Space

1. No oxygen
2. Extrme Temperature difference
3. Radiation damage from sun and deep space
4. Their space ship may broke down
5.Their bodies will become weak when staying at no gravity place for long time
......also other problems

In the space, the ultra-violet ray and the radiation from the sun is very dangerous. It will make the astronauts have cancers, or even die!

1.they haven't enought oxigen
2.their mascle are not suit to without the gravitation.

no oxygen
extremely change in temperature