Many of the
environmental conditions experienced by humans during
spaceflight are very different from those in which humans evolved; however, technology such as that offered by a
spaceship or
spacesuit is able to shield people from the harshest conditions. The immediate needs for breathable air and drinkable water are addressed by a
life support system, a group of devices that allow human beings to survive in outer space. The life support system supplies
air,
water and
food. It must also maintain temperature and pressure within acceptable limits and
deal with the body's waste products. Shielding against harmful external influences such as radiation and micro-meteorites is also necessary. Some hazards are difficult to mitigate, such as weightlessness, also defined as a
microgravity environment. Living in this type of environment impacts the body in three important ways: loss of
proprioception, changes in fluid distribution, and deterioration of the
musculoskeletal system. On November 2, 2017, scientists reported that significant changes in the position and structure of the
brain have been found in
astronauts who have taken
trips in space, based on
MRI studies. Astronauts who took longer space trips were associated with greater brain changes. In October 2018,
NASA-funded researchers found that lengthy journeys into
outer space, including travel to the
planet Mars, may substantially damage the
gastrointestinal tissues of astronauts. The studies support earlier work that found such journeys could significantly damage the brains of
astronauts, and age them prematurely.
Research Space medicine is a developing
medical practice that studies the
health of astronauts living in outer space. The main purpose of this academic pursuit is to discover how well and for how long people can survive the extreme conditions in space, and how fast they can re-adapt to the Earth's environment after returning from space. Space medicine also seeks to develop
preventive and
palliative measures to ease the suffering caused by living in an environment to which humans are not well adapted.
Ascent and re-entry During takeoff and re-entry, space travelers can experience several times normal gravity. An untrained person can usually withstand about 3g, but can black out at 4 to 6g.
G-force in the vertical direction is more difficult to tolerate than a force perpendicular to the spine because blood flows away from the brain and eyes. First the person experiences a temporary loss of vision and then at higher g-forces loses consciousness. G-force training and a
G-suit which constricts the body to keep more blood in the head can mitigate the effects. Most spacecraft are designed to keep g-forces within comfortable limits.
Space environments The environment of space is lethal without appropriate protection: the greatest threat in the vacuum of space derives from the lack of oxygen and pressure, although temperature and radiation also pose risks. The effects of space exposure can result in
ebullism,
hypoxia,
hypocapnia, and
decompression sickness. In addition to these, there is also
cellular mutation and
destruction from high energy
photons and
sub-atomic particles that are present in the surroundings. Decompression is a serious concern during the
extra-vehicular activities (EVAs) of astronauts. Current Extravehicular Mobility Unit (EMU) designs take this and other issues into consideration, and have evolved over time. A key challenge has been the competing interests of increasing astronaut mobility (which is reduced by high-pressure
EMUs, analogous to the difficulty of deforming an inflated balloon relative to a deflated one) and minimising
decompression risk. Investigators have considered pressurizing a separate head unit to the regular 71 kPa (10.3 psi) cabin pressure as opposed to the current whole-EMU pressure of . In such a design, pressurization of the torso could be achieved mechanically, avoiding mobility reduction associated with pneumatic pressurization. Exposure to vacuum for up to 30 seconds is unlikely to cause permanent physical damage. Animal experiments show that rapid and complete recovery is normal for exposures shorter than 90 seconds, while longer full-body exposures are fatal and resuscitation has never been successful. There is only a limited amount of data available from human accidents, but it is consistent with animal data. Limbs may be exposed for much longer if breathing is not impaired. In December 1966,
aerospace engineer and test subject Jim LeBlanc of
NASA was participating in a test to see how well a pressurized
space suit prototype would perform in vacuum conditions. To simulate the effects of space, NASA constructed a massive
vacuum chamber from which all air could be pumped. At some point during the test, LeBlanc's pressurization hose became detached from the space suit. Even though this caused his suit pressure to drop from to in less than 10 seconds, LeBlanc remained conscious for about 14 seconds before
losing consciousness due to hypoxia; the much lower pressure outside the body causes rapid de-oxygenation of the blood. "As I stumbled backwards, I could feel the saliva on my tongue starting to bubble just before I went unconscious and that's the last thing I remember", recalls LeBlanc. A colleague entered the chamber within 25 seconds and gave LeBlanc oxygen. The chamber was repressurized in 1 minute instead of the normal 30 minutes. LeBlanc recovered almost immediately with just an earache and no permanent damage. Another effect from a vacuum is a condition called
ebullism which results from the formation of bubbles in body fluids due to reduced ambient pressure. The steam may bloat the body up to twice its normal size and slow down circulation, but
tissues are elastic and porous enough to prevent rupture. Technically, ebullism is considered to begin at an elevation of around or pressures less than 6.3
kPa (47
mm Hg), known as the
Armstrong limit. Swelling from ebullism can be reduced by containment in a
flight suit which is necessary to prevent ebullism above 19 km. The only humans known to have died of exposure to vacuum in space are the three crew-members of the
Soyuz 11 spacecraft;
Vladislav Volkov,
Georgi Dobrovolski, and
Viktor Patsayev. During preparations for re-entry from orbit on June 30, 1971, a pressure-equalisation valve in the spacecraft's
descent module unexpectedly opened at an altitude of , causing rapid depressurisation and the subsequent death of the entire crew.
