Interplanetary radiation makes roundtrip spaceflight to mars difficult (heavy) as plain old water is the shielding of choice. A Case for Mars said 5000KG of water to protect against interstellar radiation and the nuclear submarine reactor that you need to make return fuel and energy from the surface of mars.
Space X challenge is to re integrate light weight 500KW-1MW nuclear power back into the surface mission on mars. Solar power is fine for a 4 astronauts kept warm by a toaster oven on mars, but very quickly a mars base is going to want to make ceramic materials to build things to build into the surface to get away from lack of radiation protection provided by the very tiny mars atmosphere.
More than the thin atmosphere, Mars has no electromagnetic field like Earth does to shield if from the Solar Wind and all the other nasty particles and radiations that are sleeting death just over our heads. Any outposts on Mars will have to be deep underground. Hell of an engineering project.
I think we should do it, though. Take the $ out of our UN, welfare and federal employee salary budgets