Effect of spaceflight on the human body facts for kids

This page is about the medical consequences of spaceflight on humans. For the general study, see Bioastronautics.

American astronaut Marsha Ivins shows how her hair floats in space because of microgravity.

Have you ever wondered what happens to your body when you go to space? Space travel is super exciting, but it also has some big effects on astronauts. These effects can be both short-term (while they are in space) and long-term (even after they return to Earth).

One of the biggest challenges is weightlessness, also called microgravity. This can make muscles weaker and bones thinner, similar to osteoporosis. Other issues include changes to the heart and blood vessels, fewer red blood cells (called space anemia), problems with balance, blurry eyesight, and changes in the immune system.

Astronauts might also get a "moon face" because fluids shift to their upper body. They can lose weight, get stuffy noses, have trouble sleeping, and even experience more flatulence. Scientists are still learning about these changes, especially with more private space travel happening.

NASA uses the acronym RIDGE to describe the main challenges of spaceflight for humans:

Radiation from space
Isolation and being confined
Distance from Earth
Gravity fields (or lack of them)
Hostile and closed Environments

For a long time, scientists focused on building rockets and getting to space. Now, a huge challenge is figuring out how humans can live and work in space for a long time. Understanding how space affects the human body is a key step to exploring further, like going to Mars.

In 2019, NASA shared results from the Astronaut Twin Study. One astronaut twin stayed on the International Space Station for a year, while the other stayed on Earth. They found several lasting changes in the space twin, even in their DNA and thinking skills. Also in 2019, researchers found that astronauts can have serious blood flow and blood clot problems in space. This is important for future long trips, like to Mars.

How Space Affects the Body

Humans are used to living on Earth. Space is very different! Spaceships and spacesuits protect astronauts from the worst conditions. A life support system provides air, water, and food. It also keeps the temperature and pressure right and handles waste. Protection from radiation and tiny space rocks is also needed.

Some challenges are hard to avoid, like weightlessness. Living in microgravity affects the body in three main ways:

Losing your sense of where your body is in space (proprioception).
Changes in how fluids move around your body.
Weakening of your muscles and bones.

In 2017, scientists found that astronauts' brains changed shape and position after space trips. Longer trips caused bigger brain changes. In 2018, NASA-funded research suggested that long trips, like to Mars, might damage astronauts' digestive systems. This supports earlier findings that space travel could harm brains and make astronauts age faster. In 2019, NASA reported that hidden viruses in humans might become active during space missions. This could add more risks for astronauts on deep-space missions.

Space Medicine Research

Space medicine is a special type of medicine that studies astronauts' health in space. Its goal is to find out how long people can survive in space's extreme conditions. It also looks at how fast they can get used to Earth again after returning. Space medicine also tries to find ways to prevent or treat problems caused by living in space.

Takeoff and Landing

During takeoff and landing, astronauts feel much heavier than normal. This is called G-force. An untrained person might pass out at 4 to 6 times Earth's gravity. It's harder to handle G-forces that push blood away from your brain and eyes. People might first lose their vision temporarily, then pass out. Special training and a G-suit (which squeezes the body to keep blood in the head) can help. Spacecraft are designed to keep G-forces safe for astronauts.

Dangers of Space

Space is deadly without protection. The biggest dangers are the lack of oxygen and pressure. Temperature and radiation are also risks. Being exposed to space can cause problems like bubbles in body fluids (ebullism), lack of oxygen (hypoxia), and decompression sickness. High-energy radiation can also damage cells.

Decompression is a big worry during spacewalks (EVAs). Spacesuits are designed to handle this. A challenge is making suits that allow astronauts to move easily but also keep them safe from decompression.

Vacuum

This 1768 painting from 1768 shows an experiment by Robert Boyle in 1660. He tested what happens to a living bird in a vacuum.

Our bodies are made for Earth's atmosphere, which has oxygen. In the vacuum of space, your lungs would lose all gases, including oxygen, from your blood. After about 9 to 12 seconds, your brain wouldn't get enough oxygen, and you would pass out. Being in a vacuum for up to 30 seconds probably won't cause lasting harm. Animal tests show that quick recovery is normal for exposures shorter than 90 seconds. Longer exposures are usually deadly.

