By Austin Little
Without gravity pulling on you, your body starts to change within days: fluids drift to your head, muscles and bones weaken, your spine stretches, and some astronauts' eyes change shape. Here's what scientists know about each change, how astronauts fight back, and what happens when they come home.
AI disclosure. This page was drafted with AI assistance and edited for Apiary. We don't invent quotes, stats, people, or events. If something looks off, tell Austin — that's the point of a living hive.
Here's the short answer:
- Your body is built for gravity. Take gravity away, and many systems that depend on it start to change, some within hours.
- Fluids shift toward your head. Astronauts get puffy faces, stuffy noses, and thinner-looking legs.
- Muscles shrink and weaken, especially the ones that hold you upright and move your legs.
- Your spine stretches, so astronauts get a bit taller in orbit, and back pain is common.
- Some astronauts' eyes change, with swelling at the back of the eye and shifts in vision. NASA calls this spaceflight-associated neuro-ocular syndrome, or SANS.
- Space radiation, isolation, and disrupted sleep add other risks.
- Most changes improve after returning to Earth, though some, like bone loss, can take a long time to recover and may not fully return.
Let's walk through each, in plain English.
Why gravity matters so much to your body
Every second of your life, gravity has been pulling on you. You don't notice, but your body does. It's constantly working against it:
- Your heart pumps blood uphill to your brain.
- Your leg veins have valves and muscles squeeze them to push blood back up.
- Your bones carry your weight and respond by staying strong.
- Your postural muscles (calves, thighs, back, neck) hold you upright all day.
- Your inner ear senses which way is down and helps you balance.
In orbit, astronauts aren't actually free of gravity. The International Space Station is close enough to Earth that gravity there is still strong. From the body's point of view, it's as if gravity has mostly disappeared.
The body is very good at adapting. That's the problem. It adapts to space, and then has to adapt all over again back on Earth.
The first days: space sickness and the "puffy face, bird legs" look
Space motion sickness
The cause is a mismatch between senses. Your inner ear has tiny organs that detect gravity and motion. In microgravity, they stop getting the usual "down" signal. Meanwhile, your eyes see a cabin that may be oriented any which way. Your brain gets confusing messages and responds a bit like it does with car sickness.
For most people, it fades within a few days as the brain learns to rely more on vision.
Fluid shift
On Earth, gravity pulls body fluids toward your feet. In space, fluids redistribute more evenly, which means more of them move toward your chest and head.
The results:
- Puffy face. Astronauts often look rounder-faced in orbit.
- Stuffy nose and headaches, similar to having a cold.
- Thinner legs. Some people call it "bird legs" because the legs lose fluid volume.
- Changes in thirst and urination, as the body senses "extra" fluid up top and adjusts.
Over time, the body reduces its total fluid and blood volume, which becomes important when astronauts return to gravity.
Bones: losing strength without weight
Bones aren't static. They're living tissue, constantly being broken down and rebuilt by specialized cells. On Earth, the stress of carrying your weight and the pull of muscles tell your bones to stay strong.
In space, the weight-bearing load nearly disappears. The body starts breaking down bone faster than it rebuilds it, especially in:
- Hips
- Thigh bones
- Lower spine
- Heels
Why it matters
- Fracture risk goes up, especially after returning to Earth.
- Kidney stones. As bone breaks down, calcium enters the bloodstream and urine.
- Long missions. On a trip to Mars, which could take many months each way, bone loss is a major concern.
Muscles: use it or lose it
In microgravity, the muscles that work hardest on Earth suddenly have little to do. Your calves don't push you off the floor. Your thighs don't hold you up from a chair. Your back muscles don't keep you upright.
Without countermeasures, those muscles shrink (atrophy) and lose strength and endurance.
Astronauts often describe:
- Weakness on return.
- Trouble standing and walking at first.
- Soreness as muscles readapt.
The heart is a muscle too
The heart doesn't have to pump against gravity as hard in space, so it may become a bit smaller and less conditioned over long missions.
On return, that can lead to orthostatic intolerance: feeling dizzy or faint when standing up, because blood rushes toward the legs and the body can't push it back to the brain quickly enough. That's one reason some astronauts are helped out of their capsule after landing.
How astronauts fight back: exercise
The best tool against bone and muscle loss is exercise. Lots of it.
