Humans in Space: Effects on Body & Mind
- Sylvia Rose
- Mar 19
- 5 min read
Updated: Mar 21
Space has a profound effect on the human body. Human bodies, adapted to life on Earth, undergo constant stressors in orbit. Astronauts show bone loss, muscle weakness, eye swelling and mental health problems.
Gravity: Bone Loss and Muscle Weakness
On Earth, gravity keeps muscles and bones working. In space, lack of this constant load leads to rapid bone loss, often at a rate of 1-2% per month, a rate far exceeding that seen in osteoporosis on Earth.
Similarly, muscles begin to atrophy in the weightless environment. Without the need to constantly support the body against gravity, muscles, particularly those in the legs and back, weaken.
Astronauts combat this with rigorous exercise routines, often spending hours each day using specialized resistance equipment to mimic the effects of gravity.
Despite these efforts, muscle weakness and shrinkage remains a persistent issue. Astronauts can lose up 20% of muscle mass and often need extensive rehabilitation on return to Earth.
Fluid & Cardiovascular Changes
On Earth, gravity pulls fluids towards the lower body. In space, absence of this force causes fluids to redistribute upwards. This results in a puffy face and distended neck veins, a common sight in astronaut photos.
The fluid shift puts pressure on the eyes and strains the cardiovascular system. The heart exerts less energy to pump blood up, leading to potential heart muscle weakness and even decrease in size.
Astronauts also experience a decrease in blood volume in space, a consequence of the body interpreting the fluid shift as an overabundance. This translates to orthostatic intolerance on return to Earth, with blood pressure fluctuations and dizziness upon standing.
Astronaut Scott Kelly explains how fluid redistribution feels, likening it to symptoms of a sinus infection. It's not a short-term issue. Many astronauts experience continued symptoms for days or weeks back on Earth.
Microgravity can also cause a slight buildup of fluid in the lungs. This can affect vital capacity, or the amount of air breathed in or out.
Cardiovascular System
The cardiovascular system undergoes remarkable transformations in microgravity. Astronauts may have a notable drop in blood pressure and changes in how the heart pumps blood. The heart can become more spherical due to fluid shifts.
Most astronauts feel dizzy and lightheaded when they first return to Earth, caused by sudden changes in blood distribution and regulation. To deal with cardiovascular concerns, NASA has pre-launch and post-flight exercise programs focused on maintaining heart health.
Radiation Exposure
Outside Earth's protective atmosphere, astronauts are exposed to high levels of radiation. Earth’s magnetic field is weaker, while solar energetic particles from the sun, and galactic cosmic rays are much stronger in space.
Without the protective ozone layer to shield humans, radiation can damage DNA. This increase the risk of cancer and other long-term health problems.
Radiation affects people at the cellular level. According to the European Space Agency, a week of radiation on the ISS is a year's worth of exposure to radiation on Earth. Solar flares and other sun phenomena exacerbate the levels.
The ISS gives some shielding. Astronauts wear dosimeters to monitor their radiation exposure, and mission planners carefully consider radiation levels when determining mission duration.
Deep space missions anticipate greater exposure. Research is ongoing to develop more effective shielding technologies and countermeasures to mitigate harmful effects of radiation.
Vision Changes
Many astronauts experience vision changes during long-duration spaceflights. This relates to the fluid shift and increased pressure in the skull, which can cause eye swelling, blurred vision and optic nerve stress.
Mechanisms behind this phenomenon are not fully understood. According to NASA, fluid can build up in a person's head because lack of gravity inhibits normal flow of blood and other fluids. Buildup of fluid can cause loss of smell and change eyeball shape.
Cognitive Effects
Along with physical changes, astronauts experience the psychological impact of being in space. Missions can last for several months, often in isolated environments with limited social contact.
Astronauts describe feelings of stress, anxiety and depression due to isolation. The stressful and confined environment can impair cognitive function.
Astronauts may experience issues with memory, attention, and decision-making. Chris Hadfield, a veteran astronaut now retired, emphasizes the importance of mental health during missions.
He recommends reading, exercising and maintaining communication with family and friends on Earth. Addressing mental health is vital for success of long-term missions, especially extended journeys, like those to Mars.
Return to Earth can trigger the overview effect. Some astronauts report an intense feeling of connection to humanity and Earth after observing the planet at a distance for a long time.
Astrophysicist Brad Tucker explains,
“Some people call it a feeling of inspiration. Some people call it feelings of inadequacy in terms of just how big the world is. It's a huge transition from living in a very inspiring environment." Re-adjusting to civilian life can make the sensation stranger.
Sleep Disturbances
Constant daylight cycles in orbit, along with stressful environment, can disrupt sleep patterns. This causes fatigue, reduced performance and increased error risk. Astronauts rely on strict sleep schedules, light therapy or medication for proper rest.
Immune system changes
Immune response can weaken in microgravity, increasing vulnerability to infections. Astronauts on the ISS report 25% higher incidence of upper respiratory infections compared to populations on Earth.
The decline in immune health means astronauts must monitor their well-being closely. Specific measures, including the regular collection of blood samples, are taken to check astronaut health throughout missions.
Management
Regular exercise on the ISS is important. Astronauts do daily workouts using equipment designed to accommodate microgravity. Resistance training and cardio exercises help maintain muscle mass and bone density.
Astronauts receive carefully planned diets rich in nutrients to counteract deficiencies caused by the lack of gravity. Supplements are provided keep up levels of essential vitamins and minerals.
Back to Earth
Returning to Earth is difficult. After long periods in space, astronauts often struggle to readjust to the heavier gravity upon landing. What once is effortless, like walking or even standing, suddenly needs acclimatization.
Physiotherapy and monitored rehabilitation are integral to post-mission recovery. Astronauts returning to non-zero gravity also show side effects in their skin.
Because people in spaceflight are used to clothing and hair not touching the skin all the time, a sensitivity to touch can develop. Astronauts get rashes on Earth just from skin contact with their clothes.
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