Human missions to Mars are often justified in terms of the perception that they can provide better and faster scientific returns than robotic missions alone. Alternatively, they may be justified as a means of preparing for future human colonization of Mars, where humans might "live off the land" and develop a separate node for the development of human civilization. Either way, such missions require robust and effective life support and extravehicular activity systems, in addition to a supporting cast of robotic adjuncts, to be successful. One particular science task that may be helped by human explorers could be the search for life on Mars, but there is both good news and bad news with respect to introducing autonomous microbial growth systems (i.e., human explorers) into such a search. With that conflict in mind, COSPAR has introduced several planetary protection principles under which human missions and their support systems might be judged, and which will need to be considered in mission design, development, and execution. First, safeguarding the Earth from potential back contamination is the highest planetary protection priority in Mars exploration; Second, the greater capability of human explorers can contribute to the astrobiological exploration of Mars only if human-associated contamination is controlled and understood; Third, for a landed mission conducting surface operations, it will not be possible for all human-associated processes and mission operations to be conducted within entirely closed systems; and Fourth, crewmembers exploring Mars, or their support systems, will inevitably be exposed to martian materials. Here I consider several different human-mission goals and capabilities that have been put forward by a number of space agencies and others, and discuss the problems and opportunities posed by these missions with respect to the planetary protection principles forwarded by COSPAR and under study by NASA in its NPI process for developing planetary protection requirements for future human Mars missions.
