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National Academies of Sciences, Engineering, and Medicine; Division on Earth and Life Studies; Division on Engineering and Physical Sciences; Board on Life Sciences; Space Studies Board; Committee on Planetary Protection. Report Series: Committee on Planetary Protection: Evaluation of Bioburden Requirements for Mars Missions. Washington (DC): National Academies Press (US); 2021 Oct 7.

Cover of Report Series: Committee on Planetary Protection

Report Series: Committee on Planetary Protection: Evaluation of Bioburden Requirements for Mars Missions.

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Since the beginning of the space age, NASA and the international scientific community have embraced and implemented two principles of planetary protection. First, terrestrial biological material carried by spacecraft should not compromise scientific studies of the origins or existence of life forms on extraterrestrial bodies. This principle establishes the objective of avoiding harmful forward contamination. Second, Earth's environment should be protected against adverse changes caused by the introduction of extraterrestrial matter carried by a spacecraft returning from another solar system body. This principle sets the goal of preventing back contamination.

The International Astronautical Federation and the U.S. National Academy of Sciences raised concerns over the potential contamination of other planetary bodies in 1956 and early 1958, respectively. In February 1958, the International Council of Scientific Unions (ICSU) established the Committee on Contamination by Extraterrestrial Exploration (CETEX) to consider a code of conduct to respond to such concerns. In May 1958, CETEX concluded that:

It is therefore of the greatest importance that space vehicles should not land either accidentally or deliberately on Mars (and possibly also Venus) unless all precautions have been taken to exclude living organisms from them. Otherwise the most challenging of all planetary studies, that of extraterrestrial life, may be put in jeopardy. The same precautions in regard to the development of complex molecules . . . apply equally to both Mars and Venus (News of Science 1958).

In October 1958, ICSU established the Committee on Space Research (COSPAR) to help coordinate international space research activities. COSPAR included international coordination of planetary protection guidelines as one of its principal responsibilities. Countries codified the two principles of planetary protection as international law in the Outer Space Treaty in 1967. Article IX of the treaty provides that, in space activities involving solar system bodies, states parties shall “conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of Earth, resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose.”1

Throughout most of the history of solar system exploration, NASA and COSPAR have based planetary protection policies on scientific reviews by the Space Studies Board (SSB) of the U.S. National Academies of Sciences, Engineering, and Medicine and its predecessor the Space Science Board of the U.S. National Academy of Sciences. The current NASA requirements and COSPAR guidelines for missions to Mars reflect advice from more than a dozen SSB studies over the past five decades.2

In 1982, NASA adopted a system of mission and target categories to specify planetary protection approaches for different solar system bodies, and COSPAR followed suit in 1984. Under this system, Mars fly-by and orbiter missions are under Category III and must satisfy appropriate combinations of certain documentation, trajectory, cleanroom assembly, and bioburden reduction requirements. Missions designed to make contact with Mars fall in Category IV, which COSPAR assigns to “certain types of missions (mostly probe and lander) to a target body of chemical evolution and/or origin of life interest and for which scientific opinion provides a significant chance of contamination which could compromise future investigations.” COSPAR uses “significant” to mean “the presence of environments where terrestrial organisms could survive and replicate, and [where there is] some likelihood of transfer to those places by a plausible mechanism” (COSPAR 2020).

Category IV missions are required to implement a bioburden reduction plan that includes partial sterilization and bioassay monitoring, all in addition to meeting Category III requirements (except for trajectory biasing). Category IV has three sub-categories that depend on whether the mission is carrying instruments to search for extant Martian life or not (Categories IVb and IVa, respectively) and whether the mission will land in a Special Region potentially favorable for the presence of life (Category IVc). Category V applies to missions that will return to Earth after making contact with Mars, and such missions must address the risks of back contamination in addition to the requirements for Category IV missions.3

In recent years, planetary protection policies have received increased attention. Since the planetary protection protocols for Mars were formulated, researchers have learned much about Mars, and about terrestrial life. The current scientific understanding of Mars remains incomplete, and any planetary protection guidelines are challenged by significant uncertainties. Even so, the best available research opens possibilities for different approaches to planetary protection for Mars that are more nuanced and, in some circumstances, more permissive of activities on the surface.

