A firm timetable for human missions to Mars doesn't exist yet. However, in 2004, President Bush outlined a vision to establish a lunar outpost by 2020 as a preliminary step for exploration beyond Earth, including Mars. Preliminary plans for lunar outpost missions are to be finalized by 2012. Precursor missions that would help develop the needed technology during the 2010-2020 decade were tentatively outlined by Adringa and others. On Sep 24 2007, Michael Griffin, the NASA administrator, hinted that NASA may be able to launch a human mission to Mars by 2037. The current plan is to withdraw $11 billion from space science missions to fund the vision for human exploration.
The European Space Agency is also planning human missions to Mars under its Aurora program. Russia has been collaborating with the ESA in this effort, the most recent being an investigation of the psychological and physical effects of prolonged isolation from Earth. A human mission to Mars is expected to take about eight months each way.
There are several key challenges that a human mission to Mars must overcome: (1) Physical effects of exposure to high energy photons, and energetic subatomic particles (2) Physical effects of a prolonged low-gravity environment (3) psychological effects of isolation from Earth (4) social effects of several humans living under crowded conditions for over one earth year (5) inaccessibility of terrestrial medical facilities. Some of these issues were estimated statistically in the HUMEX study. Ehlmann and others have reviewed political and economic concerns, as well as technological and biological feasibility aspects.
While fuel for roundtrip travel could be a challenge, mature technology exists to utilize Martian H2O (preferably as water ice instead of chemically bound water) and atmospheric CO2 to generate methane and oxygen.