The human element is the most complex element of the mission design Mars missions will pose significant physiological and psychological challenges to crew members Some challenges ( human engineering , life support ) must be overcome ( potential " non - starters " ) Some challenges ( bone , radiation ) may be show - stoppers ISS will only Indirectly address Mars questions before any " Go / No Go " decision Significant amount of ground - based and specialized flight research will be required -- Critical Path Roadmap project will direct HSLSPO 's research toward Mars exploration objectives