Temperature In a vacuum, there is no medium for removing heat from the body by conduction or convection. Loss of heat is by radiation from the 310 K temperature of a person to the 3 K of outer space. This is a slow process, especially in a clothed person, so there is no danger of immediately freezing. Rapid evaporative cooling of skin moisture in a vacuum may create frost, particularly in the mouth, but this is not a significant hazard. Exposure to the intense
radiation of direct, unfiltered
sunlight would lead to local heating, though that would likely be well distributed by the body's conductivity and blood circulation. Other solar radiation, particularly
ultraviolet rays, however, may cause severe sunburn.
Radiation on the
MSL (2011–2013). Without the protection of Earth's
atmosphere and
magnetosphere astronauts are exposed to high levels of
radiation. High levels of radiation damage
lymphocytes, cells heavily involved in maintaining the
immune system; this damage contributes to the lowered
immunity experienced by astronauts. Radiation has also recently been linked to a higher incidence of
cataracts in astronauts. Outside the protection of low Earth orbit,
galactic cosmic rays present further challenges to human spaceflight, as the
health threat from cosmic rays significantly increases the chances of cancer over a decade or more of exposure. A
NASA-supported study reported that radiation may harm the
brain of
astronauts and accelerate the onset of
Alzheimer's disease.
Solar flare events (though rare) can give a fatal radiation dose in minutes. It is thought that protective shielding and protective drugs may ultimately lower the risks to an acceptable level. Crew living on the
International Space Station (ISS) are partially protected from the space environment by Earth's magnetic field, as the
magnetosphere deflects
solar wind around the Earth and the ISS. Nevertheless, solar flares are powerful enough to warp and penetrate the magnetic defences, and so are still a hazard to the crew. The crew of
Expedition 10 took shelter as a precaution in 2005 in a more heavily shielded part of the station designed for this purpose. However, beyond the limited protection of Earth's
magnetosphere, interplanetary human missions are much more vulnerable. Lawrence Townsend of the University of Tennessee and others have studied
the most powerful solar flare ever recorded. Radiation doses astronauts would receive from a flare of this magnitude could cause acute radiation sickness and possibly even death. , which is caused by high-energy particles in the space environment. There is scientific concern that extended spaceflight might slow down the body's ability to protect itself against diseases. Radiation can penetrate living tissue and cause both short and long-term damage to the bone marrow stem cells which create the blood and immune systems. In particular, it causes '
chromosomal aberrations' in
lymphocytes. As these cells are central to the
immune system, any damage weakens the immune system, which means that in addition to increased vulnerability to new exposures,
viruses already present in the body—which would normally be suppressed—become active. In space,
T-cells (a form of lymphocyte) are less able to reproduce properly, and the T-cells that do reproduce are less able to fight off infection. Over time immunodeficiency results in the rapid spread of infection among crew members, especially in the confined areas of space flight systems. On 31 May 2013, NASA scientists reported that a possible
human mission to Mars may involve a great
radiation risk based on the amount of
energetic particle radiation detected by the
RAD on the
Mars Science Laboratory while traveling from the
Earth to
Mars in 2011–2012.
Weightlessness in weightless conditions.
