The most recent decadal survey for planetary science by the National Research Council (NRC), Visions and Voyages for Planetary Science in the Decade 2013-2022, identified “Planetary Habitats: Searching for the Requirements for Life” as one of three crosscutting themes in NASA’s solar system exploration strategy.1 This theme addresses the key question, Are there modern habitats elsewhere in the solar system with necessary conditions, organic matter, water, energy, and nutrients to sustain life? From this perspective, the most interesting bodies to explore present the greatest concern for contamination with terrestrial organisms riding on spacecraft.
Life on Earth, and presumably elsewhere in the solar system, depends on the occurrence of liquid water, sources of energy (chemical and solar), and numerous elements including carbon, hydrogen, nitrogen, phosphorus, sulfur, potassium, magnesium, calcium, oxygen, and iron. NASA’s exploration program to the outer planets has provided strong evidence that some of the icy satellites harbor liquid oceans beneath outer shells of ice that may range in thickness from several kilometers to several hundred kilometers. Because of their potential to inform us about life beyond Earth, these intriguing solar system objects have attracted the attention of the astrobiology community and mission planners. Although NASA has not yet established a mission schedule, anticipated flybys and orbiters pose significant challenges to planetary protection efforts that seek to maintain the pristine nature of these bodies for future scientific investigation. If future mission designs were to include landers or penetrators, the increased likelihood of coming into contact with habitable environments might require more stringent planetary protection procedures.
Since the United states is a signatory to the United Nations Outer Space Treaty, NASA has developed and implemented policies consistent with the treaty’s requirement that “parties to the Treaty shall pursue studies of outer space including the Moon and other celestial bodies, and 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.”2 The Committee on Space Research (COSPAR) of the International Council for Science maintains a planetary protection policy representing the international consensus standard for the “appropriate measures” referred to in the treaty’s language.
The avoidance of harmful contamination to planetary environments can, in its broadest interpretation, be motivated by the protection of extraterrestrial life forms and their habitats from adverse changes and/or by the preservation of the scientific integrity of results relating to those selfsame environments. COSPAR and NASA have adopted the latter interpretation. COSPAR’s planetary protection policies are founded on the principal that “the
conduct of scientific investigations of possible extraterrestrial life forms, precursors, and remnants must not be jeopardized.”3 The findings and recommendations of the Committee on Planetary Protection Standards for Icy Bodies in the Outer Solar System resulted from the deliberations conducted within a similar motivational framework.
COSPAR’s planetary protection policy categorizes spacecraft missions according to their type (i.e., flyby, orbiter, lander, or sample return) and the degree to which the spacecraft’s destination might inform the processes of chemical evolution and/or the origin of life (Table 1.1). The policy routinely changes in response to inputs from member organizations, including the NRC, which re-evaluate advances in scientific knowledge in both the planetary and the life sciences.
One such input came in 2000 when the NRC issued the report Preventing the Forward Contamination of Europa.4 The authors of that report were unable to agree on a methodology by which COSPAR’s existing categorization system could be extended to cover spacecraft missions to Europa.5 In place of categorization, the report recommended that spacecraft missions to Europa must reduce their bioload by an amount such that the probability of contaminating a putative Europan ocean with a single viable terrestrial organism at any time in the future should not exceed 10–4 per mission.
The 10–4 criterion proposed by the authors of the NRC’s 2000 Europa report is rooted in the history of COSPAR planetary protection policy statements and resolutions. Before its revision in 1982, COSPAR’s planetary protection policies were based on a quantitative assessment of the likelihood of contaminating planetary bodies of interest. The 10–4 contamination criterion can be traced back to a COSPAR resolution promulgated in 1964 concerning “any spacecraft intended for planetary landing or atmospheric penetration” and still earlier.6 Unfortunately, the historical literature does not record the rationale for COSPAR’s adoption of the 10–4 standard. Nor, in fact, has the committee been able to come up with its own quantitative rationale for this number. Even though COSPAR has all but eliminated quantitative approaches from its policy statements, the apparently arbitrary 10–4 standard continues to guide the implementation of planetary protection regulations, particularly with respect to those pertaining to missions to Mars.7 The adoption of a particular contamination criterion raises a number of questions. First, was it appropriate for the authors of the 2000 Europa report to apply a martian standard to Europa for any other than historical reasons? The current committee argues that since the advertised purpose of planetary protection is to preserve the integrity of scientific studies relevant to the origins of life and the processes of chemical evolution, the contamination standard for a particular object is directly related to the scientific priority given to studies of that object. Recent NRC reports such as A Science Strategy for the Exploration of Europa,8 New Frontiers in the Solar System: An Integrated Exploration Strategy,9 and Vision and Voyages for Planetary Science in the Decade 2013-202210 have ranked the scientific priority of studies of Mars and Europa as being, if not equal, then a very close one and two. Thus, a contamination standard applicable to one should, to first order, be applicable to the other.
