There are certain technical problems with a manned mission to Mars that will be difficult to overcome. As one example, take the issue of keeping the spacecraft atmosphere scrubbed of CO2. In earth orbit or on moon missions that is done by using CO2 absorbent materials in canisters. These last for a given period of time and then must be replaced. A new supply has to be sent up to the International Space Station once every several weeks. That's not a problem in earth orbit, but on a Mars mission there is no easy re-supply. The CO2 absorbent system has to be able to work for over a year without re-supply, and that's a technical nut that's hard to crack. They don't have a good solution for that yet. Renewable systems are operating on the ISS, but they have conventional canisters backing them up. The only credible solution I've seen is sending up unmanned supply ships ahead of the manned mission so that CO2 absorbent canisters and other critical supplies could be replaced. But it would be very tricky because if they can't dock or rendezvous with a supply ship at some point then they are dead.