As detailed in the committee’s statement of task (see Appendix A) this interim report addresses two topics:
- What are the rationales for and goals of planetary protection policies?
- Suggest a working definition of planetary protection consistent with the aforementioned goals and rationales.
THE GOALS OF PLANETARY PROTECTION
In identifying the goals of planetary protection policies, the committee reviewed formal documents that NASA officials and staff are required to use in fulfilling their responsibilities. The most recent document is the NASA Interim Directive on Planetary Protection Provisions for Robotic Extraterrestrial Missions.1 The NASA directive explicitly addresses the following two issues:
(1) The control of terrestrial microbial contamination associated with robotic space vehicles intended to land, orbit, flyby, or otherwise encounter extraterrestrial solar system bodies, and (2) The control of contamination of the Earth and the Moon by extraterrestrial material collected and returned by robotic missions.2
These two issues are commonly referred to as the control of forward and back contamination, respectively. For consistency with NASA policy and for the purposes of this interim report, the committee defines the goals of planetary protection to be twofold, as follows:
- The control of forward contamination, and
- The control of back contamination.
These goals of planetary protection are consistent with those enunciated in prior reports from the Space Studies Board (SSB),3-6, other international scientific organizations,7 and with the obligations under Article IX of the Outer Space Treaty.
1 Planetary Protection Provisions for Robotic Extraterrestrial Missions, NASA Interim Directive 8020.109A, March 30, 2017.
2 The committee notes that the Earth-Moon system is traditionally regarded as a single entity when discussing back contamination.
3 National Research Council (NRC), Assessment of Planetary Protection Requirements for Spacecraft Missions to Icy Solar System Bodies, The National Academies Press, Washington, D.C., 2012, p. 5.
4 NRC, Assessment of Planetary Protection Requirements for Mars Sample Return Missions, The National Academies Press, Washington, D.C., 2009, pp. 12-13.
5 NRC, Preventing the Forward Contamination of Mars, The National Academies Press, Washington, D.C., 2006, pp. 11-15.
PRIMARY RATIONALES FOR PLANETARY PROTECTION
In support of the twin goals of planetary protection, the committee offers a set of two principal rationales and a possible third related rationale. They are discussed below in priority order.
- Preserve the integrity of Earth’s biosphere.
- Protect the biological and environmental integrity of other solar system bodies in their natural state for future science missions.
Although Article IX of the Outer Space Treaty does not impose a hierarchy among its several requirements, the committee believes there is consensus on priorities. The first rationale references the United States’ primary obligation under the Outer Space Treaty: to avoid “adverse changes in the environment of the Earth.” This priority reflects imperatives relating to the interdependent survival of human, animal, and plant species inhabiting planet Earth. The terrestrial environment will, for the foreseeable future, and perhaps indefinitely, remain humanity’s only sustainable habitat. Thus, placing its protection at the highest priority needs no further justification. The first rationale is directly applicable to the goal of controlling back contamination of Earth through the return of materials from extraterrestrial bodies and remains effective until the absence of extraterrestrial organisms or the risk of harm from such materials is clearly established. However, even in the absence of sample-return missions, the natural interchange of potentially biotic materials between planetary bodies (e.g., organisms hitchhiking on meteorites) precludes an absolute quarantine of Earth. Nonetheless, the presence of pieces of Mars in meteorite collections does not eliminate the need for prudence in the planning of sample-return missions.
The second rationale supports the Outer Space Treaty’s Article IX mandate for states parties to explore other extraterrestrial bodies in ways that avoid their harmful contamination. More than 50 years of governmental and non-governmental participation in COSPAR policy-making demonstrates that the scope of “harmful contamination” covers any actions likely to compromise future astrobiological8 studies of planetary environments.9 This coverage underscores the high scientific priority assigned to studies of prebiotic evolution and the origins of life. The focus on preventing biological contamination of other solar system bodies also protects the right other countries have under the Outer Space Treaty to conduct astrobiological exploration of the same bodies.