A second question is determination of the standard itself. It should be possible, in principle, to come up with a standard that is simultaneously not arbitrary and still permits exploration. For example, it could be argued that the standard be such that the likelihood of contamination by spacecraft is less than the likelihood of contamination by meteoritic delivery of Earth microbes in impact-launched meteorites (integrated over some time period, say, the interval of anticipated spacecraft launches). But the adoption of such a standard may preclude the exploration of the icy bodies of the outer solar system.11
The committee’s decision to retain use of the historical 10–4 was predicated on two factors. First, planetary protection policies are deliberately conservative and strongly influenced by historical implementation practices. The 10–4 standard is conservative, but implementable, as evidenced by the extensive efforts undertaken to ensure that the Viking missions to Mars and the Juno mission to Jupiter were compliant. Second, the committee’s charge specifically focuses on the approach taken by the NRC’s 2000 Europa report committee and subsequent COSPAR actions related to planetary protection measures for the outer solar system. The introduction of a new contamination standard into the deliberations will, in the committee’s considered opinion, complicate the resolution of more serious issues arising from the methodology contained in the 2000 Europa report.
TABLE 1.1 COSPAR Planetary Protection Categories
|Category I||Category II||Category III||Category IV|
|Type of mission||Any but Earth return||Any but Earth return||No direct contact (flyby, some orbitersa)||Direct contact (lander, probe, some orbitersa)|
|Target bodyb||Not of direct interest for understanding of chemical evolution or the origin of life;
|Of significant interest relative to chemical evolution and the origin of life, but where there is only a remotec chance of contamination;
|Of interest relative to chemical evolution and the origin of life, but where there is a significantd chance of contamination;
|Of interest relative to chemical evolution and the origin of life, but where there is a significantd chance of contamination;
|Degree of concern||None||Record of planned impact probability and contamination control measures||Limit on impact probability; passive bioburden control||Limit on non-nominal impact probability; active bioburden control|
|Planetary protection policy requirements||None||Documentation: planetary protection plan, pre-launch report, post-launch report, post-encounter report, end-of-mission report||Documentation: Category II plus: contamination control, organics inventory (as necessary)||Documentation: Category III plus: probability of contamination analysis plan, microbial reduction plan, microbial assay plan, organics inventory|
|Implementing procedures such as: trajectory biasing, cleanroom, bioburden reduction (as necessary)||Implementing procedures such as: partial sterilization of contacting hardware (as necessary), bioshield, monitoring of bioburden via bioassay|
NOTE: Category V missions—all Earth-return—have not been included because they are not relevant to this study.
a The lifetime of a Mars orbiter must be such that it remains in orbit for a period in excess of 20 years or 50 years from launch with a probability of impact of 0.01 or 0.05, respectively.
bTarget body (Icy bodies mentioned in this report are in boldface):
Group 1: Flyby, Orbiter, Lander: Undifferentiated, metamorphosed asteroids; Io; others to be determined.
Group 2: Flyby, Orbiter, Lander: Venus; Moon (with organic inventory); Comets; carbonaceous chondrite asteroids; Jupiter; Saturn; Uranus; Neptune; Ganymede*; Callisto; Titan*; Triton*; Pluto/Charon*; Ceres; Large Kuiper belt objects (more than half the size of Pluto)*; other Kuiper belt objects; others to be determined.
Group 3: Flyby, Orbiters: Mars; Europa; Enceladus; others TBD.
Group 4: Lander Missions: Mars; Europa; Enceladus; others TBD.
