What Spaceflight Does to Bone, Vision, and Muscle: The Body's Silent Breakdown in Orbit

Astronauts aboard the International Space Station lose roughly 1 to 2 percent of bone mineral density per month in weight-bearing bones, the hips, spine, and femur. That rate is about ten times faster than a postmenopausal woman with osteoporosis loses bone on Earth. A six-month mission can strip the skeleton of density that takes years to partially recover, and research published in the journal PLOS ONE tracking ISS crew members found that some never fully regained it, even after a year of rehabilitation on the ground.
The mechanism is not mystery. On Earth, the skeleton is constantly loaded by gravity. Bone cells called osteoclasts resorb old bone, and osteoblasts build new bone in response to mechanical stress. Remove the stress, and osteoclasts keep working while osteoblasts slow down. The body reads weightlessness as a signal that the skeleton is excess infrastructure. It begins dismantling it.
Calcium shed from bone floods the bloodstream and exits through urine. Kidney stone risk rises sharply, NASA has documented kidney stones as one of the more common medical events in long-duration spaceflight. For Gaganyaan, India's crewed mission being prepared by ISRO, bone protection protocols will be among the most demanding physiological requirements the crew must meet.Roughly 70 percent of astronauts on long ISS missions report some degree of vision impairment. The condition has a clinical name: Spaceflight-Associated Neuro-ocular Syndrome, or SANS. In microgravity, body fluids shift toward the head. The resulting pressure in the skull pushes against the optic nerve, flattening the back of the eyeball and causing the optic disc, where the nerve meets the retina, to swell.
The visual changes range from mild blurring to a measurable shift in prescription. Scott Kelly, who spent 340 days on the ISS in 2015 and 2016, reported lasting vision changes after his mission. MRI scans of returning astronauts show structural changes to the optic nerve sheath and the eyeball itself. Some of these changes persist years after landing.

SANS is now considered one of the most serious unresolved medical risks for deep-space missions. A Mars transit would last seven to nine months one way. No crew would return to Earth if vision became seriously compromised en route.Skeletal muscle loss begins within days of entering microgravity. Without gravity to work against, the postural muscles, those that hold the spine upright and keep the legs working, begin to atrophy at a rate of roughly 20 percent over six months in the most affected groups, according to data from NASA's Human Research Program. Astronauts exercise two hours daily on the ISS specifically to slow this, using resistance devices like the Advanced Resistive Exercise Device (ARED) that simulate weight-bearing loads. Even so, muscle loss is not fully prevented.
The heart changes too. It is a muscle, and in microgravity it does not need to pump blood upward against gravity with the same force. Cardiac output drops. The heart itself becomes slightly more spherical in shape, a change documented in ultrasound studies of ISS crew members. On return to Earth, this reduced cardiac capacity contributes to orthostatic intolerance: astronauts frequently cannot stand without dizziness for days or weeks after landing.Earth's magnetic field and atmosphere block most cosmic radiation. In low Earth orbit, the ISS sits within the magnetosphere but still receives radiation doses roughly ten to twenty times higher than on the surface. Beyond low Earth orbit, on a Moon mission or a Mars transit, exposure increases dramatically. The Parker Solar Probe, launched in 2018, carries radiation instruments that have helped map the solar wind environment the crew of any future deep-space mission would face.

NASA estimates that a Mars mission would expose crew members to radiation doses approaching the career limits it currently sets for astronauts. The primary risks are elevated lifetime cancer probability and potential central nervous system damage. A 2019 study in Science Advances, using data from the Mars Science Laboratory's Curiosity rover, calculated a round-trip Mars mission would expose crew to about 1.01 sieverts of radiation, enough to raise lifetime cancer risk by roughly 5 percent above baseline.
Radiation also damages DNA in ways that can affect cognition. Studies on rodents exposed to simulated galactic cosmic rays showed measurable impairment in memory and spatial reasoning. Human data is harder to gather, but cognitive assessments of long-duration ISS crew members have shown some decline in processing speed and attention that persists after return.

For ISRO's Gaganyaan, the initial missions will stay in low Earth orbit, limiting radiation exposure to manageable levels. The harder radiation problem belongs to the generation of missions that follows.Each of these systems, bone, eye, muscle, heart, DNA, fails for the same underlying reason: they were built for a world with a specific gravitational load and a specific radiation shield, and spaceflight removes both simultaneously. The countermeasures developed so far, exercise, fluid management, shielding, can slow the damage but not stop it. A six-month ISS mission is survivable and recoverable. A three-year Mars mission, with current technology, is a different calculation entirely. The body does not adapt to space. It accommodates it, up to a point, and then the accommodation runs out.