1. Body Temperature Becomes Harder to Control

In space, the absence of gravity disrupts how the body senses and regulates heat. Astronauts often find it difficult to feel when they’re too hot or too cold, and sweating doesn’t cool them down effectively. As a result, they can overheat or feel chilled even inside the spacecraft’s carefully maintained climate.

2. Bones Lose Strength

Bones stay strong through regular pressure and movement. Without gravity pulling on the body, that mechanical stress disappears, causing bone tissue to deteriorate. Astronauts can lose around 1% of bone mass each month, especially in the hips and legs — far faster than the natural aging process on Earth. This increases fracture risk and poses a serious concern for future long-distance missions.

3. Fluids Shift Toward the Head

On Earth, gravity keeps bodily fluids distributed evenly, but in orbit, they move upward. This causes puffy faces, stuffy noses, and a constant “head cold” sensation. The buildup of pressure in the skull can also affect eyesight — one of the biggest health challenges astronauts face during long-term missions.

4. Gut Microbes Change Their Balance

Months in microgravity can alter the human microbiome — the complex ecosystem of bacteria in the digestive system. These changes may affect how astronauts digest food, absorb nutrients, and regulate immunity. Some studies also suggest a link between gut bacteria and mood, meaning long missions could influence emotional well-being as well.

5. The Spine Expands — and Astronauts Get Taller

Without gravity compressing the spine, the vertebrae naturally separate slightly, making astronauts grow up to 3% taller within just a few days in orbit. However, this effect is temporary — once back on Earth, gravity quickly restores their height to normal.

6. The Heart Grows Weaker in Weightlessness

In space, the heart doesn’t need to pump blood upward against gravity, so it gradually loses muscle mass and volume. When astronauts return to Earth, they may experience low blood pressure, dizziness, or fainting. Fortunately, with physical therapy and time, the cardiovascular system usually recovers.

7. Vision Can Be Affected by Pressure on the Eyes

Extended stays in space can lead to Spaceflight-Associated Neuro-ocular Syndrome (SANS) — a condition caused by fluid pressure on the optic nerve. This can flatten the eyeball, blur vision, and sometimes cause lasting eye changes. 

8. Muscles Shrink Without Regular Exercise

Because astronauts don’t use their muscles to support their weight in microgravity, those muscles quickly weaken. Without consistent training, they can lose up to 20% of muscle mass within days. To reduce this risk, crew members exercise for about two hours daily using resistance machines and treadmills .