Effect of spaceflight on the human body

• 1. Introduction

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  The effect of spaceflight on the human body is one of the most pressing medical questions of this century. Imagine returning from a year on the International Space Station to find that your blood volume has dropped by up to 22%, your bones have been quietly dissolving, and viruses your immune system once kept dormant have woken up. These are not science-fiction scenarios. They are documented findings from real astronauts.

  NASA packages these hazards under the acronym RIDGE: space radiation, isolation and confinement, distance from Earth, gravity fields, and hostile and closed environments. Each category opens its own chapter of human vulnerability. And in October 2015, the NASA Office of Inspector General issued a formal health hazards report on exactly these risks, aimed squarely at the prospect of a crewed mission to Mars.

  A 2024 assessment noted that what is happening at a molecular level has not always been clear, even as the bone loss, cancer risk, vision impairment, and mental health consequences are well established. That gap between what we can observe and what we can explain is what drives the field of space medicine forward. The questions this story will pursue are deceptively simple: what does space actually do to a human body, and how close are we to having answers good enough to send people to another planet?

• 2. The Vacuum and the Body

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  In December 1966, aerospace engineer Jim LeBlanc was inside a NASA vacuum chamber testing a pressurized spacesuit prototype when his pressurization hose detached from the suit. The suit pressure dropped from 3.8 psi to 0.1 psi in under ten seconds. LeBlanc remained conscious for roughly 14 seconds before losing consciousness from hypoxia. A colleague entered the chamber within 25 seconds and provided oxygen. The chamber was repressurized in one minute rather than the normal 30. LeBlanc recovered almost immediately, reporting only an earache. His own account of those final seconds: "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."

  That bubbling sensation is a glimpse of a condition called ebullism, the formation of bubbles in body fluids when ambient pressure drops. The process begins technically at an altitude of around 19 km, at pressures below 6.3 kPa. Swelling from ebullism can bloat the body to twice its normal size, but tissues are elastic enough to prevent rupture. A rough estimate gives a person about 90 seconds to be recompressed before death becomes unavoidable. During the Space Shuttle program, astronauts wore a fitted elastic garment called the Crew Altitude Protection Suit, which prevented ebullism at pressures as low as 2 kPa.

  The only people known to have died from exposure to vacuum in space are the three crewmembers of the Soyuz 11 spacecraft: Vladislav Volkov, Georgi Dobrovolski, and Viktor Patsayev. On the 30th of June 1971, during preparations for re-entry from orbit at an altitude of 168 km, a pressure-equalisation valve in the descent module opened unexpectedly. The rapid depressurisation killed all three. Their deaths remain the only confirmed human fatalities caused by vacuum exposure in space.

• 3. Radiation Without a Shield

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  Crew on the International Space Station receive partial protection from Earth's magnetic field, which deflects solar wind around the planet. Even so, solar flares are powerful enough to pierce those defences. The crew of Expedition 10 took shelter in 2005 in a more heavily shielded section of the station as a precaution during one such event.

  Beyond low Earth orbit, the situation is more serious. Lawrence Townsend of the University of Tennessee has studied the most powerful solar flare ever recorded and concluded that astronauts exposed to a flare of that magnitude could experience acute radiation sickness and possibly death. A NASA-supported study connected radiation to a higher incidence of cataracts and to potential acceleration of Alzheimer's disease onset.

  On the 31st of May 2013, NASA scientists reported on radiation data collected by the RAD instrument aboard the Mars Science Laboratory during its transit from Earth to Mars in 2011-2012. Their findings raised serious concerns about the radiation risk for any future human mission to Mars. Then in September 2017, NASA reported that radiation levels on the Martian surface temporarily doubled during a massive, unexpected solar storm, accompanied by an aurora 25 times brighter than any previously observed.

  At the cellular level, radiation causes chromosomal aberrations in lymphocytes, the cells central to the immune system. T-cells become less able to reproduce properly, and those that do reproduce fight infection less effectively. In a confined spacecraft, immunodeficiency accelerates the spread of illness among the crew. Beyond the immune system, radiation can penetrate the bone marrow stem cells that generate blood, with both short and long-term consequences that researchers are still working to quantify.

• 4. Bones, Muscles, and Microgravity

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  Without regular exercise in space, astronauts can lose up to 20% of their muscle mass in as little as 5 to 11 days. The muscle fibres that change first are the slow-twitch endurance fibres used to maintain posture, which are replaced by fast-twitch fibres inadequate for heavy work. This is not a gradual decline but a rapid structural shift.

