Long space missions cause lasting harm to bones, heart, and eyes, twin study finds
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Extended time in microgravity triggers bone mineral loss of 1 to 1.5 percent a month, muscle atrophy, and vision-threatening pressure changes inside the skull, according to NASA’s Twins Study and other research. Some of the alterations, including eye nerve flattening, still had not fully reversed months after astronauts returned to Earth. The findings sharpen one of the biggest open questions for a multi-year trip to Mars.
The Weightless Body
Without gravity, about two litres of fluid shift upward into the chest and head, leading to puffy faces and a drop in blood volume. Weight-bearing bones lose mineral at 1 to 1.5 percent a month, while calf muscles shrink by roughly 15 percent despite a daily two-hour exercise regimen. The heart becomes smaller and less efficient, and the uncompressed spine extends, temporarily adding a few centimetres of height.
Spaceflight-Associated Neuro-ocular Syndrome
The headward fluid shift raises intracranial pressure, which can swell the optic nerve, flatten the back of the eye, and measurably thicken the retinal nerve fiber layer. NASA terms the cluster of vision problems spaceflight-associated neuro-ocular syndrome. Many astronauts return needing stronger glasses, and structural eye changes sometimes persist after landing. On a multi-year Mars mission, such vision deterioration could become a critical risk.
Twins Study Revelations
NASA’s study of identical twin astronauts Scott and Mark Kelly, published in Science in 2019, provided a rare controlled comparison. Scott spent 340 days aboard the International Space Station while Mark remained on Earth. Researchers found that several of Scott’s physiological and genetic markers—including those related to bone, eyes, and immune function—had not fully returned to baseline months after his return, signaling that some spaceflight adaptations may endure much longer than expected.
What's Next
NASA and partner agencies are testing countermeasures such as artificial gravity and more targeted exercise devices to better protect crews. Whether the human body can complete a round-trip to Mars without permanent impairment remains an unresolved challenge.
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Long space missions cause lasting harm to bones, heart, and eyes, twin study finds
