For duration of their mission, astronauts experience conditions that affect their the health. For example, their bones get weaker and they may lose muscle. Being in a weightless environment means that astronauts tend not to use the muscles they rely upon on Earth, so their muscles gradually atrophy.
This, combined With the shift of fluid to the upper body and the resulting loss of essential minerals (such as calcium)
causes bones to weaken.
As a result, many astronauts are unable to walk properly for a few days upon their return to Earth.
In addition, they are exposed to radiation from the sun, which is more intense without the Earth’s atmosphere to protect them.
This puts them at a higher risk of cancer.