In 1966, NASA engineer Jim LeBlanc was testing a spacesuit. His hose came off, and his suit lost pressure quickly. He stayed awake for about 14 seconds. He felt the saliva on his tongue start to bubble before he passed out. Colleagues quickly helped him, and he recovered with no lasting damage.

Another effect of vacuum is ebullism. This is when bubbles form in body fluids because of the low pressure. Your body might swell up to twice its normal size, but your tissues are stretchy enough not to burst. This happens at about 19 kilometers (12 miles) above Earth. Experiments show that freezing of body fluids can happen. Serious symptoms like lack of oxygen in tissues and heart failure would happen in about 30 seconds. Your lungs would also collapse. It's estimated that a human has about 90 seconds to be repressurized before death might be unavoidable. Special flight suits can prevent ebullism.

The only humans known to have died from vacuum exposure in space were the three crew members of the Soyuz 11 spacecraft in 1971. A valve opened unexpectedly, causing rapid depressurization and the death of the crew.

Temperature

In space, there's no air to transfer heat. So, your body loses heat slowly through radiation. You wouldn't freeze instantly, especially if you're wearing clothes. Moisture on your skin might freeze quickly, but it's not a big danger.

Direct sunlight in space is very strong. It could cause severe sunburn.

Radiation

This chart compares radiation doses. It includes the amount found on the trip from Earth to Mars by the RAD on the MSL (2011–2013).

Without Earth's atmosphere and magnetosphere, astronauts face high levels of radiation. High radiation can damage cells that help your immune system. This makes astronauts' immunity weaker. Radiation has also been linked to more cataracts (cloudy eyes) in astronauts. Beyond low Earth orbit, galactic cosmic rays increase the risk of cancer over many years. A NASA study suggested radiation might harm astronauts' brains and speed up Alzheimer's disease. Rare but powerful solar flares can give a deadly radiation dose in minutes. Scientists hope that shielding and medicines can lower these risks.

Astronauts on the International Space Station (ISS) get some protection from Earth's magnetic field. But strong solar flares can still get through. In 2005, the crew of Expedition 10 took shelter in a more protected part of the station during a solar flare. Far from Earth's magnetic field, like on a trip to Mars, astronauts are much more vulnerable.

Scientists worry that long spaceflights might weaken the body's ability to fight diseases. Radiation can damage the cells that make blood and immune systems. This weakens the immune system, making astronauts more likely to get sick. It also means that viruses already in their bodies, which are normally controlled, can become active. In space, certain immune cells (T-cells) don't work as well. This can lead to infections spreading quickly among crew members in the small space of a spacecraft.

In 2013, NASA scientists reported that a trip to Mars could involve a big radiation risk. This was based on measurements from the RAD on the Mars Science Laboratory during its trip to Mars in 2011–2012. In 2017, NASA reported that radiation levels on Mars' surface temporarily doubled. This happened during a huge, unexpected solar storm.

Weightlessness

Astronauts on the ISS in weightlessness. Michael Foale is exercising in the front.

Living in weightlessness for a long time has harmful effects on human health. Humans are made for Earth's gravity. So, in space, many body systems start to change and even weaken. These changes are usually temporary, but some can have long-term effects on health.

Short periods of microgravity can cause space adaptation syndrome. This is like motion sickness and causes nausea. Long-term exposure causes many health problems. A big one is losing bone and muscle mass. Over time, these changes can make astronauts perform worse, increase their risk of injury, and slow down their heart and blood vessels.

Our bodies are mostly fluid. On Earth, gravity pulls fluids to the lower body. In space, this pull is gone, so fluids shift to the upper body. This causes the "puffy face" seen in astronauts. This fluid shift can also cause balance problems, blurry vision, and a loss of taste and smell.

A 2006 Space Shuttle experiment found that Salmonella typhimurium, a bacteria that causes food poisoning, became stronger when grown in space. In 2013, NASA-funded scientists reported that microbes in space seem to adapt in ways "not observed on Earth." In 2017, bacteria were found to be more resistant to antibiotics and grow better in near-weightlessness. Microorganisms have even survived the vacuum of space.

