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Preventing the Forward Contamination of Mars 1 Introduction In its 2003 strategic plan the National Aeronautics and Space Administration (NASA) cites as one of its goals “to explore the universe and search for life” (NASA, 2003). The Mars science community’s Mars Exploration Program Analysis Group (MEPAG), in its 2004 report on scientific goals, objectives, investigations, and priorities for Mars exploration (MEPAG, 2004), and NASA’s Mars Science Program Synthesis Group (MSPSG), in its published Mars Exploration Strategy (MSPSG, 2004), both identify the search for present and past life on Mars as one of four overarching goals of Mars exploration. The scientific community and NASA have thereby endorsed the investigation of the hypothesis that life may exist on Mars or may have existed previously, and they have made testing this hypothesis one of Mars exploration’s primary goals.1 As stated in NASA’s Mars exploration strategy, “NASA is currently pursuing an aggressive, science-driven agenda of robotic exploration of Mars, with the aim of concluding the current decade of research with the first landed analytical laboratory on the martian surface since the Viking missions of the 1970s. This mobile science laboratory will propel Mars exploration into the next decade for which the search for evidence of biological activity is the ultimate goal” (MSPSG, 2004, p. 1). This search necessarily brings with it the requirements of planetary protection. Planetary protection depends on a set of policies and practices designed to prevent the contamination of celestial bodies by terrestrial microorganisms that could hitchhike on a spacecraft, survive the trip, and grow and multiply on a planet, moon, asteroid, or comet—forward contamination—and to prevent the potential for any putative extraterrestrial biota that might be returned to Earth on sample return missions to contaminate Earth—back contamination. Preventing the forward contamination of Mars is the subject of this report. The possibility of such planetary cross-contamination via spacecraft cannot be easily dismissed; experiments with bacterial spores on the European Retrievable Carrier (EURECA) and NASA’s Long Duration Exposure Facility (LDEF) space missions (Horneck et al., 1994, 1995) have demonstrated that spores2 of Bacillus subtilis survived 1 year in space at the 25 percent level and 6 years in space at the 1 percent level, respectively, provided 1 Similarly, President George W. Bush’s “New Vision for Space Exploration,” announced January 14, 2004, states that the robotic exploration of Mars is to be conducted “to search for evidence of life, to understand the history of the solar system, and to prepare for future human exploration.” See President’s Commission (2004). See also NASA (2004). 2 A spore is a tough, dormant form of certain bacterial cells that is especially resistant to desiccation, heat, and radiation. Spore-forming bacteria are common on Earth, but the vast majority of microorganisms are not spore formers.
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Preventing the Forward Contamination of Mars they were shielded from solar ultraviolet light, as would be the case inside a spacecraft. Typical Earth-Mars spacecraft trajectories take less than 1 year. Spacecraft assembled within highly controlled class-100,000 clean rooms have bacterial spore densities of ~103 spores per square meter on their surfaces (Barengoltz, 2004).3 Thus, it is virtually certain that, in the absence of special measures, a large number of still-viable microbes will be present on interior spacecraft surfaces at the time a spacecraft reaches Mars from Earth. The focus then shifts to whether there are environments on Mars in which such organisms might survive and reproduce, whether these environments will be accessed by the spacecraft, and the likelihood and implications of varying answers to these questions. Planetary protection policy addresses these issues and the measures that should be taken in response. POLICY BASIS FOR PLANETARY PROTECTION The idea of planetary protection emerged with the genesis of the space program. Scientific leaders were the early proponents of planetary protection,4 and in 1958 the U.S. National Academy of Sciences (NAS) passed a resolution stating, “The National Academy of Sciences of the United States of America urges that scientists plan lunar and planetary studies with great care and deep concern so that initial operations do not compromise and make impossible forever after critical scientific experiments.”5 The NAS resolution was brought to the International Council of Scientific Unions (ICSU, now known as the International Council for Science), which in 1958 created the ad hoc Committee on Contamination by Extraterrestrial Exploration (CETEX). CETEX met for about a year and provided the first guidance for planetary protection, including recommendations that interplanetary spacecraft be sterilized, and it further stated, “The need for sterilization is only temporary. Mars and possibly Venus need to remain uncontaminated only until study by manned ships becomes possible” (CETEX, 1959). CETEX also recommended that planetary protection be transferred to the newly formed multidisciplinary, international committee of the ICSU, the Committee on Space Research (COSPAR). COSPAR continues to serve as the international policy-making body on planetary protection, and it is a consultative body to the United Nations’ Committee on the Peaceful Uses of Outer Space (Cypser, 1993). Acting on the advice of its Consultative Group on Potentially Harmful Effects of Space Experiments, COSPAR in 1964 issued Resolution 26 (COSPAR, 1964, p. 26), which affirms that the search for extraterrestrial life is an important objective of space research, that the planet of Mars may offer the only feasible opportunity to conduct this search during the foreseeable future, that contamination of this planet would make such a search far more difficult and possibly even prevent for all time an unequivocal result, that all practical steps should be taken to ensure that Mars be not biologically contaminated until such time as this search can have been satisfactorily carried out, and that cooperation in proper scheduling of experiments and use of adequate spacecraft sterilization techniques is required on the part of all deep space probe launching authorities to avoid such contamination. 