Some of The Main Problems With Space Travel
First off, there is a lot of debris in space; eventually it's going to get to a point where there is a massive risk to just sending anything into space. Second, there is a lot of radiation in space that we are only protected from by our atmosphere. If we sent someone to Mars and back, once they got back, they would loose all of their hair within a month and would also be vomiting the entire time due to radiation. If you want to see the specific effects of radiation then go here: http://www.rense.com/general96/chartmilisvrt.html
Problems that Imediately Affect the Vessel/Crew
To be able to get into just a low orbit of Earth you need to be going twenty-five-thousand miles per hour, and if you get the timing, positioning, or anything just slightly wrong then the craft could burn up before even leaving the atmosphere. Another thing is that during the space walks, if something goes wrong and they get disconnected from the craft, then they will be stuck hurtling through space until they die from dehydration. If their suit cracks or if their helmets get removed, they would swell up to almost twice their size in just about five seconds, also, their eardrums would rupture instantly and they would only be able to survive for about ninety seconds; they would also be conscientiously suffering for sixty seconds of that remaining ninety of their lives.