Weightlessness
An astronauts body can be dangerously affected by zero gravity in space, including risk of muscle and bone loss. Astronauts must exersize extensively to combat this.
Far from home
Being in space can create a lot of psychological issues, like major cases of home sickness. That is why an astronaut must go through extensive training without seeing their family for a long time
Decompression sickness
This can occur when pressure drops too fast. An astronaut must be quick and get in their spacesuits to get oxygen as soon as an astronaut notices this.