3 A class-100,000 clean room is defined as a clean room with 100,000 0.5-micron-diameter particles per cubic foot of atmosphere. Class-100,000 clean rooms typically require restricted access, positive pressurization, and perhaps other measures. Clean rooms with fewer atmospheric particles have more stringent requirements. Class-10,000 clean rooms are typical of hospital operating rooms; class-1,000 facilities are typical for making computer disk drives, and class-100 are typical for semiconductor and pharmaceutical manufacture. Class-10 and class-1 rooms also exist. Definitions of clean rooms are given in “Federal Standard 209E: Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones,” available at <www.zenobi.ethz.ch/Analytik5/USstandard.pdf>. Federal Standard 209E has been formally superseded by International Organization for Standardization (ISO) Standards in Metric Units; see “Cancellation of Fed-STD-209E,” available at <www.iest.org/publctns/fedstd209.htm>. However, U.S. usage still often refers to the imperial unit definitions. For a discussion of clean-room levels and requirements, see, for example, <www.dataclean.com>. 4 Letter from Joshua Lederberg, University of Wisconsin, to Detlev Bronk, President, National Academy of Sciences, December 24, 1957, with enclosed memorandum entitled “Lunar Biology?”, National Academy of Sciences, Records Office, Washington, D.C. 5 National Academy of Sciences, resolution adopted by the Council of the NAS, February 8, 1958. Addendum to Minutes of the Meeting of the Council of the National Academy of Sciences, February 8, 1958.
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Preventing the Forward Contamination of Mars THE OUTER SPACE TREATY Language related to planetary protection was incorporated into Article IX of the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies (known as the Outer Space Treaty), which entered into force in 1967: States 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 the Earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose.6 The United States signed and ratified the Outer Space Treaty in 1967, and so is legally bound by the treaty’s requirement to avoid harmful contamination of the Moon and other celestial bodies. The treaty was the second so-called non-armament treaty of the Cold War and was in some respects modeled on its predecessor, the Antarctic Treaty, which entered into force in 1961 (U.S. Department of State, 2004). Article IX of the Antarctic Treaty called for states that are parties to the treaty to recommend measures to further “the preservation and conservation of living resources in Antarctica.” Article IX of the Outer Space Treaty, however, is ambiguous with respect to whether its focus is on protecting celestial bodies themselves or the scientific interests of those countries exploring them. A policy review of the Outer Space Treaty concluded that, while Article IX “imposed international obligations on all state parties to protect and preserve the environmental integrity of outer space and celestial bodies such as Mars,” there is no definition as to what constitutes harmful contamination, nor does the treaty specify under what circumstances it would be necessary to “adopt appropriate measures” or which measures would in fact be “appropriate” (Goh and Kazeminejad, 2004, p. 219). An earlier legal review, however, argued that “if the assumption is made that the parties to the treaty were not merely being verbose” and “harmful contamination” is not simply redundant, “harmful” should be interpreted as “harmful to the interests of other states,” and since “states have an interest in protecting their ongoing space programs,” Article IX must mean that “any contamination which would result in harm to a state’s experiments or programs is to be avoided” (Cypser, 1993, pp. 324-325). Both reviews, and their interpretations, are unofficial. Current NASA policy states that the goal of NASA’s forward contamination planetary protection policy is the protection of scientific investigations (Rummel and Billings, 2004), declaring explicitly that “the conduct of scientific investigations of possible extraterrestrial life forms, precursors, and remnants must not be jeopardized” (NASA, 1999). This has been the approach taken by COSPAR for the past four decades, most obviously with respect to its idea of a finite “period of biological exploration” beyond which planetary protection measures need not extend.7 Consistent with this approach, the protection of the ability to perform scientific measurements without confounding them with false positives has been the focus of past National Research Council (NRC) examinations of forward contamination planetary protection policy for Mars. In its 1992 report Biological Contamination of Mars: Issues and Recommendations, the Space Studies Board (SSB) Task Group on Planetary Protection emphasized that “the philosophical intent of the 1978 committee [the SSB committee that had previously addressed the topic] to protect Mars from terrestrial contamination so as not to jeopardize future life-detection experiments on Mars is still profoundly important” (NRC, 1992, p. 57). The present committee’s statement of task reads in part, “To the maximum possible extent, the [committee’s] recommendations should be developed to be compatible with an implementation that will use the regulatory framework for planetary protection currently in use by NASA and COSPAR,”8 which the committee understood as 6 The full text of the treaty, its membership, and related documents such as the Antarctic Treaty appear in Rauf et al. (2000), pp. 138-142 (treaty text) and pp. 241-247 (membership). 7 The period of biological exploration is referred to as either a defined number of years or the time to completion of a series of robotic missions to, or experiments on, Mars, during which strict planetary protection practices must be followed to protect the planet for the conduct of scientific investigations, including the search for life. 8 See the committee’s statement of task, reproduced in the Preface to this report.