New participants in space activities are also transforming the context for planetary protection concerning Mars. The number of governmental space agencies planning, participating in, and undertaking missions to Mars has increased significantly and now includes China, the European Space Agency, India, Japan, Russia, the United Arab Emirates, and the United States. For the first time, two nations—China and the United States—are operating rovers on the Martian surface. In addition, private-sector space enterprises are engaged in activities designed to make commercial missions to Mars possible.

The nature of missions to Mars is also evolving. To date, missions to the red planet have utilized robotic technologies (e.g., orbiters, landers, and rovers) designed to engage in scientific exploration. Thus, past planetary protection measures have only applied to scientific, robotic missions to Mars. The future will feature more diversity in the types of mission technologies and purposes. As NASA's Mars Cube One (MarCO) mission demonstrated, small satellite (SmallSat) capabilities open new horizons for more and different kinds of governmental and non-governmental orbital missions. The technological developments that facilitate SmallSats also support other possibilities, as seen on Mars with NASA's Ingenuity, the first drone to fly on another solar system body. At the same time, technologies such as terrain relative navigation (TRN), which was used for the Mars 2020 landing, are enabling precise access to larger portions of the Martian surface.

The types of missions are also on the cusp of changing dramatically. Space companies are planning commercial missions designed, for example, to extract water from subsurface ice deposits that can be utilized for human consumables and producing rocket propellants. NASA, China, and commercial space enterprises, such as SpaceX, are preparing to undertake human missions to Mars, an endeavor that will require technologies far different from the present generation of robotic, science-focused capabilities.

As scientific knowledge, the number of participants, and the types of missions to Mars expand, NASA, COSPAR, and recent reports on planetary protection have recognized that planetary protection policy has to take into account the interests of scientific discovery, commercial activity, and human exploration in space.4 For this study, NASA instructed the committee to “consider the views of the broad community of stakeholders, including Mars and astrobiological scientists, government agencies dealing with spaceflight and exploration, and the aerospace industry, including emerging commercial entities.”5,6 In keeping with this aspect of its charge, the committee based its analysis and findings in this report on the science, while remaining aware of the increasingly diverse set of mission participants and objectives.


The changing context for planetary protection policy has contributed to a need to review the science informing missions to Mars and re-assess planetary protection strategies. Several prior studies provided important information, perspectives, and context for the committee's current task, including those highlighted below.

Preventing the Forward Contamination of Mars (2006)

This SSB report reviewed current scientific knowledge about Mars, the history of planetary protection policy for Mars missions, scientific understanding of the survival of terrestrial organisms in extreme environments, and techniques for life detection and spacecraft decontamination, and it recommended a strategy for adapting Mars planetary protection approaches to respond to new scientific and technological advances (NRC 2006). Among the major conclusions of the report was that

Many of the existing policies and practices for preventing the forward contamination of Mars are outdated in light of new scientific evidence about Mars and current research on the ability of microorganisms to survive in severe conditions on Earth (NRC 2006, p. 2).

In responding to this finding, the report made several recommendations, including that:

NASA should take the following steps to transition toward a new approach to assessing the bioburden on spacecraft:

  • Transition from the use of spore counts to the use of molecular assay methods that provide rapid estimates of total bioburden (e.g., via limulus amebocyte lysate (LAL) analysis) and estimates of viable bioburden (e.g., via adenosine triphosphate (ATP) analysis). These determinations should be combined with the use of phylogenetic techniques to obtain estimates of the number of microbes present with physiologies that might permit them to grow in Martian environments.
  • Develop a standard certification process to transition the new bioassay and bioburden assessment and reduction techniques to standard methods.
  • Complete the transition and fully employ molecular assay methods for missions to be launched in 2016 and beyond” (NRC 2006, p. 3).