Michael Foale can be seen exercising in the foreground. Following the advent of
space stations that can be inhabited for long periods of time, exposure to
weightlessness has been demonstrated to have some deleterious effects on human health. Humans are well-adapted to the physical conditions at the surface of the Earth, and so in response to weightlessness, various
physiological systems begin to change, and in some cases,
atrophy. Though these changes are usually temporary, some do have a long-term impact on human
health. Short-term exposure to microgravity causes
space adaptation syndrome, self-limiting nausea caused by derangement of the
vestibular system. Long-term exposure causes multiple health problems, one of the most significant being loss of bone and muscle mass. Over time these
deconditioning effects can impair astronauts' performance, increase their risk of injury, reduce their
aerobic capacity, and slow down their
cardiovascular system. As the human body consists mostly of fluids, gravity tends to force them into the lower half of the body, and our bodies have many systems to balance this situation. When released from the pull of gravity, these systems continue to work, causing a general redistribution of fluids into the upper half of the body. This is the cause of the round-faced 'puffiness' seen in astronauts, Redistributing fluids around the body itself causes balance disorders,
distorted vision, and a loss of taste and smell. A 2006 Space Shuttle experiment found that
Salmonella typhimurium, a bacterium that can cause
food poisoning, became more virulent when cultivated in space. On April 29, 2013, scientists in
Rensselaer Polytechnic Institute, funded by
NASA, reported that, during
spaceflight on the
International Space Station,
microbes seem to adapt to the
space environment in ways "not observed on Earth" and in ways that "can lead to increases in growth and
virulence". In 2017,
bacteria were found to be more resistant to
antibiotics and to thrive in the near-weightlessness of space.
Microorganisms have been observed to survive the
vacuum of outer space.
Motion sickness floating free in orbit with a
space suit and
Manned Maneuvering Unit. The most common problem experienced by humans in the initial hours of weightlessness is known as
space adaptation syndrome or SAS, commonly referred to as space sickness. It is related to
motion sickness, and arises as the
vestibular system adapts to weightlessness. Symptoms of SAS include
nausea and
vomiting,
vertigo,
headaches,
lethargy, and overall malaise. The types of
muscle fibre prominent in muscles also change. Slow-twitch endurance fibres used to maintain posture are replaced by fast-twitch rapidly contracting fibres that are insufficient for any heavy labour. Advances in research on exercise, hormone supplements, and medication may help maintain muscle and body mass.
Bone metabolism also changes. Normally, bone is laid down in the direction of mechanical stress. However, in a microgravity environment, there is very little mechanical stress. This results in a
loss of bone tissue approximately 1.5% per month especially from the lower vertebrae, hip, and femur. Due to microgravity and the decreased load on the bones, there is a rapid increase in bone loss, from 3% cortical bone loss per decade to about 1% every month the body is exposed to microgravity, for an otherwise healthy adult. The rapid change in bone density is dramatic, making bones frail and resulting in symptoms that resemble those of osteoporosis. On Earth, the bones are constantly being shed and regenerated through a well-balanced system which involves signaling of osteoblasts and osteoclasts. These systems are coupled, so that whenever bone is broken down, newly formed layers take its place—neither should happen without the other, in a healthy adult. In space, however, there is an increase in osteoclast activity due to microgravity. This is a problem because osteoclasts break down the bones into minerals that are reabsorbed by the body. Osteoblasts are not consecutively active with the osteoclasts, causing the bone to be constantly diminished with no recovery. This increase in osteoclasts activity has been seen particularly in the pelvic region because this is the region that carries the biggest load with gravity present. A study demonstrated that in healthy mice, osteoclasts appearance increased by 197%, accompanied by a down-regulation of osteoblasts and growth factors that are known to help with the formation of new bone, after only sixteen days of exposure to microgravity. Elevated blood
calcium levels from the lost bone result in dangerous calcification of soft tissues and potential
kidney stone formation. and a stationary bicycle; each astronaut spends at least two hours per day exercising on the equipment. Astronauts use bungee cords to strap themselves to the treadmill. Astronauts subject to long periods of weightlessness wear pants with elastic bands attached between waistband and cuffs to compress the leg bones and reduce osteopenia. The Human Research Program's Human Health Countermeasures Element chartered the Digital Astronaut Project to investigate targeted questions about exercise countermeasure regimes. NASA is focusing on integrating a model of the advanced Resistive Exercise Device (ARED) currently on board the
International Space Station with
OpenSim musculoskeletal models of humans exercising with the device. The goal of this work is to use inverse dynamics to estimate joint torques and muscle forces resulting from using the ARED, and thus more accurately prescribe exercise regimens for the astronauts. These joint torques and muscle forces could be used in conjunction with more fundamental computational simulations of bone remodeling and muscle adaptation in order to more completely model the end effects of such countermeasures, and determine whether a proposed exercise regime would be sufficient to sustain astronaut musculoskeletal health.