The second rationale—to assure the biological integrity of the environment of other worlds for future science missions—recognizes that two leading imperatives of planetary exploration have been, and remain, to search other solar system bodies for evidence of life, past or present, and to assess the potential of selected bodies to support life, to have harbored it in the past, or to preserve evidence of prebiotic chemical evolution. These imperatives entail taking steps to prevent live, viable terrestrial organisms from
6 NRC, Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making, National Academy Press, Washington, D.C., 1998, p. 8.
7 See, for example, European Space Science Committee, Mars Sample Return Backward Contamination—Strategic Advice and Requirements, European Science Foundation, Strasbourg, France, 2013, pp. 5-6.
8 By “astrobiological” the committee refers to scientific investigations regarding both the possibility of extant or extinct life and evidence for prebiotic-like chemistry.
9 The committee will address the scope of the meaning of “harmful contamination” more fully in its final report.
being carried into extraterrestrial environments where they could survive and propagate; an outcome that would adversely affect current and subsequent science missions searching for evidence of indigenous life.
The second rationale has new significance as scientists and mission planners contemplate the in situ exploration of the satellites of planetary bodies in the outer solar system. Substantial reservoirs of liquid water are believed to exist beneath the icy crusts of, for example, Europa and Enceladus. Whereas the inadvertent transfer of viable terrestrial organisms to Mars might cause local contamination only, the introduction of similar lifeforms to a subsurface ocean will cause the global contamination of a potential biosphere.10
ANOTHER POTENTIAL RATIONALE FOR PLANETARY PROTECTION
COSPAR and NASA documents have contained these rationales since the dawn of the space age, and they form the bedrock of planetary protection policies. Beginning in 1976, NASA planetary protection guidance for execution of certain categories of missions began to be codified. The first version codification contained only the two rationales cited above.11
It will be important to stringently avoid the possibility that terrestrial organisms, their remains, or organic matter in general could inadvertently be incorporated into sample material returned from Mars. Contamination with terrestrial material would compromise the integrity of the sample by adding confusing background to potential discoveries related to extinct or extant life on Mars. DNA and proteins of terrestrial origin could likely be unambiguously identified, but other organic material might not be so easily distinguished. The search for candidate martian organic biomarkers would be confounded by the presence of terrestrial material. Because the detection of life or evidence of prebiotic chemistry is a key objective of Mars exploration, considerable effort to avoid such contamination is justified.
In 2005, NASA created a new version of its formal planetary protection document,14 which also included further attention to sample return constraints and issues associated with sample contamination. As stated, this provision sought to avoid a “false positive” in, for example, scientific tests on the returned sample. This sample-return goal informs a third potential rationale for planetary protection:
- Preserve the astrobiological integrity of current or future scientific investigations (for certain in situ experiments and particularly returned samples) that could lead to consensus in the scientific community that life is or was present on a solar system body.
10 See, for example, NRC, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000; and NRC, Assessment of Planetary Protection Requirements for Spacecraft Missions to Icy Solar System Bodies, 2012.
11 D.L. DeVincenzi, P.D. Stabekis, and J.B. Barengoltz, A proposed new policy for planetary protection, Advances in Space Research 3:13-21, 1983.
12 The search for life on Mars has been a key scientific objective since the earliest days of space exploration. See, for example, NRC, Biology and the Exploration of Mars, National Academy of Sciences, Washington, D.C., 1966, pp. vii-ix.
13 NRC, Mars Sample Return: Issues and Recommendations, National Academy Press, 1997, p. 37.
14 NASA, “Planetary Protection Provisions for Robotic Extraterrestrial Missions,” NPR 8020.12C, NASA, Washington, D.C., April 27, 2005.
