ASTRONAUT COMMISSION #7973: the body in space — bone, blood, balance; the medical school crossover (one-coin #7971 with the teaching ward).
The Fluid Shift
Without gravity pulling blood downward, roughly two litres move toward the head in the first days. Faces puff, legs thin, and the body — reading the fullness as too much blood — sheds volume it will need on return.
Bone Is a Load-Bearing Argument
Weight-bearing bone loses roughly one to one and a half percent of mass a month in orbit — faster than post-menopausal osteoporosis. The skeleton is not a structure; it is a negotiation with load.
Two Hours a Day, Strapped Down
Exercise is not recreation aboard; it is prescribed medicine, harnessed to a treadmill or a resistive device. It slows the loss. It does not stop it.
The Eye Changes Shape
Long-duration crew return with flattened eyeballs, swollen optic discs and shifted vision. Spaceflight-associated neuro-ocular syndrome is the finding that most worries planners of a Mars transit, because it does not fully reverse.
Radiation Has No Countermeasure Yet
Beyond Earth's magnetic field, galactic cosmic rays deliver damage no practical shielding stops. This is the honest limit on human deep-space flight, and no amount of engineering optimism removes it.
The Twins Study
One brother in orbit for 340 days, his identical twin on the ground — the cleanest controlled experiment human spaceflight has produced. Most changes reversed. Some did not.
A body in the neutral posture of weightlessness: spine lengthened, fluid shifted, hair adrift.