1. INTRODUCTION

The challenges of a human mission to Mars include the cost of the effort and the potential risk to the crew (and perhaps to the Earth’s biosphere), but the costs incurred will only be supportable if there is a clear advantage to having human capabilities present on that world to accomplish mission objectives. If humans voyage to Mars to establish another outpost for our civilization, then human explorers are essential to achieving that objective. With such a goal in mind, one-way travel could accommodate mission success, and focus the crew’s attention fully on their task of survival and spread. Nonetheless, if one-way travel is not acceptable, and if humans are going to Mars to accelerate the pace of potential scientific discovery, to learn whether or not Mars is the abode of non-terrestrial life, then a human mission can only be justified if the human explorers do not erase the information they wish to discover on Mars. Earth microbes are the culprits in both cases—in the first case, human colonists will wish to avoid unpredictable changes in the martian environment caused by their own contamination, and they will not want to be surprised by the presence of live martians that have been obscured by it. In the second case, Earth organisms could easily obscure the faint signal of martian life where it is rare and most easily dealt with, and microbes will have to be kept places on Mars (a.k.a., "special regions") where they might grow and thrive, and where martian life might best exist. If there is non-terrestrial life on Mars, then human explorers will most safely encounter it under controlled conditions. Such conditions will provide both scientific objectivity, and ensure that inadvertent exposure to Mars microbes doesn’t affect the ability of the crew return to Earth.

Provisions must be taken on human missions to avoid the contamination of Mars by Earth organisms ("forward contamination") and to prevent the contamination of Earth by martian organisms ("backward," or "back contamination"). Such provisions will be essential to mission success. If Mars will become a home for a future branch of humanity, then provisions for managing the potential contamination of martian habitable zones and particularly possible martian aquifers will be especially important. If humans are intended to forward astrobiological objectives, it would be impossible to achieve them if abundant Earth life were to be introduced into every environment visited on Mars—and disastrous if the crew were unprepared to deal with martian life if they find it. As such, planetary protection provisions for future human missions to Mars are essential to mission success, and must be integrated into such missions during the earliest stages of their design and development. Spacecraft and planetary habitat designs must accommodate those provisions, and the microbial ecosystems within the spacecraft and habitats on Mars should be well known and monitored carefully throughout the mission. As a complement to microbial monitoring, provisions for the careful monitoring of the medical state of the crew will provide baseline data to ensure that health responses to disease exposure and other environmental stresses are not mistaken for the effects of the crew being exposed to martian materials.

2. PLANETARY PROTECTION REQUIREMENTS FOR MARS

In order to prevent forward and backward contamination in solar system exploration, robotic missions have well-defined requirements that are based on the biological potential of the target body visited and the type of spacecraft and activities planned for the mission. For Mars, The Committee for Space Research (COSPAR) specifies that the planet is "a target body of chemical evolution and/or origin of life interest...for which scientific opinion provides a significant chance of contamination which could jeopardize a future biological experiment" (COSPAR 2008). In other words, mission contamination can lead to Earth organisms or organics obscuring future experiments sent to detect both. Accordingly, roboticmission requirements have been developed to apply to Mars orbiters, landers that will land at places that are too dry or too cold to support the reproduction or Earth life, and to landers that may land or rove to places that are warm enough and damp enough to support Earth microbes. Assuming they are not also subjected to the lethal UV irradiation of the martian surface for very long (seconds to minutes), it can be shown that most surface locations on Mars are still of the too-cold and too-dry variety, most of the time.

The martian subsurface, on the other hand, is quite a different situation. Below the level of the pervasive solar-UV, Mars may be cold and dry for quite some distance down, but there is good, circumstantial evidence for the presence of large subsurface aquifers in the not-too-distant past, and a good possibility that they remain, today. While most evidence would suggest that such aquifers could be 100s of meters or even kilometers below the surface, there are suggestive features in some crater walls on Mars, known as "gullies" that have been explained by some researchers to be most consistent with liquid water flowing over the martian surface—either directly on the surface or on the surface underneath martian snowpack or glaciers that are remnant from a time when Mars, as a whole, appears to have been warmer and wetter. Such liquid water flows, as well as other potentially warm and wet sites, have been designated "special regions" on Mars, and the most stringent requirements for spacecraft microbial decontamination are associated with them. Robotic missions have long been subjected to planetary protection restrictions, while in contrast, few human missions have been covered by them. There have never been significant restrictions on outbound microbial contamination for such missions, and in the history of human spaceflight only the crews of Apollo 11, 12, and 14 were quarantined upon their return from the Moon. Once analysis demonstrated that lunar materials posed no biological threat, planetary protection requirements were abandoned for later lunar missions. Subsequent human missions remained in low Earth orbit (LEO), so the human spaceflight program has no recent experience with the implementation of planetary protection requirements, although they will need to understand them very well to support a human Mars mission. But we are not yet ready for such missions, and much more work is required. In the past decade recurrent workshops have begun to consider planetary protection policies and controls for Mars (cf., Criswell, et al. 2005; Hogan et al. 2007; Kminek et al. 2007), but preparation for such missions is still in its infancy.