The third rationale addresses the imperatives of the life detection investigations themselves, whether conducted in situ or via sample return. As such, this aspect of planetary protection policy focuses more on consideration of the scientific success of the mission than on safety concerns. It includes the mandates of the first rationale (e.g. ensuring that returned samples do not have a harmful impact on Earth’s biosphere) and the second rationale (e.g., to avoid the inadvertent transfer of materials that might impact the biological and environmental integrity of other solar system bodies; such as Europa and Enceladus where local contamination may spread throughout a global ocean). It also recognizes the scientific goal of releasing extraterrestrial samples returned to Earth to the broader community for study, and the programmatic implications of finding a viable life form in a returned sample. Space science and planetary exploration programs seek to establish, with the smallest feasible uncertainty, the biological nature and life potential of extraterrestrial environments and materials. Extraterrestrial materials returned to Earth are likely to engender widespread interest in, and demand for, the distribution of returned samples for analysis in laboratories outside of containment facilities.15 To effect such distribution, mission scientists must ensure the biological safety of the samples, for both humans and the terrestrial biota. This requires that spacecraft and their payloads be prepared in such a manner that samples can be collected and preserved with appropriate cleanliness and sterility with respect to terrestrial contaminants. Meeting this requirement will permit subsequent analysis to be sufficiently unambiguous and will assure the scientific community, governments, relevant international (or intergovernmental) organizations, and the public of the safety of such distribution. As noted, this rationale may primarily support mission science goals, because the programmatic option of keeping the returned materials in confinement remains. The confinement option would limit access, add time and cost, and require instrumentation for containment not currently planned.
The impacts of false-positive and false-negative results caused by terrestrial biological or organic contamination need to be carefully weighed. At best, a false positive may mean that the scientific utility of the samples may be compromised because they are retained in confinement or are sterilized (e.g., by heating or irradiation) prior to release.16 Moreover, a false positive might suggest that future sample-return missions be judged too risky to contemplate. A worse-case scenario concerns the possibility that terrestrial biological or organic contamination causes a false-negative result in a biohazard assessment—in which case, potentially hazardous material might be inadvertently released from confinement.
However, implementation of first and second rationales may already include what the third rationale covers. Avoiding forward and back contamination in missions to Mars can be viewed as addressing contamination that travels from Earth to Mars and back. From its origin in the 1997 SSB study and its implementation in COSPAR and NASA documents, the third rationale has been associated with preventing a false positive in a sample returned to Earth from a solar system body. However, molecular biology has advanced considerably in the last 20 years, and the committee needs to investigate more thoroughly whether new methods in molecular biology make false positive and negative results in biohazard assessments conducted on returned samples far less likely.17
The committee recognizes that how and to what degree planetary protection policies should safeguard mission science represents an important and debated question. The need to distinguish more clearly between sample cleanliness for scientific reasons and sample cleanliness for planetary protection also affects the roles and responsibilities of the mission science team and the planetary protection team.
15 See, for example, NRC, The Quarantine and Certification of Martian Samples, National Academy Press, Washington, D.C., 2002; and National Research Council, Assessment of Planetary Protection Requirements for Mars Sample Return Missions, The National Academies Press, Washington, D.C., 2009.
16 NRC, The Quarantine and Certification of Martian Samples, 2002, pp. 39-48.
17 It is also worth noting that theoretical and laboratory studies conducted over the same period lend credence to the notion that life on one planet may seed life on others by hitchhiking on material ejected from planetary surfaces during giant impacts. If so, it is conceivable that life on one planet may be genetically related to life on another since both had a common origin.
Rationale 3 highlights overlapping responsibilities, and these responsibilities could fall on the science team, planetary protection, or both. This overlap creates questions about the best approach to reducing the likelihood that sample cleanliness issues do not create conflicts between planetary protection efforts and mission science objectives.
ADDITIONAL WORK NEEDED
The committee agreed that more presentations from experts, discussion, and debate are needed to examine these questions. As a result, this interim report does not address roles and responsibilities of planetary protection and mission science teams. Nor does the interim report address all possible rationales for planetary protection, including the claim that the natural environments of extraterrestrial bodies should be preserved in their natural state.
The three rationales that the committee identified and used in developing the working definition reflect NASA policy, COSPAR guidance, and the planning and design of ongoing missions to, for example, Mars and Europa. The committee recognizes that the third rationale can have substantial implications, and it will explore them in its final report.
Finally, planetary protection requirements evolve over time as our initial ignorance advances to knowledge through the results of exploration and experiments. For example, we have learned that liquid water is common in the solar system and that terrestrial life inhabits a vast range of environments, including those that do not ultimately depend on sunlight as a source of energy. Thus, ensuring clear biological assessments of returned samples is important for future missions, both in the planning of scientific investigations and in setting of planetary protection measures.