What Happens to Astronauts’ Bodies in Space?

APS member and astronaut Jessica Meir, PhD, works in a microgravity environment on the International Space Station. Credit: NASA

If I asked you where is up and where is down, you would most likely be able to identify it. This is because the Earth’s gravitational pull on your body makes it easier for you to understand how you relate to your space (spatial orientation). Now imagine not knowing the difference between up or down. Imagine your hair is sticking up like you are underwater. Imagine feeling weightless. This is what it is like to be in microgravity (very weak gravity) in space.

About 623 miles away from Earth’s atmosphere is an area known as Lower Earth Orbit. This is the start of microgravity environments that are some of the of the most extreme and least explored areas of the universe. However, scientists do know that the physiological effects of microgravity are not beneficial to people. From a physiological perspective, a microgravity environment can change the body in ways that do not happen on Earth. In space, microgravity affects the entire physiological system, including two major areas: the muscles and the eyes.


On Earth, red blood cells and plasma—the fluid that makes up more than half of our blood—flow through the arms and legs with the help of gravity. In space, there is limited gravitational pull and fluid is distributed toward the head instead of being evenly distributed throughout the body. Astronauts are encouraged to exercise between two and two-and-a-half hours per day to increase blood circulation and prevent muscle atrophy.

Muscle atrophy, or the loss of muscle tissue, is common in a microgravity environment. In the first few weeks of being in a microgravity environment, plasma and red blood cells decrease significantly in the kidneys. This means that the body has less oxygen moving around the body, which can potentially cause nutritional deficiencies and problems with heat distribution to lower extremities. 


After extended short- or long-duration space flight, increased pressure in the skull causes visual changes known as spaceflight-associated neuro-ocular syndrome (SANS). SANS is a concern in the aerospace medicine industry because it leads to temporary vision impairment. There is currently no way to prevent SANS. Some symptoms of SANS include vision loss, headaches and limited visual depth perception. 

Fortunately, scientists today are working on technologies to be able to limit the effects of microgravity on the body, such as pressurized sleeping bags for the lower extremities and supplements to help the body to adapt to microgravity. The future for short- and long-duration space flights is rapidly changing from science fiction into reality. It is important to understand the effects of the microgravity environment without the romanticized vision of exploring the last frontier.

Jeska Clark is a master’s degree student at Arizona State University in the Human Systems Engineering Department. Her research focus is on human performance and optimizing decision-making in extreme environments by using technology.

3 thoughts on “What Happens to Astronauts’ Bodies in Space?

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