What Happens to the Human Body in Space Long-Term?

The International Space Station has been continuously inhabited since November 2000, making it the most sustained source of data on what prolonged exposure to microgravity does to the human body. The findings are clear enough that NASA's Human Research Programme has identified five main risk categories for long-duration space missions. What is less often discussed is how the microgravity findings translate — or do not translate — to the partial gravity of Mars.

Bone Density: The Clearest Effect

Astronauts on the ISS lose approximately 1–2% of bone mineral density per month in load-bearing bones, primarily the lower spine, hips, and legs. Over a six-month mission, this represents a loss equivalent to decades of osteoporosis progression on Earth. Exercise countermeasures — specifically resistive exercise using the Advanced Resistive Exercise Device — can reduce but not eliminate this loss. Post-mission recovery takes approximately two years for most astronauts, with some individuals showing permanent reduction in bone density.

Mars, at 0.38g, provides more mechanical loading than microgravity but less than Earth. The bone density outcome for long-term Mars residents would fall between the ISS data and Earth baseline — but where exactly depends on the exercise regimen, the diet, and individual biological variation. No direct data exists, because no human has lived at 0.38g for an extended period.

Cardiovascular Deconditioning

In microgravity, the cardiovascular system redistributes blood volume toward the upper body — the legs no longer need to work against gravity to return blood to the heart. The heart adapts by becoming smaller and less efficient at pumping blood against the gravity gradient. On return to Earth, astronauts experience orthostatic hypotension — dizziness on standing — that requires weeks of reconditioning to resolve.

At Martian gravity, the cardiovascular deconditioning would be less severe but still present. A colony born and raised on Mars would have cardiovascular systems calibrated to 0.38g — functional and efficient for Mars, potentially inadequate for Earth gravity without medical support.

Vision Changes: The Intracranial Pressure Problem

One of the most unexpected findings from long-duration ISS research has been structural changes to astronauts' eyeballs and optic nerves. Approximately two-thirds of astronauts on long-duration missions develop some degree of visual impairment associated with intracranial pressure changes — the fluid redistribution of microgravity increases pressure on the optic nerve, causing the eyeball to flatten and the optic nerve sheath to expand. These changes are partially reversible but can be permanent.

The Child Born on Mars: The Unknown Variable

All the ISS data is from adults who developed under 1g and are now living at 0g. The critical unknown for Mars colonisation is what happens to a child who develops from birth under 0.38g. The developmental windows for bone mineralisation, cardiovascular architecture, vestibular calibration, and lung development all occur in the first two decades of life. A child who goes through those windows under Mars gravity will emerge with a body calibrated to Mars — not to Earth, and not to the intermediate value of adult adaptation.

In Children of Dust, Alina's Year Five developmental assessment of Nova Donnelly-Vasquez does not use the word "adapted." It uses "native condition." This is the scientifically honest classification: Nova's body is not an Earth body that adjusted to Mars. It is a body that was built by Mars, for Mars, from the beginning.