*The mission-specific assignment of these bodies to Category II must be supported by an analysis of the “remote” potential for contamination of the liquid-water environments that may exist beneath their surfaces (a probability of introducing a single viable terrestrial organism of < 1 × 10–4), addressing both the existence of such environments and the prospects of accessing them. The probability target of 10–4 was originally proposed on the basis of historical precedents in the 2000 NRC report Preventing the Forward Contamination of Europa. NASA’s formal planetary protection policy has adopted this value as defined in NASA Procedural Requirements (NPR) document 8020.12C. COSPAR has discussed 10–4 as the acceptable risk for contamination and formally adopted this value in March 2011 for missions to icy bodies in the outer solar system
c In COSPAR usage, the term “remote” specifically implies the absence of environments where terrestrial organisms could survive and replicate, or that there is a very low likelihood of transfer to environments where terrestrial organisms could survive and replicate.
d In COSPAR usage, the term “significant” specifically implies the presence of environments where terrestrial organisms could survive and replicate, and some likelihood of transfer to those places by a plausible mechanism.
In 2009, COSPAR’s Panel on Planetary Protection held two workshops to explore how the NRC’s europan criterion and its underlying methodology might extend to other icy bodies of the outer solar system and simultaneously retain consistency with COSPAR’s existing categorization scheme.12,13 These workshops—held on April 15-17 and December 9-10 in Vienna, Austria, and Pasadena, California, respectively—evaluated new scientific evidence and information not available to the authors of the 2000 Europa report. The deliberations at the workshops led COSPAR’s Panel on Planetary Protection (PPP) to adopt an extended, but simplified, version of the approach previously recommended by the NRC. The key feature of the PPP’s proposal was the division of the icy bodies of the outer solar system into three groups:
1. A large group of objects including small icy bodies that were judged to have only a “remote” chance of contamination by spacecraft missions of all types (Table 1.1; see note c for COSPAR’s definition of “remote”);
2. A group consisting of Ganymede, Titan, Triton, Pluto/Charon, and those Kuiper belt objects with diameters greater than one half that of Pluto that were also thought to pose a “remote” concern for contamination provided that the implementers of a specific spacecraft mission could demonstrate consistency with the 10–4 criterion;14 and
3. A group consisting of Europa and Enceladus that were believed to have a “significant” chance of contamination by spacecraft missions (see Table 1.1; see note d for COSPAR’s definition of “significant”).
The significant chance of contamination implies that specific measures, including bioburden reduction, need to be implemented for flybys and for orbiter and lander missions to Europa and Enceladus so as to reduce the probability of inadvertent contamination of bodies of water beneath the surfaces of these objects to less than 1 × 10–4 per mission. In March 2011 COSPAR officially adopted the proposed revisions to planetary protection policy advocated by the PPP.
Based on the findings of the 2009 workshops and the growing scientific data supporting exploratory missions for extant life or clues to the origin and evolution of life on outer planets and icy bodies, NASA asked the NRC (Appendix A) to revisit the conclusions contained in the 2000 Europa report and to review, update, and extend its recommendations to cover the entire range of icy bodies—i.e., asteroids, satellites, Kuiper belt objects, and comets.
At one time, COSPAR defined the time period for planetary protection to coincide with the so-called period of biological exploration or, simply, the period of exploration.15,16 This period refers to the time necessary for robotic missions to determine whether biological systems occur on a potentially habitable planetary body. The committee recognizes that some in the scientific community would support longer periods of planetary protection, perhaps bordering on perpetuity. Indeed, the authors of the 2000 Europa report explicitly made this assumption.17 However, the committee adopts the position that an indefinite time horizon for planetary protection will lead to ad hoc practical solutions that may differ for each mission. The concept of a period of exploration lives on in COSPAR policy, explicitly, only in a single section entitled “Numerical Implementation Guidelines for Forward Contamination Calculations” of an appendix on implementation guidelines.18 In this context, “the period of exploration can be assumed to be no less than 50 years after a Category III or IV mission arrives at its protected target.”19 However, the first planetary space probes were launched almost 50 years ago, and the exploration of the solar system is still in its infancy. Clearly 100 years is too short, given the multi-decade pace of outer planet missions. Yet the pace of technological change and the length of human civilizations do not provide a sound justification for a period of planetary protection of 10,000 years or more. It is not possible to know with certainty the timeframe of exploration of the solar system, and therefore the committee assumes arbitrarily that it will extend for the next millennium.