Fluid redistribution observes as a water bubble floats in front of him on the
Space Shuttle Discovery. Water
cohesion plays a bigger role in microgravity than on Earth In space, astronauts lose fluid volume—including up to 22% of their blood volume. When the astronauts return to Earth, low blood volume can cause orthostatic intolerance or dizziness when standing. Under the influence of the earth's
gravity, when a person is standing, blood and other body fluids are pulled towards the lower body, increasing pressure at the feet. In a microgravity environment, hydrostatic pressures throughout the body are removed and the resulting change in blood distribution is analogous to an individual changing from standing up to lying down. The persistent change in the redistribution of blood volume may result in facial
edema and other unwelcome side effects. Upon return to Earth, the reduced blood volume creates
orthostatic hypotension. Orthostatic tolerance after spaceflight has been greatly improved by fluid loading countermeasures taken by astronauts before touchdown.
Disruption of senses Vision In 2013 NASA published a study that found changes to the eyes and eyesight of monkeys with spaceflights longer than 6 months. Noted changes included a flattening of the eyeball and changes to the retina. Another effect is known as
cosmic ray visual phenomena. Since dust can not settle in zero gravity, small pieces of dead skin or metal can get in the eye, causing irritation and increasing the risk of infection. Long spaceflights can also alter a space traveler's eye movements (particularly the
vestibulo-ocular reflex).
Intracranial pressure Because weightlessness increases the amount of fluid in the upper part of the body, it has been hypothesized that astronauts experience pathologically elevated
intracranial pressure. This would increase pressure on the backs of the eyeballs, affecting their shape and slightly crushing the
optic nerve. This was noticed in 2012 in a study using
MRI scans of astronauts who had returned to Earth following at least one month in space. However, direct evidence of pathologically elevated intracranial pressures in microgravity has yet to be obtained. Invasive measures of intracranial pressure on parabolic flights showed that pressures were actually reduced relative to supine levels and slightly higher than seated levels, meaning pressures were within normal physiological variation. Without elevated intracranial pressures, a force that flattens the posterior of the eye is still created by the removal of hydrostatic gradients in the intracranial and intraocular spaces. Such eyesight problems could be a major concern for future deep space flight missions, including a
crewed mission to the planet
Mars. If indeed elevated intracranial pressure is the cause, artificial gravity might present one solution, as it would for many human health risks in space. However, such artificial gravitational systems have yet to be proven. More, even with sophisticated artificial gravity, a state of relative microgravity may remain, the risks of which remain unknown.
Taste One effect of weightlessness on humans is that some astronauts report a change in their sense of
taste when in space. Some astronauts find that their food is bland, others find that their favorite foods no longer taste as good (one who enjoyed coffee disliked the taste so much on a mission that he stopped drinking it after returning to Earth); some astronauts enjoy eating certain foods that they would not normally eat, and some experience no change whatsoever. Multiple tests have not identified the cause, and several theories have been suggested, including food degradation, and psychological changes such as boredom. Astronauts often choose strong-tasting food to combat the loss of taste.
Additional physiological effects Within one month the human skeleton fully extends in weightlessness, causing height to increase by . After two months, calluses on the bottoms of feet
molt and fall off from lack of use, leaving soft new skin. Tops of feet become, by contrast, raw and painfully sensitive, as they rub against the handrails feet are hooked into for stability. Tears cannot be shed while crying, as they stick together into a ball. In microgravity odors quickly permeate the environment, and NASA found in a test that the smell of
cream sherry triggered the gag reflex. Various other physical discomforts such as back and abdominal pain are common because of the readjustment to gravity, where in space there was no gravity and these muscles could freely stretch. These may be part of the
asthenization syndrome reported by
cosmonauts living in space over an extended period of time, but regarded as anecdotal by astronauts. Fatigue, listlessness, and psychosomatic worries are also part of the syndrome. The data is inconclusive; however, the syndrome does appear to exist as a manifestation of the internal and external stress crews in space must face. ==Psychological effects==