How the Human Body Changes in Space

Your body is built for life under gravity.

Your bones, muscles, heart, inner ear and even your eyes all depend on that steady downward pull.

Take it away for weeks or months, and the body starts to adapt in ways that range from mildly uncomfortable to seriously challenging.

Decades of research on astronauts have revealed what changes, why it happens and how crews cope.

Here is a system-by-system tour of the body in space, along with what happens when astronauts return home.

It describes general findings from spaceflight research, not medical guidance.

The First Days: Space Motion Sickness

Deep inside your ears, tiny structures filled with fluid and small crystals sense gravity and motion.

They tell your brain which way is down.

In weightlessness, those signals suddenly stop making sense, while your eyes may be telling your brain something completely different.

The result, for many astronauts, is space motion sickness: nausea, headaches, disorientation and sometimes vomiting during the first few days in orbit.

Most people adapt within a few days as the brain learns to rely more on vision and less on the inner ear.

Crews often move slowly and keep their heads still at first to ease the transition.

Fluids on the Move

On Earth, gravity pulls blood and other body fluids toward your feet.

In space, that pull disappears, and roughly two liters of fluid shift toward the upper body and head.

Astronauts develop puffy faces and thinner legs, a combination they sometimes call puffy face, bird legs.

Many feel stuffed up, as if they had a head cold, which also dulls the senses of taste and smell.

The body reads the extra fluid in the chest and head as a sign of too much fluid overall, so it gets rid of some.

Blood volume drops, and the heart, no longer working against gravity to push blood up to the brain, has an easier job.

Over time it can lose a little muscle and become slightly rounder in shape.

Bones and Muscles Without Gravity

Bones stay strong because they carry weight.

In weightlessness, the body starts breaking down bone faster than it rebuilds it, especially in the hips, spine and legs.

Without countermeasures, astronauts can lose about 1 to 1.5 percent of the mineral density in these weight-bearing bones for every month in space.

The calcium released from bone also raises the risk of kidney stones.

Muscles follow the same logic.

The large muscles in the legs and back, which normally keep you upright, have little to do when you float,

and they can lose mass and strength quickly.

That is why crews on long missions exercise for about two hours a day, using treadmills, stationary bikes and resistance machines that simulate weightlifting.

Exercise does not stop all the loss, but it greatly reduces it.

Changes to the Eyes and Brain

One of the more surprising discoveries of long-duration spaceflight involves vision.

Many astronauts have developed a set of eye changes now called spaceflight-associated neuro-ocular syndrome.

It can include swelling of the optic nerve where it enters the eye,

a flattening of the back of the eyeball and folds in the tissue behind the retina.

Some astronauts notice that distant or near objects become blurrier as their focus shifts.

Scientists suspect the upward fluid shift plays a major role,

but the exact cause is still being studied. Astronauts often carry glasses with adjustable prescriptions to handle changes during a mission,

and some of the effects can persist after they return.

Brain scans taken before and after long missions have also shown that the brain can shift slightly upward within the skull,

and that the fluid-filled spaces inside it can enlarge.

Researchers are still working out what these changes mean over the long term.

Radiation Exposure

On the ground, Earth's atmosphere and magnetic field block most radiation from space.

Astronauts in low orbit are still partly protected by the magnetic field,

but they receive considerably more radiation than people on the ground,

and the dose would be much higher on trips beyond Earth's magnetic shield.

Some astronauts, going back to the Apollo era, have reported seeing brief flashes of light when their eyes are closed,

thought to be caused by energetic particles passing through the eye or brain.

The main long-term concern is an increased lifetime risk of cancer.

Every astronaut wears a radiation badge, and agencies track each person's total exposure over their career.

Sleep, Immunity and the Rhythm of the Day

On a space station, the sun rises and sets about every 90 minutes, which can confuse the body's internal clock.

Noise, excitement and a demanding schedule add to the challenge,

and studies have found that astronauts tend to sleep less in orbit than they do at home.

Spaceflight can also change how the immune system behaves.

Some immune cells respond differently, and dormant viruses that most people carry,

such as the ones behind cold sores and chickenpox, can reactivate, usually without causing symptoms.

Stress, radiation and disrupted sleep may all contribute.

Other research has looked at shifts in the gut's microbes and in how genes are switched on and off during flight.

Coming Back to Gravity

Returning to Earth reverses many of these changes, but not instantly.

In the first days after landing, astronauts commonly deal with:

• Dizziness on standing: The heart and blood vessels must relearn how to push blood uphill to the brain.
• Wobbly balance: The inner ear has to recalibrate to gravity, so walking in a straight line or turning quickly can be tricky.
• A heavy feeling: Many say their bodies, and even small objects like a pen, feel strangely heavy.
• Aches and stiffness: Muscles and the spine suddenly have to carry the body's full weight again.

Astronauts go through structured reconditioning programs lasting several weeks, working with specialists to rebuild strength, balance and endurance.

Muscle strength usually returns within weeks to months.

Bone takes much longer to rebuild, and some studies suggest it may not fully recover even after a year or more back on Earth.

Frequently Asked Questions

Do astronauts really grow taller in space?

Yes, temporarily.

Without gravity compressing the spine, the discs between the vertebrae expand, and astronauts can gain up to about 3 percent in height.

The extra height disappears after they return, and back pain during flight is a common complaint.

Can people catch a cold in space?

It is possible, which is why crews spend a period in quarantine before launch to avoid carrying an infection into orbit.

Because the immune system can behave differently in space, flight doctors on the ground keep a close eye on crew health throughout a mission.

Does spaceflight change a person's DNA?

NASA's Twins Study compared astronaut Scott Kelly, who spent about a year in space in 2015 and 2016,

with his identical twin brother Mark, who stayed on Earth.

Researchers found many changes in how Scott's genes were switched on and off during flight,

but most returned to normal within about six months of landing.

His underlying genetic code did not fundamentally change, and the two remained identical twins.

What It All Adds Up To

Spaceflight touches nearly every part of the human body, from the inner ear and eyes to bones, muscles, blood and brain.

Many of these changes reverse after landing, while others, like bone loss and some eye changes, can linger.

Every mission adds to what scientists know, and that knowledge will shape how future crews stay healthy on longer journeys, including a trip to Mars.