On the International Space Station, astronauts typically exercise around two hours a day, using equipment designed for weightlessness:
- A treadmill with a harness and bungee cords that pull the astronaut down onto the belt.
- A stationary bike (no seat needed; astronauts clip in and strap down).
Exercise helps a lot.
Your spine: getting taller (and achier)
On Earth, gravity squeezes the soft discs between your spinal bones. That's why you're slightly shorter at the end of the day than when you wake up.
In space, without that squeeze, the discs expand and the spine lengthens.
It sounds fun, but:
- Disc problems.
- Spacesuit fit. Engineers have to account for height changes when designing suits and seats.
Back on Earth, astronauts return to their normal height as gravity compresses the spine again.
Eyes: one of the most puzzling problems
This is one of the most important health concerns for long missions, and one of the least understood.
What happens
Changes seen in some astronauts include:
- Swelling of the optic disc, where the optic nerve enters the back of the eye.
- Folds in the layers at the back of the eye.
- Shifts in vision, often toward farsightedness, meaning trouble seeing close up.
Not every astronaut is affected, and the severity varies.
Why it might happen
Scientists are still working this out. Leading ideas involve the headward fluid shift changing pressure in and around the brain and eyes.
What astronauts do
- Regular eye exams and imaging before, during, and after missions.
- Special glasses with adjustable prescriptions on board.
For a mission to Mars, protecting vision matters a lot. You can't swing by an eye doctor on the way.
Radiation: the invisible risk
On Earth, the planet's magnetic field and thick atmosphere shield us from much of the radiation in space.
Beyond Earth's protection, on trips to the Moon or Mars, exposure increases further. Sources include:
- Galactic cosmic rays: high-energy particles from outside our solar system, hard to shield against.
- Solar particle events: bursts of particles from the Sun during solar storms.
The immune system, the gut, and the genes
Immune changes
Research suggests spaceflight can change how the immune system behaves.
The NASA Twins Study
Researchers looked at many measures, including genes, the immune system, gut bacteria, cognition, and more. They found a range of changes during the mission.
One thing that was widely misreported: headlines claimed Scott's DNA had "changed" so he was no longer an identical twin. That's not what the study found.
A study of one pair of twins can't tell us everything, but it gave researchers a useful roadmap.
Mind, sleep, and isolation
Space is hard on the mind, too.
- Sleep. Noise, workload, and lack of normal day-night cues can disrupt sleep. Lighting systems are designed to help regulate body clocks.
- Isolation and confinement. Living in a small space with the same few people for months is stressful.
- Distance from home. On the ISS, astronauts can talk to family. On a trip to Mars, communication delays would make real-time calls impossible.
Space agencies select astronauts carefully, train them in teamwork and stress management, and provide psychological support.
A rough timeline: what changes when
Every astronaut is different, but the changes tend to arrive in a loose order. Here's the general picture, without precise numbers.
Minutes to hours after reaching orbit: Fluids start shifting toward the head. The face feels full, the nose stuffs up, and some people get a headache. The inner ear loses its sense of "down."
First few days: Space motion sickness peaks for those who get it, then usually fades. Astronauts learn to move around by pushing off gently instead of walking. The body begins shedding some fluid. The spine starts lengthening, and back pain may show up.
First weeks: Muscles that used to hold you up start losing size and strength if they aren't exercised hard. Bone breakdown speeds up. Blood volume settles at a lower level. Sleep patterns may still be adjusting.
Months: Bone and muscle loss continue unless exercise and nutrition keep up. For some astronauts, eye changes linked to SANS become noticeable on exams. Radiation exposure keeps adding up day by day.
Landing day and after: Gravity returns all at once. Balance, blood pressure control, and strength all have to readjust, and rehabilitation begins.
Small changes people don't expect
Not every effect of space is dramatic. Some are just strange.
- Taste and appetite. Many astronauts say food tastes blander in orbit, possibly because a stuffy nose dulls smell, which is a big part of flavor.
- Feet that go soft. With no walking, the thick skin on the soles of the feet can soften and peel.
- Hair and nails keep growing, and trimmings have to be caught (often with a vacuum) so they don't float into equipment.
- Sneezing and tears behave differently. Without gravity, tears don't fall; they collect in a watery blob on the eye.