Space Sickness

Bruce McCandless II floating freely in orbit with a space suit and Manned Maneuvering Unit.

The most common problem for astronauts in the first few hours of weightlessness is space adaptation syndrome (SAS), or space sickness. It's like motion sickness. It happens as the inner ear (which controls balance) adjusts to weightlessness. Symptoms include nausea and vomiting, dizziness, headaches, tiredness, and feeling unwell. The first case was reported by cosmonaut Gherman Titov in 1961. About 45% of all people who have flown in space have had it.

Weak Bones and Muscles

On the International Space Station, astronaut Frank De Winne uses bungee cords to attach himself to the COLBERT treadmill.

A major effect of long-term weightlessness is losing bone and muscle mass. In space, astronauts don't put much weight on their back or leg muscles. These muscles then get weaker and smaller. Astronauts can lose up to 20% of their muscle mass in just 5 to 11 days without regular exercise.

Bone health also changes. On Earth, bones are constantly being broken down and rebuilt. In microgravity, there's less stress on bones, so they break down faster than they rebuild. Astronauts can lose about 1.5% of their bone tissue each month, especially from the spine, hips, and thigh bones. This rapid bone loss makes bones fragile, similar to osteoporosis. The body reabsorbs minerals from the broken-down bone, which can lead to high calcium levels in the blood. This can cause dangerous calcification of soft tissues and kidney stones.

It's not fully known if bones recover completely. Unlike people with osteoporosis, astronauts usually regain their bone density. After a 3–4 month trip, it takes about 2–3 years to get back lost bone density. Scientists are developing new ways to help astronauts recover faster. Diet, exercise, and medication might help grow new bone.

To fight these effects, the ISS has two treadmills and a special exercise device (aRED). Astronauts spend at least two hours a day exercising. They use bungee cords to strap themselves to the treadmill. Astronauts on long missions also wear special pants with elastic bands to compress their leg bones and reduce bone loss.

NASA is using computer tools to figure out the best exercise plans for astronauts. They are trying to understand how the ARED device affects muscles and bones. The goal is to create exercise routines that keep astronauts' bones and muscles healthy in space.

Fluid Shift

This image shows how fluids shift in the body in microgravity (it's a bit exaggerated).

Astronaut Clayton Anderson watches a water bubble float in front of him on the Space Shuttle Discovery. Water's stickiness (cohesion) is more important in microgravity than on Earth.

In space, astronauts lose fluid volume, including up to 22% of their blood. When they return to Earth, this low blood volume can make them dizzy when standing up. On Earth, gravity pulls blood to the lower body. In microgravity, this pull is gone, so blood shifts to the upper body. This causes a puffy face and other side effects. When astronauts return to Earth, their bodies have to readjust to gravity, which can cause dizziness. Drinking fluids before landing helps a lot.

Changes to Senses

Vision

In 2013, NASA published a study showing changes to the eyes and eyesight of monkeys after spaceflights longer than 6 months. Their eyeballs flattened, and their retinas changed. Astronauts' eyesight can become blurry after too much time in space. Another effect is seeing flashes of light caused by cosmic rays.

[a] NASA survey of 300 male and female astronauts, about 23 percent of short-flight and 49 percent of long-flight astronauts said they had experienced problems with both near and distance vision during their missions. Again, for some people vision problems persisted for years afterward.

In zero gravity, tiny pieces of dead skin or metal can float into the eye, causing irritation and increasing the risk of infection. Long spaceflights can also change how astronauts' eyes move.

Pressure in the Head

Because weightlessness increases fluid in the upper body, some scientists think astronauts might have higher pressure inside their heads. This could put pressure on the back of the eyeballs and slightly squeeze the optic nerve. This was seen in 2012 in MRI scans of astronauts after at least one month in space. However, direct proof of high pressure in the head in microgravity is still being sought. Some studies have shown that pressure in the head might actually be lower than when lying down, and only slightly higher than when sitting. This means the pressure might be within normal limits.