Finally, in recognition of the expected continued advances in science and technology that were likely to influence the evolution of planetary protection policy, the report recommended that:

NASA should establish an independent review panel that meets every 3 years to (1) consider the latest scientific information about Mars, as well as about Earth microorganisms, and recommend to NASA appropriate modifications to NASA's planetary protection implementation requirements as needed in light of new knowledge; and (2) identify and define the highest-priority measurements needed at Mars to inform future assessments and possible modifications of planetary protection requirements (NRC 2006, p. 6).

An Astrobiology Strategy for the Exploration of Mars (2007)

This SSB report considered scientific findings from robotic missions conducted between 1997 and 2006. It recommended a research and exploration strategy to guide the study of “potential targets for Mars exploration particularly suited for elucidating the prebiotic and possibly biotic history of Mars, and methods for identifying these targets” (NRC 2007). The report recommended that the highest priority targets for advancing astrobiological science objectives are “places where liquid water might exist today or might have existed in the past and where organic carbon might be present or might have been preserved.” More specifically, the report stated that:

Sites pertinent to present-day or geologically recent water include the following:

  • The surface, interior, and margins of the polar caps;
  • Cold, warm, or hot springs or underground hydrothermal systems; and
  • Source or outflow regions associated with near-surface aquifers that might be responsible for the “gullies” that have been observed.

Sites pertinent to geologically ancient water include the following:

  • Source or outflow regions for the catastrophic flood channels;
  • Ancient highlands that formed at a time when surface water might have been widespread (e.g., in the Noachian); and
  • Deposits of minerals that are associated with surface or subsurface water or with ancient hydrothermal systems or cold, warm, or hot springs (NRC 2007, p. 6).

The committee has taken this report's analysis and recommendations into account in its findings on the characteristics of potential sites that may not be of astrobiological interest in Chapter 3 below.

Review and Assessment of Planetary Protection Policy Development Processes (2018)

This SSB report responded to NASA's request for a comprehensive review of the history of planetary protection policy and an assessment of the current policy development process (NASEM 2018). The report made recommendations for improving the management of policy implementation, increasing investment in research and technology development to enable state-of-the-art approaches for planetary protection, and engaging all stakeholders, including private-sector entities, in policy development. The report offered the following specific recommendation concerning future Mars missions:

NASA's process for developing a human Mars exploration policy should include examination of alternative planetary protection scenarios and should have access to the necessary research that informs these alternatives. It should also include plans to engage with other nations on the policy and legal implications of missions to Mars.

The analysis supporting this recommendation discussed alternative approaches to enabling human missions to Mars while still preventing contamination of regions of high astrobiological interest.7 The report emphasized the importance of further research into the risks of atmospheric transport of contaminants from one part of Mars to another.

NASA Planetary Protection Independent Review Board Report (2019)

In response to the 2018 SSB report, NASA formed a Planetary Protection Independent Review Board (PPIRB) “to develop U.S. policies that properly balance the legitimate need to protect against the harmful contamination of Earth or other celestial bodies with the scientific, social, and economic benefits of public and private space missions” (PPIRB 2019). The PPIRB report specifically recommended that NASA evaluate the categorization of missions to Mars:

NASA should reconsider how much of the Martian surface and subsurface could be Category II versus IV by revisiting assumptions and performing new analysis of transport, survival and amplification in order to reassess the risk of survival and propagation of terrestrial biota on Mars.

All past U.S. landed missions have been treated as though there is a “significant” chance that terrestrial organisms can survive and be transported to areas where life or bio-signature detection experiments would be performed. Rummel et al. (2014) have shown that many areas of the surface are not locations of PP (planetary protection) concern. Similarly, although there may be subsurface regions that continue to warrant additional special PP consideration, this need not be the case for all subsurface regions. NASA should revisit the categorization of areas that are not considered to be ‘Special Regions’ and determine limits on terrestrial bioload transport and amplification from current landing sites.