It is worth noting that the values assigned to the period of exploration and the contamination standard are related. The former allows an upper limit to be placed on the acceptable per-mission likelihood of contamination. In other words, the product of the number of spacecraft missions to a particular body during the period of exploration
and the contamination standard must be less than one. Thus, the values of 1,000 years and 10–4 are self consistent if no more than one mission is dispatched per decade to each icy body of concern.20
The approach adopted by COSPAR for assessing compliance with its 10–4 standard for missions to Europa and Enceladus (and to a lesser degree for missions to Ganymede, Titan, Triton, Pluto-Charon, and large Kuiper belt objects) makes use of a methodology—the so-called Coleman-Sagan approach (see Chapter 2)21,22,23—that involves the multiplication of conservatively estimated, but poorly known, parameters. In the case of Europa, the following factors, at a minimum, appear in the calculation:24
• Bioburden at launch;
• Cruise survival for contaminating organisms;
• Organism survival in the radiation environment adjacent to Europa;
• Probability of landing on Europa;
• The mechanisms and timescales of transport to the europan subsurface; and
• Organism survival and proliferation before, during, and after subsurface transfer.
It is notable that COSPAR’s approach leaves open the possibility of including additional parameters in the calculation. Indeed, the Juno mission to Jupiter was determined to be compliant with the 10–4 standard only after the inclusion of an additional parameter related to the probability that organisms on the Juno spacecraft would survive a high-velocity impact with Europa. The impact-survival parameter was determined via modeling and numerical simulations.
If COSPAR’s requirement cannot be met, the spacecraft must be subject to rigorous cleaning and microbial reduction processes until it reaches a terminal, or Viking-level, bioload specification. As its name implies, the terminal specification is that to which the Viking Mars orbiter/landers of the 1970s were subjected. This terminal specification was achieved by sealing the Viking spacecraft in a biobarrier and dry heating the entire assembly to a temperature of >111°C for a period of 35 hours.
The long-standing NASA standard assay procedure determines the number of cultivable aerobic bacterial spores that may exist on flight hardware in order to meet a bioburden distribution requirement. The assay technique originally developed for the Viking missions uses a standard culture/pour plate technique to determine the number of spores in any given sample. The spores serve as a “proxy” representation of the total microbial bioburden on the spacecraft.
Over the past decades, research has greatly expanded the understanding and techniques for finding and culturing microbes, providing a greater depth of knowledge about their viability and adaptability within a variety of environments. Surveys of conserved genes from environmental DNA preparations reveal that the sum of all cultivated microorganisms represents <1 percent of naturally occurring microbial diversity.25 Extrapolation from the observation that 99 percent of all microorganisms in nature do not readily grow under laboratory conditions suggests that the standard NASA spore assay detects only a small fraction of the different kinds of heat-resistant organisms on a spacecraft (see Chapter 2). This inference implies that measurements of initial bioloads and the adequacy of bioload reduction almost certainly will underdetermine the total number of viable microbes on spacecraft by at least two orders of magnitude.
In addition to the recent changes in COSPAR policy for the icy bodies (see above), significant scientific and programmatic changes warrant a reconsideration of the 2000 Europa report. The scientific factors include the following:
• Significant advances in understanding of Europa and the other Galilean satellites. The 2000 Europa report preceded the conclusion of remote-sensing observations of Europa and the other Galilean satellites by the Galileo spacecraft in 2003. On the basis of more extensive analysis of Galileo data and associated theoretical and modeling studies, the planetary science community has a much better understanding of Europa’s internal structure and
the thickness and dynamics of its ice shell. The same can be said concerning understanding of the two other icy Galilean satellites, Ganymede and Callisto. See Chapter 4.
• The discovery of Enceladus’s polar plumes. The 2000 Europa report was drafted prior to the beginning of intensive in situ and remote-sensing studies of the Saturn system by the Cassini-Hyugens spacecraft in 2004. Prior observations of Enceladus by the Voyager spacecraft in 1980 and 1981 had revealed that this 500-km-diameter satellite possessed an unusually smooth surface and a circumstantial association with Saturn’s tenuous E ring. Cassini observations in 2005 revealed plumes of icy material emanating from discrete points along fissures located near to Enceladus’s South Pole. The identification of the plumes not only confirmed that this satellite was the source of the material forming the E ring, but also transformed Enceladus into one of the prime locations of astrobiological interest in the solar system. Whereas an ice shell several kilometers to tens of kilometers thick surrounds Europa’s ocean, Enceladus’s internal water may communicate directly with the satellite’s surface. See Chapter 4.