If high pressure in the head is the cause, artificial gravity might be a solution. This would also help with many other health risks in space. However, artificial gravity systems are still being developed. Even with artificial gravity, some level of microgravity might remain, and its risks are unknown.

Taste

Some astronauts report that their sense of taste changes in space. Some find food bland, while others dislike foods they usually love. Some enjoy foods they wouldn't normally eat, and some notice no change. Scientists haven't found a clear reason. Theories include food going bad or psychological changes like boredom. Astronauts often choose strong-tasting foods to help with the loss of taste.

Other Body Effects

In weightlessness, the human spine fully extends, making astronauts grow about an inch taller. After two months, the calluses on the bottom of their feet fall off because they aren't used. The tops of their feet can become sore from rubbing against handrails used for stability. Tears can't fall when crying; they stick together in a ball. In microgravity, smells spread quickly. NASA found that the smell of cream sherry triggered a gag reflex in one test. Other discomforts like back and stomach pain are common when returning to Earth. This is because the body readjusts to gravity after these muscles could stretch freely in space.

Mind Effects

Studies of Russian cosmonauts, like those on Mir, give scientists information about the long-term effects of space on the human body.

Research

The mental effects of living in space are not fully understood. But we can compare them to situations on Earth, like living in Arctic research stations or submarines. The huge stress on the crew, along with their bodies adapting to new environments, can cause anxiety, trouble sleeping, and depression.

Stress

There is strong evidence that stress is a major problem for astronauts' morale and performance. NASA first studied psychological stress when their crewed missions began. It became a focus again when American astronauts joined Russian cosmonauts on the Mir space station. Early American missions caused stress from being watched by the public and being away from family. On the ISS, being isolated from family is still a common cause of stress. For example, astronaut Daniel Tani's mother died in a car accident while he was in space.

Sleep

Astronauts often don't get enough good sleep in space. This is because of changing light and dark cycles on the spacecraft and poor lighting during the day. Even looking out the window before bed can confuse the brain, leading to bad sleep patterns. These sleep problems affect how astronauts think and act. They also make the psychological stress worse. Sleep on the ISS is often interrupted by mission tasks, like when new spacecraft arrive or depart. The station is also noisy because fans must run constantly to move air around in zero gravity. Fifty percent of Space Shuttle astronauts took sleeping pills but still got two hours less sleep each night in space than on Earth. NASA is researching ways to improve sleep, as better sleep reduces tiredness and increases productivity.

Length of Space Travel

A study of the longest spaceflight found that the first three weeks are a critical time. Astronauts' attention is negatively affected as they adjust to the extreme changes. Skylab's crews stayed in space for 1, 2, and 3 months. Long-term crews on Salyut 6, Salyut 7, and the ISS stay for about 5–6 months. MIR expeditions often lasted even longer. The ISS working environment adds more stress. Astronauts live and work in cramped spaces with people from different cultures who speak different languages. Earlier space stations had crews who spoke one language. The ISS has crews from many cultures and languages.

Future of Space Travel

Humans have spent a total of 58 years in space. This has given us a much better understanding of how the body adapts. In the future, as we build things in space and explore other planets, humans will need to spend longer and longer periods in space. Most of our current information comes from short missions. So, some long-term effects of living in space are still unknown. A round trip to Mars with today's technology is estimated to take at least 18 months just for travel. Knowing how the human body reacts to such long times in space is very important for these journeys.

Spacecraft will need good medical facilities. They will need to handle any injuries or emergencies. They will also need many diagnostic and medical tools to keep the crew healthy for a long time. These will be the only medical facilities available.

Right now, only very carefully tested humans have gone to space. If people start living off-world someday, many different types of people will face these dangers. The effects on very young children are completely unknown. In 1998, John Glenn, one of the first American astronauts, returned to space at age 77. His 9-day flight gave NASA important information about how space affects older people. Things like food needs and physical environments that haven't been studied yet will become important. Overall, we still have limited information about all the effects of living in space. This makes it hard to reduce the risks during long stays in space. Testbeds like the ISS are being used to research some of these risks.

The space environment is still largely unknown, and there will likely be new dangers we don't know about yet. However, future technologies like artificial gravity and more complex life support systems might someday help reduce some of these risks.