  Bone loss proceeds at a different pace but is equally alarming. In a microgravity environment, bone tissue is lost at approximately 1.5% per month, concentrated in the lower vertebrae, hip, and femur. To put that in context, cortical bone loss on Earth happens at around 3% per decade. In space, the equivalent loss occurs every month. The mechanism involves osteoclasts, the cells that break bone down into minerals reabsorbed by the body. In microgravity, osteoclast activity increases sharply while the osteoblasts that rebuild bone fail to keep pace. One study found that in healthy mice, osteoclast presence increased by 197% after only sixteen days of microgravity exposure. The elevated blood calcium that results from dissolving bone creates additional risks, including the calcification of soft tissues and kidney stone formation.

  Unlike people with osteoporosis, astronauts do eventually regain their bone density after returning to Earth, but the process is slow. After a 3-4 month trip to space, recovery takes roughly 2-3 years. To slow the loss during missions, the ISS is equipped with two treadmills, including one called the COLBERT, an advanced Resistive Exercise Device, and a stationary bicycle. Each astronaut is expected to spend at least two hours per day on this equipment. Astronauts use bungee cords to strap themselves to the treadmill in zero gravity, and some wear pants with elastic bands attached between the waistband and cuffs to apply compression to leg bones.

• 5. The Fluid Body in Freefall

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  Fluid redistribution is one of the most visually obvious signs that the body is in space. On Earth, gravity pulls blood and other fluids toward the lower half of the body, and the circulatory system actively works to balance that. In microgravity, those balancing systems keep working, pushing fluids upward with nothing to counteract them. The result is the round-faced puffiness seen in photographs of astronauts in orbit, a condition researchers call facial edema.

  By some measures, astronauts lose up to 22% of their blood volume during spaceflight. When they return to Earth, that reduction can trigger orthostatic hypotension, a drop in blood pressure when standing upright that causes dizziness. Fluid loading countermeasures taken before landing have significantly improved orthostatic tolerance after return.

  In November 2019, researchers reported that a six-month study of 11 healthy astronauts aboard the ISS found serious blood flow and clotting problems. The findings have direct implications for long-duration missions, including a future trip to Mars. Fluid redistribution also appears to affect speech motor control, and it contributes to vision disorders by altering pressure on the backs of the eyeballs. A NASA survey of 300 male and female astronauts found that about 23% of short-flight astronauts and 49% of long-flight astronauts reported problems with both near and distance vision during their missions, with some reporting that problems persisted for years after returning to Earth.

• 6. The Mind Under Pressure

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  Astronaut Valery Ryumin, writing about the Salyut 6 mission, quoted a line from O. Henry's "The Handbook of Hymen": "If you want to instigate the art of manslaughter just shut two men up in an eighteen by twenty-foot cabin for a month. Human nature won't stand it." Ryumin offered this not as a joke but as a description of what confined spaceflight actually felt like.

  NASA identified psychological stress as a major concern from the beginning of crewed missions. Stressors in early American missions included maintaining peak performance under public scrutiny and isolation from family and peers. On the ISS, personal losses have made the isolation acute. NASA astronaut Daniel Tani's mother died in a car accident while he was on board, and Michael Fincke was forced to miss the birth of his second child.

  Sleep is a persistent problem. Fifty percent of Space Shuttle astronauts took sleeping pills and still got two hours less sleep each night in space than they did on the ground. Sound levels on the ISS are unavoidably high because fans must run continuously to circulate the atmosphere, which would stagnate in freefall. Light cycles on flight decks are highly variable, and even looking out a window before bed can send conflicting signals to the brain. On the 12th of April 2019, NASA reported results from the Astronaut Twin Study, in which one twin spent a year on the ISS while the other remained on Earth. The study documented lasting changes in DNA and cognition, adding molecular detail to what had previously been observed only in behavior and performance.

• 7. Toward Mars and Beyond

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  A round trip to Mars with current technology is estimated to require at least 18 months in transit alone, not counting time on the surface. That is far longer than most ISS missions, and most of the long-term physiological data we have comes from missions of relatively short duration.

  On the 29th of October 1998, John Glenn, one of the original Mercury 7 astronauts, returned to space at the age of 77. His nine-day mission provided NASA with a rare window into how spaceflight affects older bodies, a population almost entirely absent from the existing data. The broader gaps are even larger: no children have ever been to space, reproductive research is still in early stages with animal studies, and the effects on the very young are completely unknown. SpaceBorn United, a company aiming to enable human reproduction in space, was as of 2023 planning preliminary rat research in orbit.

  The cumulative sum of human time in space now stands at 58 solar years, which has produced a much better understanding of adaptation but has also clarified the scale of what remains unknown. In October 2018, NASA-funded researchers found that lengthy journeys into space may substantially damage the gastrointestinal tissues of astronauts, compounding earlier findings on brain aging. In March 2019, NASA reported that latent viruses in the human body may be activated during space missions, adding a further risk category to an already long list. The Digital Astronaut Project, using computational models integrated with the ARED exercise device and OpenSim musculoskeletal software, represents one effort to close that gap without waiting for the data that only a Mars mission could provide.