Human Mars missions: Why? How? When?

More than half a century passed since the earliest serious plans for a human Mars mission and the perspectives are still unclear. While exploring Mars remains a fundamental goal for human space exploration, nobody is at present in the position of forwarding a serious prediction about a possible timeframe. In the last years, however, the several Space Agencies have forwarded new plans, the International Space Exploration Coordination Group (ISECG) has issued a Global Exploration Roadmap and even private organizations and companies made assessments on the subject. The aim of the present paper is summarizing the present situation with the aim of understanding the basic choices which planners of a human Mars mission must d