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Introduction
STUDY BACKGROUND
Planetary protection has two goals; namely, to protect the biological integrity of other solar system bodies for future science missions (preventing forward contamination)1 and to preserve the integrity of Earth’s biosphere (preventing back contamination). These goals are incorporated into National Aeronautics and Space Administration (NASA) policy,2 international law,3 and international scientific consensus guidance from the Committee on Space Research (COSPAR) of the International Council for Scientific Unions.4 The NASA and COSPAR policies include specific guidelines for different types of solar system exploration missions (e.g., flybys, orbiters, landers) and for different solar system bodies (i.e., planets, asteroids, comets, etc.).
For most of the history of the space age, missions to solar system bodies beyond Earth have been conducted by a few government agencies for purely scientific purposes. That situation is changing now, especially in the sense that nongovernmental entities, including private-sector enterprises, are preparing to conduct missions to other solar system bodies for diverse reasons, such as accessing and using extraterrestrial resources. Changes such as these, as well as recent advances in the scientific understanding of solar system bodies, have led NASA and the international scientific community to conclude that reassessments of planetary protection policies are timely and important.
The Committee on Planetary Protection (CoPP) of the Space Studies Board (SSB) has responded to the need to reassess planetary protection policy implications in a 2020 report regarding lunar lander missions5 and a 2021 report regarding Mars robotic lander missions.6 The present CoPP report extends the committee’s assessments to missions to small solar system bodies.
PLANETARY PROTECTION POLICY FOR SMALL SOLAR SYSTEM BODIES
Both NASA and COSPAR policies provide for assigning each solar system exploration mission to a specific planetary protection category. For missions that are not intended to return samples to Earth, these
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1 In planetary protection policy, the goal of preventing forward contamination focuses exclusively on avoiding interference with searches for evidence of life or the origin of life, and it does not encompass other esthetic, environmental, or ethical issues relating to contamination of solar system bodies.
2 NASA, 2021, “NASA Procedural Requirements: Planetary Protection Provisions for Robotic Extraterrestrial Missions,” NPR 8715.24, September 24.
3 “Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies,” 18 U.S.T. 2410, 610 U.N.T.S. 205, opened for signature January 27, 1967.
4 Committee on Space Research (COSPAR), 2021, “The COSPAR Policy on Planetary Protection,” approved by the COSPAR Bureau on June 3, 2021, https://cosparhq.cnes.fr/assets/uploads/2021/07/PPPolicy_2021_3-June.pdf.
5 National Academies of Sciences, Engineering, and Medicine (NASEM), 2020, Report Series: Committee on Planetary Protection: Planetary Protection for the Study of Lunar Volatiles, Washington, DC: The National Academies Press, https://doi.org/10.17226/26029.
6 NASEM, 2021, Report Series: Committee on Planetary Protection: Evaluation of Bioburden Requirements for Mars Missions, Washington, DC: The National Academies Press, https://doi.org/10.17226/26336.
range from Category I—for missions to objects for which there are no concerns about planetary protection—to Category IV—for missions to bodies for which there is a significant concern that contamination could be harmful to scientific investigations of evidence of life on the target body. There is a Category V for missions that will return samples from solar system bodies back to Earth.
The current policy places missions to undifferentiated, metamorphosed asteroids in Category I, for which there are no planetary protection requirements beyond identifying the target of the mission. NASA assigns missions to other asteroids and to Kuiper Belt objects to Category II, for which there are only information requirements for planetary protection.7 COSPAR specifies that missions to carbonaceous asteroids and the dwarf planet Ceres also should fall in Category II.8 COSPAR also recommends that forward contamination controls for small bodies other than those cited above may not be necessary and that those missions should fall into Category I or II, because there are so many members of each class of the other solar system objects. These planetary protection policies for small-body missions mean that missions are not required to comply with spacecraft cleanliness protocols or provide spacecraft organic inventories.
The committee discusses the distinctions between Categories I and II for small-body missions, and the implications of the committee’s findings, in more detail in Chapter 4.
RELEVANT PRIOR REPORTS
There are several past advisory reports that are relevant to the committee’s assessment of planetary protection policy for missions to small bodies, and they are summarized below.
The 1998 SSB report Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making9 focused on recommendations for planetary protection provisions to prevent back contamination from sample-return missions and did not address forward contamination. However, the report did consider the likelihood of finding living organisms on small solar system bodies, and those conclusions are relevant to motivating forward contamination policies as well.
With regard to asteroids, the report concluded:
For samples returned from C-type asteroids, undifferentiated metamorphosed asteroids, and differentiated asteroids, the potential for a living entity in a returned sample is extremely low, but the task group could not conclude that it is zero.
For comets, the report concluded:
It is extremely unlikely that life could exist on comets, but only in a few cases can the possibility be totally ruled out, such as in the outer layers of Oort Cloud comets entering the solar system for the first time.... The task group concluded that cometary nuclei are unlikely to contain organisms capable of self-replication.
These conclusions are consistent with the present NASA and COSPAR guidelines for small-body missions.
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7 The characteristics of these different objects are discussed in Chapter 2.
8 COSPAR, 2021, “The COSPAR Policy on Planetary Protection,” approved by the COSPAR Bureau on June 3, 2021, https://cosparhq.cnes.fr/assets/uploads/2021/07/PPPolicy_2021_3-June.pdf.
9 National Research Council, 1998, Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making, Washington, DC: National Academy Press, https://doi.org/10.17226/6281.
The 2019 SSB report An Astrobiology Strategy for the Search for Life in the Universe10 reviewed the field of astrobiology from the perspective of relationships between searches for evidence of life in the solar system and the study of extrasolar planetary systems. The report noted the scientific synergy between understanding the role of small bodies in delivery of volatiles and organics necessary for the origin of life to the early Earth and other solar system planets and understanding similar processes for other evolving planetary systems. Thus, the report highlighted the importance of small bodies in prebiotic chemistry in the solar system.
