Life on Earth has evolved under the constant pull of gravity. But when astronauts travel into space, they experience microgravity, a condition in which gravity is so weak that objects appear to float. Though exciting for exploration, microgravity causes profound changes in the human body—some temporary, others with potential long-term health consequences.
Understanding these effects is essential for the safety of astronauts on long missions, such as to the Moon or Mars, and has even contributed to medical knowledge back on Earth.
Muscle and Bone Loss
In space, the musculoskeletal system weakens rapidly. Without the need to support body weight or resist gravity:
- Muscles shrink, especially in the legs and back
- Bone density decreases by up to 1–2% per month, similar to accelerated osteoporosis
- Calcium released from bones may increase the risk of kidney stones
Astronauts must exercise for 2+ hours per day to reduce these effects using resistance machines, treadmills, and bikes.
Fluid Shift and Puffy Face
On Earth, gravity pulls bodily fluids downward. In space, fluids shift toward the upper body, resulting in:
- Swollen face and nasal congestion
- Increased intracranial pressure, which can flatten the eyeball and reduce vision
- Possible long-term visual impairment called Spaceflight-Associated Neuro-ocular Syndrome (SANS)
This fluid redistribution also stresses the cardiovascular system, sometimes causing orthostatic intolerance (fainting) upon return to gravity.
Cardiovascular Changes
The heart becomes more spherical in shape and may weaken due to reduced effort in microgravity. Blood volume drops, and astronauts may develop arrhythmias or low blood pressure upon returning to Earth.
To counteract this, astronauts wear compression garments and increase salt intake before re-entry.
Immune System and Cellular Changes
The immune system becomes less effective in microgravity. Some immune cells function abnormally, making astronauts more vulnerable to infection and reactivation of dormant viruses like herpes.
Additionally, gene expression and cell behavior can change, as observed in twin studies involving astronauts. Space may even accelerate certain aspects of aging.
Psychological Effects
Confined quarters, disrupted sleep cycles, and social isolation can lead to:
- Mood changes
- Sleep disorders
- Cognitive impairments
Mission planners now emphasize mental health support, including virtual communication with family, structured schedules, and access to psychological tools.
Radiation Exposure
Earth’s atmosphere and magnetic field protect us from most cosmic radiation. In orbit or deep space, astronauts are exposed to high-energy particles, increasing the risk of:
- Cancer
- Neurodegeneration
- Reproductive damage
Radiation shielding is essential, but current technologies are not sufficient for long-duration deep space missions.
Glossary
- Microgravity – a condition where gravity is extremely weak, experienced in orbit
- Bone density – the amount of mineral matter per square centimeter of bones
- Orthostatic intolerance – inability to stand without dizziness or fainting
- SANS – a condition involving visual changes due to fluid shift in space
- Radiation exposure – contact with energetic particles harmful to human tissue
- Immune suppression – weakening of the immune system’s ability to defend the body
