Astronauts living on the International Space Station face significant health challenges from microgravity, impacting their bones, blood, and eyes. Extended stays in orbit cause bones to lose density, red blood cells to be destroyed faster, and can lead to a serious vision impairment syndrome. These body changes pose major concerns for future long-duration missions to the Moon and Mars.
Space Takes a Toll on Astronaut Bones
Life without Earth's gravity dramatically weakens astronauts' bones. In microgravity, the weight-bearing bones in the legs, hips, and spine do not carry the same load as on Earth. This lack of stress causes bones to lose density at a rapid rate. Astronauts typically lose between 1% and 1.5% of their bone density each month during four-to-six-month missions. This means a six-month stay in space can result in about a 10% loss of bone mass, particularly in the proximal femoral bone.[nasa+2]
This bone loss rate is about ten times faster than that seen in osteoporosis on Earth. The body also increases its mineral content elsewhere, leading to issues like higher urinary calcium and a greater risk of kidney stones. Recovery after returning to Earth can take at least three or four years, and some bone density may never fully return. A study found that the shinbone density of nine astronauts had not fully recovered even a year after landing. For potential Mars missions, which could last up to three years, astronauts could lose a third of their bone mineral density, putting nearly all at risk for osteopenia and a third at risk for osteoporosis. To combat this, astronauts exercise for two and a half hours daily, six times a week, and take nutritional supplements like calcium and vitamin D. Medications like bisphosphonates are also being studied to help mitigate bone loss.[esa+12]
Space Anemia: Red Blood Cell Destruction
Space travel also profoundly affects astronauts' blood. A condition known as "space anemia" develops due to prolonged exposure to microgravity. This condition is marked by decreased levels of red blood cells, hemoglobin, and hematocrit. A groundbreaking study found that astronauts destroy 54% more red blood cells in space than they normally would on Earth. On Earth, our bodies create and destroy about 2 million red blood cells per second; in space, astronauts destroy 3 million per second.[pmc+7]
This increased destruction of red blood cells continues for the entire mission, not just as an initial adaptation to fluid shifts. Dr. Guy Trudel, a lead author of the study, noted, "Our study shows that upon arriving in space, more red blood cells are destroyed, and this continues for the entire duration of the astronaut's mission". While astronauts' bodies likely produce more red blood cells to compensate, this continuous high turnover has nutritional implications. Anemia can cause fatigue, weakness, and shortness of breath, which could be dangerous during emergency procedures or when dealing with gravity again upon returning to Earth or landing on another planet. Researchers are actively investigating effective countermeasures for space anemia.[nasa+8]
Spaceflight Associated Neuro-ocular Syndrome (SANS)
Astronauts often experience significant changes to their eyes and vision, a condition called Spaceflight Associated Neuro-ocular Syndrome, or SANS. This syndrome affects at least 70% of crew members on the International Space Station who spend six to 12 months in orbit. SANS symptoms include optic disc edema (swelling of the optic nerve), flattening of the back of the eye, choroidal folds, and shifts towards hyperopia (farsightedness). Some astronauts also report blurred vision and difficulty focusing on close objects.[en+11]
The primary suspected cause of SANS is the shift of bodily fluids toward the head in microgravity. Without gravity pulling fluids down, blood and cerebrospinal fluid move upwards, increasing pressure around the brain and eyes. Santiago Costantino, an ophthalmologist from Université de Montréal, explained, "Weightlessness alters the distribution of blood in the body, increasing blood flow to the head and slowing venous circulation in the eye". While many visual changes often reverse after astronauts return to Earth, some can persist for years, with globe flattening seen up to seven years post-flight. NASA is researching potential solutions, including nutritional supplements and mechanical interventions like Lower Body Negative Pressure (LBNP) devices, which aim to pull fluids back to the lower body.[nasa+12]
Broader Impact and Future Missions
Beyond bones, blood, and eyes, microgravity also affects other body systems. Astronauts experience significant muscle atrophy, losing up to 20% of muscle mass in short missions if not for intense daily exercise. The heart, also a muscle, can decondition and decrease in size because it does not have to work as hard in microgravity. This can lead to reduced aerobic capacity and orthostatic intolerance, making it difficult to stand without dizziness upon returning to Earth.[pmc+8]
Understanding these complex physiological changes is crucial as space agencies like NASA plan longer missions to the Moon and eventually Mars. Such missions will expose astronauts to microgravity for extended periods, potentially exacerbating these health risks. Ongoing research on the International Space Station, involving daily exercise and medical monitoring, is vital to develop effective countermeasures to protect astronaut health during humanity's expanding journey into space.[nasa+17]
