The human body spent four billion years learning to live with gravity — then astronauts started leaving it behind for months at a time. What happens next is the strangest medical experiment ever run, and every crew member is a data point.
The puffy face and the bird legs
The changes start within hours. On Earth, gravity drags your blood and fluids downward and your body constantly fights back; in freefall the fight continues with no opponent, so a couple of liters of fluid shift up into your chest and head. The result is the classic astronaut look — puffy faces, stuffy sinuses, and skinny 'bird legs'. Food tastes bland through congested sinuses, which is why the Station's pantry runs deep on hot sauce and wasabi. Meanwhile your inner ear's balance organs stop making sense, and about half of all astronauts spend their first days queasy while the brain relearns which way is up: it decides, eventually, that 'up' is wherever your head is pointing.
Use it or lose it
The serious changes are slower. Bones, no longer loaded, shed roughly 1% of their density per month in the hips and spine — a rate ten times faster than osteoporosis on Earth. Muscles waste; hearts, with less work to do, decondition. The countermeasure is brute force: about two hours of exercise every single day on a treadmill (with bungee harness), a stationary bike, and a machine that fakes weightlifting with vacuum cylinders. It mostly works — crews return in remarkable shape — but 'mostly' is the operative word, and a Mars mission's crew would live this experiment for three years.
Taller, but it costs you
Some effects sound like superpowers. Freed from compression, astronauts' spines stretch — crews return up to a few centimeters taller (it reverses in weeks, often with backaches). Others are stranger: fluid pressure reshapes some astronauts' eyeballs, blurring vision — a syndrome discovered only when crews started staying long enough to notice, and now one of NASA's most-studied risks for Mars. And back on Earth, gravity comes as a shock: after months away, returning astronauts relearn standing balance, and rehab runs for weeks. Every bit of it is measured, logged, and published — because the only way to learn what deep space does to a body is to keep sending bodies.