The biggest obstacle that needs to be resolved for any potention mission.
1. A demonstrated technology that solves the radiation exposure problem. Right now the estimated risk is just too high i.e. a trip to the Mars would kill potential astronaut prematurely. One of the Apollo missions missed a deadly solar storm by only few days. NASA will not go the route of waiver agreements.
Radiation exposure is the modern day sea monsters. A lot of nonsense. Sure there's radiation in space but its a minor engineering and/or medical issue. Besides, space travel is dangerous; that's just the way it is and always will be. An astronaut could just as easily die in a car wreck.
The biggest obstacle that needs to be resolved for any potention mission is fear.