In order to plan a manned space expedition there are a lot of challenges to be addressed both technoligical and psychological
So far the only manned missions beyond Low Earth Orbit have been Apollo missions. Mission beyond Low Earth Orbit presents the issue of exposure to cosmic radiation since the protection of Van Allen belts is no longer present. While Apollo missions had a duration of a dozen of days a mission to Mars or other bodies of our planetary systems require with current technology several months of travel. A several month trip requires an additional overhead in term of supplies/oxygen to allow astronaut to survive. Unlike ISS if a problem arises there is almost no escape route for a mission, in comparison with Apollo 13 every B-plan will still require month of voyage. Atmosphere of Mars is much more ostile of the lunar one: e.g. wind-storm that have potential to fatally damage space-suit or human habitat (in this respect a mission to asteroid might be easier). So far no human-made vehicle has ever taken off from Mars so this poses the issue of reentry. Also it should be taken into account the psychological effect of such a journey
Experience shows that in some cases a human presence on-board would have averted crash. Signal time-of-flight between probe and ground control (Earth) is in terms of minutes (20 min. for Mars). This makes time-critical operation such as lan ding impossible in real-time. As a result probe are required to be autonomous and this makes them extremely vulnerable in face of expected conditions and possible programming errors. In the famous Apollo 11 expedition Neil Armstrong had to over-ride on board computers to perform Lunar landing.