Traveling in space, outside of Earth’s gravity, exposes humans to a microgravity environment that profoundly affects the body. The physiological adaptations required to survive in orbit challenge nearly every system, from balance and fluid regulation to bone and muscle integrity. The absence of gravity unmasks a series of acute and chronic health concerns. These health issues stem from two primary sources: the body’s internal reaction to weightlessness and external threats originating from the space environment.
Acute Adaptation: Space Motion Sickness
The most immediate and common reaction to entering orbit is Space Adaptation Syndrome (SAS), often called space motion sickness, which affects approximately 70% of astronauts in the first three days. This acute disorder stems from a sensory conflict between the visual system and the vestibular system within the inner ear. On Earth, vestibular organs sense gravity and linear movement, but in microgravity, they fail to provide the expected input, creating a sensory mismatch.
Symptoms include headache, lightheadedness, nausea, vomiting, and spatial disorientation. Severe cases can impair concentration and motor control, leading to the delay of mission-critical activities until adaptation occurs. Simultaneously, the body experiences a rapid fluid shift as hydrostatic pressure is removed. This causes blood and other fluids to migrate from the lower extremities toward the head, resulting in the characteristic “puffy face” and “bird legs” phenomenon, often accompanied by nasal congestion.
Chronic Effects: Musculoskeletal and Cardiovascular Health
Over weeks and months, the lack of mechanical load in microgravity leads to structural degradation in the musculoskeletal and cardiovascular systems. The skeletal system, especially the weight-bearing bones of the lower extremities and lumbar spine, begins to lose mineral density rapidly. Astronauts can lose as much bone mass in the proximal femur in one month as a postmenopausal woman loses in an entire year. This imbalance occurs because the cells that build new bone slow down, while the cells responsible for bone breakdown continue their normal activity, causing breakdown to outpace formation.
Muscle atrophy also progresses swiftly, particularly in the anti-gravity muscles used for posture and movement on Earth. Muscle mass and strength decline because they no longer work against gravity, with some astronauts experiencing up to a 20% decrease in muscle mass. The cardiovascular system also deconditions as the heart works less strenuously to pump blood in the weightless environment, leading to decreased blood volume. This deconditioning makes the return to Earth’s gravity challenging, often resulting in orthostatic intolerance—an inability to maintain blood pressure when standing upright, which can cause fainting.
External Health Threats: Radiation and Immune Function
Beyond the effects of microgravity, the space environment poses external threats, primarily from ionizing radiation and compromised immune function. Space radiation is composed of high-energy particles, mainly Galactic Cosmic Rays (GCRs) and Solar Particle Events (SPEs). GCRs are high-energy protons and heavy ions from outside the solar system that can pass through spacecraft shielding. Exposure to GCRs increases the lifetime risk for cancer and can cause damage to the central nervous system.
SPEs are bursts of high-energy protons from the sun that are less frequent but can deliver a high dose of radiation quickly, potentially causing acute radiation sickness. The combination of stress, radiation, and altered sleep cycles in space suppresses the immune system. This immune dysregulation reduces the effectiveness of immune cells. Consequently, latent viruses, such as those in the herpes family, frequently reactivate. This viral shedding serves as a biomarker of compromised immune status, suggesting increased susceptibility to onboard pathogens.
Mitigating Health Risks in Orbit
To counteract the profound physical changes of spaceflight, an intensive countermeasure program is implemented throughout the mission. Exercise is the primary defense against bone and muscle loss, requiring astronauts to spend approximately two hours daily on specialized equipment. This includes the Advanced Resistive Exercise Device (ARED), which simulates free-weight lifting using vacuum cylinders to provide up to 600 pounds of resistance.
Aerobic fitness is maintained using a cycle ergometer (CEVIS) and a specialized treadmill (T2/COLBERT), where astronauts are strapped down to simulate body weight. Pharmacological interventions are also being explored, such as bisphosphonates, to prevent bone mineral density loss, often combined with nutritional supplements like calcium and Vitamin D. The external threat of radiation is managed through careful monitoring and the use of temporary “storm shelters” with increased shielding during predicted Solar Particle Events. GCRs, however, remain a persistent challenge that requires ongoing research.