In 2019, NASA’s Planetary Protection Independent Review Board (PPIRB) report11 included a minor recommendation that called for review of planetary protection categorization for small-body missions, as follows:
In cases of missions to Solar System destinations where there is a large population of similar Category I and II objects (e.g., comets, asteroids, Kuiper Belt objects), NASA should allow classification of individual objects as Category I to simplify missions to them.
The PPIRB report explained that:
In the case of small bodies where there are numerous potential targets, the contamination of any individual does not cause significant contamination to the class as a whole. If chemical evolution or origin of life experiments are planned for such objects, there are myriad to choose from that will not have been previously visited by robotic probes.
This recommendation reflects a concern that meeting planetary protection requirements for small-body missions might be unnecessarily complex or costly. The present CoPP report provides an in-depth scientific assessment of that PPIRB recommendation for small bodies.
The 2022 decadal survey for planetary science and astrobiology, Origins, Worlds, and Life,12 recommended eight high-priority mission themes for consideration in the next round of competitions for NASA’s medium-class New Frontiers missions. Three of those priorities addressed the science of small bodies, as follows:
- Centaur Orbiter and Lander to orbit and land on an ice-rich planetesimal in the outer solar system,
- Ceres Sample Return to collect and return samples from salt deposits on the dwarf planet Ceres, and
- Comet Surface Sample Return to map the nucleus of a Jupiter family comet and bring a sample of the surface back to Earth.
All three mission concepts would emphasize studies to understand small bodies as reservoirs of prebiotic material and potential environments that may have played a role in the origin of life in the solar system.
CHARGE TO THE COMMITTEE AND STRUCTURE OF THIS REPORT
As a standing discipline committee of the SSB to advise NASA on important matters relating to planetary protection, the committee’s charge includes the preparation of short assessment reports detailing progress in areas relating to NASA’s planetary protection guidelines or new scientific and technical
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10 NASEM, 2019, An Astrobiology Strategy for the Search for Life in the Universe, Washington, DC: The National Academies Press, https://doi.org/10.17226/25252.
11 Planetary Protection Independent Review Board (PPIRB), 2019, NASA Planetary Protection Independent Review Board (PPIRB): Report to NASA/SMD: Final Report, Washington, DC: NASA, p. 13.
12 NASEM, 2022, Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023–2032, Washington, DC: The National Academies Press, https://doi.org/10.17226/26522.
developments. Such reports may include consensus findings and discussion of the basis for the committee’s conclusions, but short reports do not include specific, formal recommendations. The Statement of Task is as follows:
The Committee on Planetary Protection (CoPP) of the Space Studies Board (SSB) shall conduct a study on planetary protection categorization of outbound-only missions to small bodies that addresses the following topics. In what follows, an “identifiable population” of solar system small bodies refers to a subset of solar system small bodies defined by ranges of measurable known parameters, such as (a) orbital elements, (b) spectroscopic classification, (c) activity, (d) composition, and/or (e) size. Objects yet to be discovered, whose properties fall into the defining ranges, are to be considered members of the corresponding identifiable population.
- Are there identifiable populations of solar system small bodies that are sufficiently numerous, of sufficiently similar accessibility, and/or of sufficiently low relevance to the study of chemical evolution related to the search for extraterrestrial life that the contamination of one object in the population would reasonably be expected to have no tangible effect on the potential for scientific investigation using other objects in the population? If so, provide a list of these identifiable populations and their defining parameters;
- For the populations identified in #1, is it appropriate to categorize all missions to objects in these as planetary protection Category I?
- If, after the publication of the study, new information indicates that a previously defined identifiable population is sufficiently inhomogeneous with regard to planetary protection to warrant reassessment, what protocols should be followed in order to revise the defining parameter ranges and corresponding planetary protection categorizations?
The implications of the report findings will be consistent with the budget limitations provided by NASA at the time of study initiation. The study will gather input from stakeholders, including the planetary and astrobiology science communities, government agencies dealing with spaceflight and exploration, and the aerospace industry, including emerging commercial entities.
This report responds to NASA’s request. The report is based on expert briefings and discussions during the committee’s five virtual open meetings from December 2021 through April 2022 and the committee’s review of publicly available material.
In Chapter 2, the committee provides an overview of some key characteristics of the different types and populations of small solar system bodies, including information regarding their possible relevance to astrobiological studies.13Chapter 2 also presents the committee’s findings about the habitability of small bodies and the risk of terrestrial microbial contamination of small bodies. Chapter 3 discusses the committee’s criteria for assessing the extent to which missions to different small bodies need to implement planetary protection protocols, or not, and the chapter presents the committee’s specific findings on mission categorization. Chapter 4 delves into the distinctions between Category I and II missions and the implications of those distinctions in terms of mission complexity, cost, and objectives. Chapter 5 highlights issues about implications for private-sector missions to small bodies and about a lack of clarity regarding government policy for implementing planetary protection in private-sector missions.
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13 There are a few types of small bodies that the committee did not consider in its analysis, including Trans-Neptunian objects at heliocentric distances beyond 50 AU and dwarf planets in the Kuiper Belt. Planetary protection assessment for missions to Pluto and other Kuiper Belt object dwarf planets will no doubt need to be considered once such a mission is prioritized; the committee anticipates that any such mission will be a scientific mission and thus scientific requirements will likely outweigh planetary protection cleanliness requirements. Finally, the committee did not consider either missions to planetary satellites or planetary defense missions designed to deflect or destroy an asteroid that might become a hazard to Earth. These were considered to be out of the scope of this particular study.
