NASA Space Station
What weightlessness does to people
Bone loss, fluid shift, weakened hearts and eyesight that does not always come back.
Without gravity to load them, bones lose about 1% of their density a month and muscles waste, which is why station crews exercise two hours a day against resistance machines. Fluid shifts toward the head, puffing the face and flattening the back of the eyeball — the resulting vision changes are the most concerning long-duration effect and do not always fully reverse. The immune system dulls, the heart weakens and radiation exposure accumulates. This is the real obstacle to a Mars mission: not propulsion, but the human in the can.
Where it has come up lately
Payload
NASA
NASA Space Station
NASA Space Station
NASA Space Station
Related terms
EVA (spacewalk)
Working outside the spacecraft, in a suit that is itself a spacecraft.
Life support (ECLSS)
The machinery that keeps the air breathable and the water drinkable, indefinitely.
Docking and berthing
Two ways to attach a spacecraft to a station: it flies in, or it is caught and bolted on.
Expedition
The numbered crew rotation aboard the space station, roughly six months at a time.
Questions
What is What weightlessness does to people?
Bone loss, fluid shift, weakened hearts and eyesight that does not always come back.
Why does it matter?
The immune system dulls, the heart weakens and radiation exposure accumulates. This is the real obstacle to a Mars mission: not propulsion, but the human in the can.