• New understanding of Titan’s complexity. In situ observations conducted by the Hyugens lander in 2005, augmented by subsequent remote-sensing studies by the Cassini orbiter, have transformed understanding of Titan’s complex environment. Discoveries include the presence of the methane analog of Earth’s water cycle and the likelihood of an internal water-ammonia ocean. See Chapter 4.
• The diversity and complexity of Kuiper belt objects. Although the discovery of more than 100 Kuiper belt objects (KBOs) significantly smaller than Pluto dates back to the 1990s, new observations have detected several KBOs with diameters comparable to or greater than that of Pluto. Moreover, an anomalously large number of KBOs appear to have satellites, which raises the possibility of tidal heating. Neptune’s largest satellite Triton is thought to be a captured KBO that has undergone extensive tidal heating. Images of Triton from Voyager 2 revealed geyser-like activity and an extremely young surface, raising the possibility of geologic activity on other tidally heated KBOs. See Chapter 4.
• Significant advances in microbial ecology and the biology of extremophiles. Investigations of extremophiles and novel cultivation techniques have improved understanding of the amazing physiological diversity of microbes and their requirements for growth under nominal and extreme environmental conditions. The sequencing of individual microbial genomes and the mixed genomic analysis (metagenomics) of complex microbial communities has demonstrated unanticipated levels of diversity and the evolutionary significance of horizontal transfer of genes between microbes in reshaping their genomes. Microbes take advantage of this versatility to adapt to new environments, but at the same time these studies inform researchers about the limited range of conditions that individual microbial taxa can tolerate. See Chapter 5.
The programmatic factors include the following:
• The high priority given to missions to Europa and Enceladus in the first and second planetary science decadal surveys. The NRC released its first planetary science decadal survey 2 years after the completion of the 2000 Europa report.26 The survey’s highest-priority non-Mars mission described the Europa Geophysical Explorer, a flagship-class mission that would orbit Europa and determine whether an internal ocean exists. A Europa orbiter retained its position as the highest-priority non-Mars mission in the most recent planetary decadal survey.27 Moreover, the decade-plus of study and planning behind the current mission concept, the Jupiter Europa Orbiter, has resulted in a mission far more robust and capable than the minimal orbiter NASA considered at the time of the 2000 Europa report. See Appendix B.
• The internationalization of missions to Jupiter’s moons. The days when NASA alone could conceive, plan, and successfully execute missions to Jupiter and beyond have ended. The European Space Agency (ESA), the Japan Aerospace Exploration Agency (JAXA), and the Russian Federal Space Agency have developed plans for future exploration of the Jupiter system. Most attention has focused on the development of a joint NASA-ESA Europa Jupiter System Mission (EJSM). This concept envisages a combination of independent and coordinated studies of Jupiter and its satellites by a NASA-supplied Jupiter Europa Orbiter and an ESA-supplied Jupiter Ganymede Orbiter. Another possible mission would include a JAXA-supplied Jupiter Magnetospheric Orbiter. The international nature of these missions will require agreed upon criteria and procedures for satisfying planetary protection requirements.
• Planning for future exploration of Titan and Enceladus. Interest in a follow-on mission to Cassini-Huygens has focused on the development of the NASA-ESA Titan Saturn System Mission. This concept envisages the deployment of two ESA-supplied in situ elements—a lake lander and a hot-air balloon—delivered by a large and complex NASA-supplied orbiter. Studies of Enceladus could occur before or after orbiting Titan. An alternative mission plan describes a stand-alone Enceladus orbiter. See Appendix B.
• The initiation of the New Frontiers mission line. The initiation of the New Frontiers line of principal investigator-led, medium-cost missions represents an important legacy of the first planetary science decadal survey. New Frontiers missions selected by NASA that will target the outer solar system include the New Horizons mission to Pluto-Charon and the Juno mission to Jupiter. The latter will invoke a planetary protection plan that relies on the findings and recommendations of the NRC’s 2000 Europa report. The most recent planetary decadal survey identified several additional New Frontiers candidates relevant to the subject matter of this report.