The report also recommended that NASA make a distinction between high priority zones on Mars for astrobiologically relevant scientific studies and regions where human missions could be conducted with less rigorous planetary protection constraints:

NASA should consider establishing (i) high priority astrobiology zones, i.e., regions considered to be of high scientific priority for identifying extinct or extant life, and (ii) human exploration zones, i.e., regions where the larger amounts of biological contamination inevitably associated with human exploration missions, as compared to robotic scientific missions, will be acceptable.

The present charge to the committee directly reflects the PPIRB recommendation to reconsider the categorization scheme for Mars missions. This report contains the committee's findings on the subject.

Assessment of the Report of NASA's Planetary Protection Review Board (2020)

NASA requested that the SSB undertake a study to assess the consistency between the findings and recommendations in the 2018 SSB report and those in the PPIRB report. In its 2020 report, the review committee noted that the 2018 SSB report and the PPIRB report both emphasized that research is needed to explore ways to develop a more flexible planetary protection approach to Mars missions (NASEM 2020). The report reiterated the recommendations in the 2018 report on the need for research on the likelihood of transport and survival of biological contaminants between human exploration sites and sites of high scientific interest.

In addition to the attention paid to Mars missions, the 2018, 2019, and 2020 reports agree that: (1) planetary protection is an essential element of solar system exploration; (2) the Outer Space Treaty is the international foundation for planetary protection; (3) that COSPAR is a recognized international coordination forum for planetary protection standards; and (4) the same standards should be applied to both government-sponsored and private-sector missions.


The Committee on Planetary Protection (CoPP) is a standing discipline committee of the SSB to advise NASA on important matters relating to planetary protection. In fulfilling this mandate, the CoPP will:

Carry out its charge at its in-person and virtual meetings by gathering evidence from experts, deliberating, and, when necessary, by preparing short assessment reports detailing progress in areas relating to NASA's planetary protection guidelines or new scientific and technical developments. Such reports may include findings and discussion of key activities undertaken by NASA as well as the status of its actions that relate to the state of implementation of priority missions and programs.

This report responds to NASA's request for a study to identify criteria for determining locations or regions on Mars that are potentially suitable for missions of less restrictive bioburden than the current requirements for Category IV.8 The report is based on briefings and discussions during the committee's six virtual open meetings in March through June 2021 and the committee's review of publicly available material.

In Chapter 2, the committee briefly considers the scientific priority of astrobiological studies at Mars and the consequent importance of planetary protection. Chapter 3 discusses the conditions required for survival and proliferation of terrestrial life (p. 16) and Special Regions on Mars (p. 27) to lead into the committee's findings with respect to the principal elements of the study charge—the criteria for landing sites with relaxed spacecraft bioburdens, means to validate that the criteria can be met, and examples of potentially acceptable sites (p. 40). Chapter 4 contains findings on Mars planetary protection implementation issues and offers a risk management approach for meeting planetary protection objectives as an alternative to the current planetary protection category bioburden requirements. The committee's findings apply specifically to missions for which NASA has responsibility for planetary protection. For commercial missions in which NASA has no role or connection, the U.S. government still needs to designate a regulatory agency with responsibility to authorize and continually supervise the space activities of nongovernmental entities in accordance with the Outer Space Treaty. Finally, Chapter 5 comments briefly on planetary protection implications for human missions to Mars.



Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, opened for signature January 27, 1967, 18 U.S.T. 2410, 610 U.N.T.S. 205, art. IX.


See Appendix C and the COSPAR category definitions in Table 2.1 of NASEM (2018).


The U.S. National Strategy for Planetary Protection reflects this need in balancing U.S. “interests in promoting scientific discovery, human exploration, and the growth of private sector space activities, all with due consideration for public safety and applicable obligations” White House and National Space Council (2020, p. 1).


The statement of task is reprinted in Appendix A.


The committee heard from some representatives of industry groups, but despite attempts, it was unable to secure meetings with companies planning private-sector Mars missions.


The committee uses the term “regions of biological interest” to mean any location on Mars that might be relevant to scientific studies of evidence for extant or extinct life on Mars.


See Appendix A for the full statement of task.

Copyright 2021 by the National Academy of Sciences. All rights reserved.
Bookshelf ID: NBK576461


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