Crewed space travel isn’t just an engineering feat. It’s a test of the endurance and resilience of the human body. In microgravity, astronauts' bodies undergo dramatic changes: muscles weaken, bones lose density, fluids shift, and vision can be affected. Beyond that, deep space exposes them to radiation, which can damage DNA and increase long-term health risks.
As space agencies and private companies plan for missions to the Moon, Mars, and beyond, understanding exactly how space travel impacts the human body is key. That’s why researchers are continuously developing exercise regimens, protective technologies, and medical countermeasures informed by countless studies. But still, space remains an unforgiving frontier — one that continues to push the limits of human biology with every mission.
Microgravity’s Toll on the Body
Gravity is a silent force shaping the human body. On Earth, it strengthens muscles, maintains bone density, and regulates fluid distribution. In microgravity, however, that balance is lost. Without the need to support body weight, muscles begin to weaken — especially in the legs, back, and core. Bones suffer, too. NASA research shows that astronauts typically lose 1 percent to 1.5 percent of their bone density per month, increasing the risk of fractures and osteoporosis.
To combat these effects, astronauts on the International Space Station (ISS) follow a strict exercise routine, spending about two hours a day on resistance-based workouts using specialized equipment. Some are even prescribed bone-strengthening medications, such as bisphosphonates, to help slow deterioration. While these measures help, they don’t completely prevent bone loss, making long-duration missions a challenge for human physiology.
Read More: The International Space Station May be Too Clean - But These Microbes Could Help
Fluid Shifts and Vision Changes
Microgravity doesn’t just weaken muscles and bones. It disrupts the way fluids move throughout the body. On Earth, gravity pulls internal fluids downward. But in space, fluids shift toward the upper body and head. This redistribution of about two liters of fluid causes facial puffiness, head congestion, and increased pressure inside the skull.
For about 70 percent of astronauts, these changes are dramatic enough that they lead to Spaceflight-Associated Neuro-ocular Syndrome (SANS), a condition where excess fluid in the head deforms the shape of the eye. The result? Structural changes in the brain, blurry vision, and, in some cases, permanent eyesight changes.
Scientists are still working to understand why some astronauts develop SANS while others don’t, but the condition could pose an even more significant challenge for deep-space missions.
The Threat of Cosmic Radiation
Earth’s magnetic field acts as a shield, largely protecting us from high-energy radiation from the Sun and galactic cosmic rays. But in space, especially beyond Earth’s orbit, astronauts are more exposed to these particles, which can damage DNA, increase cancer risk, and accelerate cellular aging.
Radiation may also negatively impact brain function. Some research even suggests that prolonged exposure to space radiation could speed up how quickly beta-amyloid plaques build up in the brain, potentially increasing the risk of cognitive diseases like Alzheimer’s.
To protect astronauts, scientists are working on better spacecraft shielding, radiation-resistant materials, and even pharmaceutical treatments that could reduce cellular damage. But until these defenses improve, radiation exposure remains one of the biggest barriers to crewed interplanetary exploration.
Read More: Spending Time in Space Slows Down Astronauts’ Thinking
The Psychological and Emotional Toll
Space travel is not only a physical challenge, but also a major mental test. Isolation, confinement, and the pressures of high-stakes decision-making can weigh heavily on astronauts.
Sleep is another challenge. The ISS orbits Earth every 90 minutes, meaning astronauts experience 16 sunrises and sunsets per day. This rapidly changing light cycle can disrupt circadian rhythms, which can adversely affect awareness, concentration, and performance.
To help astronauts maintain their mental and physical well-being, space agencies implement carefully structured schedules that regulate work, exercise, and rest. NASA also uses strategic LED lighting systems that are tuned to brighter, bluer light during the “morning” hours and dimmer, redder light during the “evening” hours. This helps the astronauts synchronize their internal clocks and improves their sleep.
Preparing for the Future: Safeguarding Human Health in Space
As we plan to push humans farther into space, researchers are racing to develop new ways to protect astronauts that will undertake long-term missions to the Moon, or even other planets. Some of the most promising advancements include:
Pharmaceutical solutions: Scientists are exploring medications that can slow bone loss, preserving skeletal strength in microgravity. Other drugs aim to protect cells from radiation damage, potentially reducing long-term health risks such as cancer and neurodegenerative diseases.
Enhanced exercise technology: Engineers are developing advanced resistance and vibration-based exercise equipment to better mimic the effects of gravity on muscles and bones. These improvements could help astronauts maintain strength and endurance more effectively during extended missions.
Improved spacecraft shielding: Researchers are testing new materials and innovative detection and shielding techniques to absorb or deflect cosmic radiation. Future spacecraft may incorporate multi-layered protective barriers or even water as built-in radiation shields.
Artificial gravity research – Scientists are investigating rotating spacecraft designs that could generate artificial gravity through centrifugal force. This technology could help counteract the harmful effects of prolonged weightlessness by providing astronauts with a more Earth-like environment.
Moreover, the integration of wearable technology is already revolutionizing how researchers monitor astronaut health in real time. These devices track everything from muscle activity to sleep patterns, providing critical data that help tailor countermeasures to individual needs. As we look to future missions, including potential voyages to Mars, all these medical and technological innovations will be central to ensuring crew safety and mission success.
The human body is adaptable, but space presents challenges unlike any we’ve faced before. Each mission expands our understanding of how we function beyond Earth’s boundaries, inching us closer to a future where humans can thrive in space.
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
NASA. Astronaut Exercise
NASA Technical Brief. Bone Loss
National Library of Medicine. Fluid shifts, vasodilatation and ambulatory blood pressure reduction during long duration spaceflight
National Library of Medicine. Navigating the Unknown: A Comprehensive Review of Spaceflight-Associated Neuro-Ocular Syndrome
National Library of Medicine. Space–brain: The negative effects of space exposure on the central nervous system
National Library of Medicine. Translating current biomedical therapies for long duration, deep space missions
Jake Parks is a freelance writer and editor who specializes in covering science news. He has previously written for Astronomy magazine, Discover Magazine, The Ohio State University, the University of Wisconsin-Madison, and more.
