I think the Radiation beyond earth will be the greatest challenge for space exploration. what are other factors we face in this area?
There are many dangers in space and some hurdles are:
- food supplies: linked with the long distances, the time taken to travel implies lots of food for the crew
- metheorites and other obstacles: they can be a real pain in the neck and cause quite a lot damage
- distances: everything is far away in space
- the return voyage: if needed
- costs: who will pay for all the technology and materials?
- low gravity: can a human go a very long time with nearly no gravity?
- time: We don't live long enough to reach anywhere far away
To name just a few.
Funding! Funding! Funding!
From an engineering perspective, there is not any problem which can not be overcome. Ironically, the hugest problem causing the most and longest discussions in every (deep) space project every single time is cash. How are we going to build this or that with this little amount of money? It is the hardest "engineering" constrain of all.