• Possibility of Discovery-class missions to outer solar system bodies. With the exception of New Horizons and Juno, all expeditions to the outer solar system launched to date correspond to flagship-class missions. The complex power and communications systems required for spacecraft that venture beyond the asteroid belt generally exceed the cost caps of principal investigator-led Discovery missions. The need to flight-test the newly developed Advanced Stirling Radioisotope Generator (ASRG) has opened the outer solar system to smaller missions. The most recent competition for Discovery missions allowed for the potential use of two ASRGs at no expense to the principal investigator. One of the three proposals selected for additional study was the Titan Mare Explorer (TIME), a lake lander. The potential selection of TIME and the possibility of future ASRG-powered Discovery missions to destinations in the outer solar system raise important questions. The one most relevant to this study concerns the compatibility between the financial and temporal constraints placed on the development and launch schedule of Discovery missions and the constraints placed by the potential implementation of complex planetary protection measures. See Appendix B.
1. National Research Council, Vision and Voyages for Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., 2011, pp. 11 and 75-78.
2. United Nations, Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies, U.N. Document No. 6347, Article IX, January 1967.
3. COSPAR, “COSPAR Planetary Protection Policy (20 October 2002; As Amended to 24 March 2011),” COSPAR, Paris, p. 1, available at http://cosparhq.cnes.fr/Scistr/PPPolicy%20(24Mar2011).pdf.
4. National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000.
5. National Research Council, A Review of Space Research, National Academy of Sciences, Washington, D.C., 1961, p. 10.9.
6. National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000, p. 23.
7. COSPAR, “COSPAR Planetary Protection Policy (20 October 2002; As Amended to 24 March 2011),” COSPAR, Paris, p. A1, available at http://cosparhq.cnes.fr/Scistr/PPPolicy%20(24Mar2011).pdf.
8. National Research Council, A Science Strategy for the Exploration of Europa, National Academy Press, Washington, D.C., 1999, p. 64.
9. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 5 and 196-199.
10. National Research Council, Vision and Voyages for Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., 2011, pp. 269-271.
11. Personal communication to the committee, Christopher Chyba, October 2011.
12. COSPAR Panel on Planetary Protection, COSPAR Workshop on Planetary Protection for Outer Planet Satellites and Small Solar System Bodies, European Space Policy Institute, Vienna, Austria, 2009.
13. COSPAR Panel on Planetary Protection, COSPAR Workshop on Planetary Protection for Titan and Ganymede, COSPAR, Paris, France, 2010.
14. COSPAR Panel on Planetary Protection, COSPAR Workshop on Planetary Protection for Titan and Ganymede, COSPAR, Paris, France, 2010, p. 30.
15. COSPAR. 1969. COSPAR Decision No. 16, COSPAR Information Bulletin, No. 50, pp. 15-16. COSPAR, Paris.
16. For a recent discussion of the concept of the period of biological exploration see, for example, National Research Council, Preventing the Forward Contamination of Mars, The National Academies Press, Washington, D.C., 2006, pp. 13-14, 17, 22-23, and 25.
17. National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000, pp. 2, 22, and 25.
18. COSPAR, “COSPAR Planetary Protection Policy (20 October 2002; As Amended to 24 March 2011),” COSPAR, Paris, p. A-1, available at http://cosparhq.cnes.fr/Scistr/PPPolicy%20(24Mar2011).pdf.
19. COSPAR, “COSPAR Planetary Protection Policy (20 October 2002; As Amended to 24 March 2011),” COSPAR, Paris, p. A-1, available at http://cosparhq.cnes.fr/Scistr/PPPolicy%20(24Mar2011).pdf.
20. Personal communication with committee, Christopher Chyba, October 2011.
21. C. Sagan and S. Coleman, Spacecraft sterilization standards and contamination of Mars, Astronautics and Aeronautics 3(5), 1965.
22. C. Sagan and S. Coleman, “Decontamination standards for martian exploration programs,” pp. 470-481 in National Research Council, Biology and the Exploration of Mars, National Academy of Sciences, Washington, D.C., 1966.
23. J. Barengoltz, A review of the approach of NASA projects to planetary protection compliance, IEEE Aerospace Conference, 2005, doi:10.1109/AERO.2005.1559319.
24. COSPAR, “COSPAR Planetary Protection Policy (20 October 2002; As Amended to 24 March 2011),” COSPAR, Paris, p. A-6, available at http://cosparhq.cnes.fr/Scistr/PPPolicy%20(24Mar2011).pdf.
25. N.R. Pace, A molecular view of microbial diversity and the biosphere, Science 276(5313):734-740, 1997.
26. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.
27. National Research Council, Vision and Voyages of Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., 2011.