- Posture. At rest, floating bodies drift into a relaxed, slightly crouched "neutral body posture," with arms floating up and knees bent.
These quirks sound small, but on a long mission they all affect comfort, hygiene, and the design of everything astronauts use.
Does partial gravity help? The Moon and Mars question
Honest answer: scientists don't fully know yet. Humans have spent only short periods on the Moon, and nobody has been to Mars.
Ideas researchers have discussed include spinning spacecraft or centrifuges to create artificial gravity, stronger exercise programs, and better ways to monitor health on long trips. Those are still largely in research and planning. This is one of the big open questions for future missions, and it's part of why time on the ISS and on future Moon missions matters so much.
Coming home: readjusting to gravity
Landing back on Earth is its own challenge.
Common experiences after long missions:
- Dizziness and balance problems. The inner ear and brain have to relearn gravity. Walking may feel like being drunk at first.
- Feeling heavy. Everything, even a phone or your own head, feels heavier.
- Fainting risk when standing, because of reduced blood volume.
- Muscle soreness and weakness.
- Back pain as the spine compresses again.
- Clumsiness, like letting go of objects in midair, expecting them to float.
Astronauts work with rehabilitation specialists after returning, with gradual exercise to rebuild strength, balance, and endurance.
What about space tourists?
Commercial spaceflight is opening space to people who aren't career astronauts.
Short trips mean less time for bone, muscle, and eye changes. But the stresses of launch and landing, motion sickness, and fluid shifts still apply. Providers typically require medical screening.
If you're ever considering a spaceflight, talk to your doctor first, especially if you have heart conditions, high blood pressure, a history of fainting, back or spine problems, eye conditions, are pregnant, or have other health concerns. The provider's medical team should evaluate you too. This is not medical advice; it's a reminder to bring your real doctor into the decision.
What space teaches us about health on Earth
Studying astronauts isn't only about space. A lot of what happens in orbit looks like faster versions of things that happen on Earth:
- Bone loss resembles osteoporosis and the bone loss from long bed rest.
- Muscle loss resembles what happens with immobility, injury, or aging.
- Fluid shifts and fainting on return relate to conditions where people get dizzy standing up.
If you're dealing with bone loss, prolonged bed rest, muscle weakness, or dizziness when standing, those are real medical issues. See a doctor or physical therapist rather than relying on space articles.
Frequently asked questions
Do astronauts get taller in space?
Yes, temporarily. Without gravity compressing the spinal discs, the spine lengthens. Astronauts return to their normal height after coming home.
How long does it take bones to recover after space?
It varies by person and mission length. Recovery can take months to years, and some studies suggest bone may not fully return to its preflight state in every area.
Why do astronauts' faces look puffy?
Without gravity pulling fluids toward the feet, more fluid moves toward the head and chest, causing a rounder face and stuffy nose.
Can astronauts' eyesight be permanently damaged?
Some changes from SANS resolve after returning to Earth, but some have lasted. It's an active area of research and one of NASA's priorities for long missions.
Why do astronauts have trouble walking after landing?
Their muscles have weakened, their blood volume has dropped, and their inner ear and brain have adapted to weightlessness. It takes time to readjust to gravity.
How do astronauts exercise in space?
With a treadmill that uses harnesses to hold them down, a stationary bike, and a resistance machine that simulates weightlifting. They exercise roughly two hours a day.
Could humans survive a trip to Mars?
Researchers think it's possible, but bone and muscle loss, vision changes, radiation, and isolation are major challenges. Much current research focuses on solving them before crewed missions.
The takeaway
Your body is built around gravity. In space, without it, fluids drift to your head, your inner ear gets confused, bones and muscles weaken from lack of use, your spine stretches, your heart deconditions, and some astronauts' eyes change shape. Radiation, sleep disruption, and isolation add more strain.
Astronauts fight back with daily exercise, careful nutrition, medical monitoring, and lots of research. Most changes improve after returning to Earth, though some, especially bone loss and certain eye changes, can take a long time or linger.
Every astronaut is also a kind of research volunteer, and what we learn from them helps people back on the ground, from older adults to anyone stuck in a hospital bed.
If a detail here has changed with new research, tell Austin. Space medicine moves fast, and we'll update.