Recently, COSPAR (2008) has promulgated guidelines for human missions to Mars (Table 1), with the intention of guiding the future development of requirements for national and international missions to Mars, and their supporting systems. Among the provisions in these guidelines is the statement that "safeguarding the Earth" is the "highest planetary protection priority" for Mars exploration, and that exposure of humans to Mars, and Mars to humans, is inevitable—but that "the intent of this planetary protection policy is the same whether a mission to Mars is conducted robotically or with human explorers." Largely, that intent can be realized by providing a comprehensive set of requirements to prevent the contamination of "special regions" on Mars and uncontrolled exposure to materials from such regions, and by providing appropriate measures to be taken if astronauts retrieve or are exposed to materials from "special regions." One other important principle stated in the guidelines is that "an onboard crewmember should be given primary responsibility for the implementation of planetary protection provisions affecting the crew during the mission."

With these COSPAR guidelines in mind, there will be a number of protocols and systems that need to be developed to support their successful implementation on a Mars mission. In a number of instances, these protocols and systems will dovetail well with those developed to implement space medicine support for the mission.

3. PROTOCOLS AND SYSTEMS

Human missions will require a different approach to planetary protection controls on forward contamination than those used by robotic missions. The human body hosts large, dynamic populations of microorganisms and cannot be sterilized or subjected to traditional microbial reduction methods such as dry heat. Instead, human missions will likely emphasize protective measures to minimize direct and indirect contact with the planet, and to understand contamination when it takes place. Such a strategy will require in-depth knowledge of the microbes carried inside and outside of the crewmembers’ bodies, as well as very detailed knowledge of the crewmembers’ health day-to-day. Health knowledge will be established by space medical systems on the mission, which will also have to deal with changes to the health of the human crew due to the long duration of the spaceflight to Mars (Baisden et al. 2008). A particular challenge will be to correctly identify the cause of those changes—and not to confuse spaceflight-induced health changes with those that may be caused by exposure to martian materials, including, potentially, martian life.

A conservative approach will be warranted to protect crew health in light of continuing uncertainties about possible martian microbes and the potential for Earth microbes to survive or mutate under martian conditions. Although the risks from possible martian microbes are presumed to be exceedingly small, they are not demonstrably zero. The possibility of past or extant life on Mars and other targets of interest—a main motivation for exploring those bodies—is tantalizing enough that we should assume that extraterrestrial biological materials exist until proven otherwise. Alternatively, there is a concern about the potential for Earth microbes carried by the mission either to develop heightened pathogenicity (Nickerson et al. 2000) or to become more dangerous to the crew because of reduced immune-system function during spaceflight and through long-term isolation from the wider population of humankind. One particularly important step will be to firmly establish a census of the kinds and numbers of microbes making the voyage to Mars, both inside the crew and in the spacecraft environment. This task will most likely be something that can be initiated during a preflight "health stabilization" program. As the mission proceeds, however, the microbial population on the spacecraft and in the crew should be carefully monitored for important changes related to crew health and safety.

Elsewhere, a wide variety of concerns about human-associated contamination will affect operations on a daily basis, including gas venting from habitats and spacesuits, airlock procedures, sample transportation and handling, and in-situ resource utilization. Limitations on where astronauts might go, and where they will have to be supported by robotic systems, will have to be defined and enforced. Some of the same measures to prevent forward contamination should also protect astronauts from exposure to potential biohazards.

Environmental control and life support systems must ensure that surface materials do not enter into the air and water supply, whether these consumables are brought from Earth or produced in-situ. Additional research will be needed to understand potential exposures, their monitoring, and what to do after an exposure especially if it results in unusual symptoms. Despite preventive measures, there may be unmitigatable risks to the astronauts. The nature of those risks will need to be investigated, and measures to deal with them developed and again, enforced. As a result of the convergence of the medical requirements to protect the crew, and the environmental requirements to protect Mars, it would be logical to assign an inflight medical specialist the responsibility to enforce planetary protection requirements on such a mission.

Ultimately, Earth’s safety will be most important. Returned samples taken from "special regions" will require containment and special handling. To prevent a mistaken diagnosis of "Marslife- exposure" the crew’s status with respect to pathogen defenses will need to be monitored throughout the mission, and on Earth post-return during a return "health stabilization program" that can also serve as an effective quarantine, if needed. The location, protocols, and duration of such an isolation period will need to be considered carefully.

4. AREAS FOR FURTHER RESEARCH, DEVELOPMENT, AND TESTING

A comprehensive medical care capability for a transit mission has to support the crew without the possibility of an immediate return to the Earth, while having to deal with limited, asynchronous communications during much of the mission (cf., Davis 1999; Nicogossian and Pober 2001; Thirsk et al. 2009).

Along with having access to such a comprehensive medical care system, key areas of planetary protection (PP) concern and importance to human missions include:

The process of planning a human mission to Mars has only just begun. It will take decades to balance the limitations of our transport and life support systems against the need for complex medical care during the mission, and the overall hazards of the martian environment. The potential for human contamination to spread on Mars could destroy scientific and resource-use opportunities for generations to come, while the potential to encounter unknown martian life forms (which may be more dangerous if they turn out to be closely related to us) generates the concern that such a mission could endanger even the Earth, itself. For such a mission to be truly successful, and for its planning to be truly robust given the potential for robotic discoveries on Mars, it is essential that planetary protection requirements be developed from the first, and that future medical care systems for Mars embody planetary protection support as a critical requirement.

ACKNOWLEDGEMENTS: Thanks to Gerhard Kminek of the European Space Agency for his support of this work and for a close and productive working relationship on the COSPAR Panel on Planetary Protection and in the interagency application of NASA’s and ESA’s planetary protection policies.