Prolonged exposure to microgravity during missions in space, such as the recent nine-month stay of astronauts Sunita Williams and Butch Wilmore aboard the International Space Station (ISS), profoundly affects the human body. One of the most significant changes is muscle atrophy, where astronauts can lose up to 20% of their muscle mass within just two weeks due to the lack of gravitational force. The muscles that are most affected are those responsible for posture, including those in the back and legs. In addition, bone density decreases at an alarming rate of about 1% per month, leading to an increased risk of fractures and longer recovery times upon returning to Earth. To combat these effects, astronauts engage in rigorous exercise routines, including resistance training and aerobic workouts for approximately 2.5 hours each day, supplemented with dietary aids to promote bone health.

Fluid redistribution is another notable effect experienced by astronauts during extended space missions. In microgravity, bodily fluids tend to shift towards the upper body and face, resulting in a "moon face" appearance. This redistribution can lead to orthostatic hypotension, where astronauts experience dizziness upon standing due to their bodies struggling to adjust to Earth's stronger gravitational pull after returning. The vestibular system, which helps maintain balance, is also affected; astronauts may experience disorientation and motion sickness as their brains receive conflicting signals from their inner ear and visual cues.

Radiation exposure poses a significant risk for astronauts on long-duration missions. While the ISS provides some protection from Earth's magnetic field, astronauts still encounter radiation levels that are ten times higher than on Earth. This chronic exposure can increase the risk of developing cancer and may lead to cognitive impairments over time. Additionally, studies indicate that about 7% of genes may remain disrupted even after returning to Earth, highlighting potential long-term effects on health. The immune system also suffers; astronauts often exhibit decreased immune cell counts and increased inflammation levels while in space, making them more susceptible to infections.

Psychological and cognitive effects are also prevalent during extended missions in space. The isolation and confinement experienced by astronauts can lead to stress and anxiety, impacting mental health. Moreover, cognitive functions may be impaired due to radiation exposure, raising concerns about conditions like Alzheimer's disease developing later in life. Sleep patterns are frequently disrupted as well; many astronauts report difficulties in achieving restful sleep while aboard the ISS due to changes in their circadian rhythms. As research continues into these effects, understanding how long-term spaceflight alters human physiology remains crucial for future explorations beyond low Earth orbit.