al. Bisphosphonates as a supplement to exercise to protect bone during long-duration spaceflight. Osteoporosis International 2013 Jul; 24(7): 2105–14.

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spaceflight with a mean of 5.5 months’ duration. Compared with once-standard countermeasures, bone is better pre-served with heavy resistive exercise, with further preservation when bisphosphonates are added. Material

served with heavy resistive exercise, with further preservation when bisphosphonates are added. e versus post; 2, p<0.05 (bisphosphonate group significantly different from pre-

Material

e versus post; 2, p<0.05 (bisphosphonate group significantly different from pre-sphosphonate group significantly different from

Material

sphosphonate group significantly different from

From Leblanc A, Matsumoto T, Jones J, et al. Bisphosphonates as a supplement to exercise to protect bone during

Material From Leblanc A, Matsumoto T, Jones J, et al. Bisphosphonates as a supplement to exercise to protect bone during

Osteoporosis International

Material Osteoporosis International 2013 Jul; 24(7): 2105–14.

Material 2013 Jul; 24(7): 2105–14.Osteoporosis International 2013 Jul; 24(7): 2105–14.Osteoporosis International

Material Osteoporosis International 2013 Jul; 24(7): 2105–14.Osteoporosis International

- a result, seated height and standing height are increased; - a result, seated height and standing height are increased; seated height may increase by as much as 6 per cent. Another - seated height may increase by as much as 6 per cent. Another Taylor

a result, seated height and standing height are increased; Taylor

a result, seated height and standing height are increased; seated height may increase by as much as 6 per cent. Another Taylor

seated height may increase by as much as 6 per cent. Another measureable change is the typical biphasic decrease in lower Taylor

measureable change is the typical biphasic decrease in lower extremity diameter. The fluid shift and redistribution of the

Taylor extremity diameter. The fluid shift and redistribution of the first few days in weightlessness cause a loss of volume from

Taylor first few days in weightlessness cause a loss of volume from diminished venous capacitance and interstitial fluid from

Taylor diminished venous capacitance and interstitial fluid from leg musculature, which is complete within seven to ten days.

Taylor leg musculature, which is complete within seven to ten days. Thereafter a further gradual decline may occur, dependent

Taylor Thereafter a further gradual decline may occur, dependent on exercise-supported maintenance of postural muscula

Taylor on exercise-supported maintenance of postural muscula& leg musculature, which is complete within seven to ten days. & leg musculature, which is complete within seven to ten days. Thereafter a further gradual decline may occur, dependent & Thereafter a further gradual decline may occur, dependent on exercise-supported maintenance of postural muscula& on exercise-supported maintenance of postural musculature in the calf and thigh, and measurement of calf volume

& ture in the calf and thigh, and measurement of calf volume Francis

ture in the calf and thigh, and measurement of calf volume Francis

ture in the calf and thigh, and measurement of calf volume has long served as a surrogate inflight muscle mass assessFrancis

has long served as a surrogate inflight muscle mass assessment. This has been attenuated somewhat with the recent

Francisment. This has been attenuated somewhat with the recent introduction of heavy resistive exercise.

Francisintroduction of heavy resistive exercise.

Periodic assessment of body mass has proven to be a use

FrancisPeriodic assessment of body mass has proven to be a use

ful measure of health and nutrition, and is typically per

Francisful measure of health and nutrition, and is typically performed on a monthly basis during long-duration flight.

Francisformed on a monthly basis during long-duration flight. Undue ‘weight’ loss is an indication of inadequate energy

FrancisUndue ‘weight’ loss is an indication of inadequate energy