Effect of spaceflight on the human body
The effects of spaceflight on the human body are complex and largely harmful over both short and long term. Significant adverse effects of long-term weightlessness include muscle atrophy and deterioration of the skeleton. Other significant effects include a slowing of cardiovascular system functions, decreased production of red blood cells, balance disorders, eyesight disorders and changes in the immune system. Additional symptoms include fluid redistribution, loss of body mass, nasal congestion, sleep disturbance, and excess flatulence. A 2024 assessment noted that "well-known problems include bone loss, heightened cancer risk, vision impairment, weakened immune systems, and mental health issues... et what's going on at a molecular level hasn't always been clear", arousing concerns especially vis a vis private and commercial spaceflight now occurring without any scientific or medical research being conducted among those populations regarding effects.
Overall, NASA refers to the various deleterious effects of spaceflight on the human body by the acronym RIDGE.
The engineering problems associated with leaving Earth and developing space propulsion systems have been examined for more than a century, and millions of hours of research have been spent on them. In recent years, there has been an increase in research on the issue of how humans can survive and work in space for extended and possibly indefinite periods of time. This question requires input from the physical and biological sciences and has now become the greatest challenge facing human space exploration. A fundamental step in overcoming this challenge is trying to understand the effects of long-term space travel on the human body.
In October 2015, the NASA Office of Inspector General issued a health hazards report related to space exploration, including a human mission to Mars.
On 12 April 2019, NASA reported medical results from the Astronaut Twin Study, where one astronaut twin spent a year in space on the International Space Station, while the other spent the year on Earth, which demonstrated several long-lasting changes, including those related to alterations in DNA and cognition, after the twins were compared.
In November 2019, researchers reported that astronauts experienced serious blood flow and clot problems while on board the International Space Station, based on a six-month study of 11 healthy astronauts. The results may influence long-term spaceflight, including a mission to the planet Mars, according to the researchers.
Physiological effects
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.
In March 2019, NASA reported that latent viruses in humans may be activated during space missions, adding possibly more risk to astronauts in future deep-space missions.
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 of astronauts. Current Extravehicular Mobility Unit 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 and minimising decompression risk. Investigators have considered pressurizing a separate head unit to the regular 71 kPa 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.Vacuum
Human physiology is adapted to living within the atmosphere of Earth, and a certain amount of oxygen is required in the air we breathe. If the body does not get enough oxygen, then the astronaut is at risk of becoming unconscious and dying from hypoxia. In the vacuum of space, gas exchange in the lungs continues but results in the removal of all gases, including oxygen, from the bloodstream. After 9 to 12 seconds, the deoxygenated blood reaches the brain, and it results in the loss of consciousness. 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, known as the Armstrong limit. Experiments with other animals have revealed an array of symptoms that could also apply to humans. The least severe of these is the freezing of bodily secretions due to evaporative cooling. Severe symptoms, such as loss of oxygen in tissue, followed by circulatory failure and flaccid paralysis would occur in about 30 seconds. The lungs also collapse in this process, but will continue to release water vapour leading to cooling and ice formation in the respiratory tract. A rough estimate is that a human will have about 90 seconds to be recompressed, after which death may be unavoidable. Swelling from ebullism can be reduced by containment in a flight suit which is necessary to prevent ebullism above 19 km. During the Space Shuttle program astronauts wore a fitted elastic garment called a Crew Altitude Protection Suit which prevented ebullism at pressures as low as 2 kPa.
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.