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Preventing the Forward Contamination of Mars a call for it to restrict its formal recommendations, to the maximum extent possible and consistent with the mandate of previous SSB committees, to the goal of protecting current and future scientific investigations. Scientific investigations could in principle be jeopardized either by biological contaminants carried on a spacecraft intended to perform certain life-detection or life-related experiments itself, or by the establishment and growth of terrestrial organisms on Mars that could then interfere with scientific investigations of subsequent missions to the planet. Certain life-detection techniques could also be jeopardized by the delivery to Mars of nonliving material, either remnants of organisms or, possibly, chemicals that were not biological in origin. Because Mars and other celestial bodies are “not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means” (Article II of the Outer Space Treaty), planetary protection is inherently international and a matter of concern for all existing and future spacefaring nations. PROTECTING SCIENCE AND PROTECTING MARS The committee is aware that some in the scientific community have increasingly voiced concerns about ensuring the environmental integrity of other celestial bodies, aside from protecting scientific investigations for their own sake. In particular, the SSB Task Group on the Forward Contamination of Europa noted in its report Preventing the Forward Contamination of Europa that “future spacecraft missions to Europa must be subject to procedures designed to prevent its contamination by terrestrial organisms. This is necessary to safeguard the scientific integrity of future studies of Europa’s biological potential and to protect against potential harm to europan organisms, if they exist, and is mandated by obligations under the [Outer Space Treaty]” (NRC, 2000, p. 13). That is, the Europa task group declared that in europan planetary protection policy, protection of europan organisms was as important as protection of scientific studies. Virtually identical concerns about protecting a possible europan biosphere had been articulated by the SSB Committee on Planetary and Lunar Exploration (NRC, 1999, p. 72) in its report A Science Strategy for the Exploration of Europa. The present committee considered at length the importance of protecting scientific research at Mars for the period of biological exploration versus protecting the planet Mars, potentially in perpetuity. Although an assessment of the protection of planet Mars goes beyond the statement of task for this study, the committee was conscious of the fact that recent Mars exploration has revealed a planet that now appears more potentially hospitable to terrestrial microbes than was envisioned by the 1978 or 1992 NRC studies. Whether the predominant rationale for planetary protection is to protect the science or to protect the planet involves differing viewpoints and significant uncertainties that are likely to continue for many years. The committee considered both aspects of planetary protection, although the committee’s statement of task, and thus its data gathering, deliberations, and conclusions, centered on the current COSPAR policy of protecting the planet for science. Human Missions to Mars In January 2004, President George W. Bush announced “A Renewed Spirit of Discovery: The President’s Vision for U.S. Exploration,” which set the goal of human exploration of the Moon, Mars, and the solar system.9 That vision underscored the importance of the Mars robotic program as a testbed for demonstrating technological capabilities that are “key to enabling future human Mars missions.” Although the committee’s charge focuses on preventing forward contamination of Mars by varied future spacecraft missions and activities, “including orbiters, atmospheric missions, landers, penetrators, and drills,”10 upcoming exploration by human missions seems likely in light of the national vision for space exploration put forth by President Bush and the subsequent Moon-Mars initiative (President’s Commission, 2004; NASA, 2004). Future human or robotic-aided human missions are likely to present significant forward contamination challenges in both planning and implementation. 9 See President’s Commission on Implementation of the United States Space Exploration Policy (2004). 10 See the charge to the committee in the Preface to this report.
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Preventing the Forward Contamination of Mars Human missions will inevitably introduce considerations that go beyond those covered by the forward contamination controls and policies discussed in this report. Furthermore, they are likely to involve examination of COSPAR policies and questions about minimizing potential contamination that could be introduced through human operations, exploration, construction, sampling, and sequencing of activities. Today, there are no official COSPAR or NASA policies encompassing forward contamination of solar system bodies during human missions. Although significant study will be necessary before planning and implementing contamination controls for human missions, the committee recognizes that planetary protection considerations will be important in all phases of future missions, whether robotic or human. The committee notes that previous NRC reports—Biological Contamination of Mars: Issues and Recommendations (NRC, 1992) and Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface (NRC, 2002)—have addressed human missions to Mars and have concluded that information from precursor robotic missions is critical for planning safe, productive human missions that will have a minimal impact on Mars. In anticipating the long-term potential for expansion of human activities on Mars, it may be prudent to consider forward contamination policies in the context of analogous policies for sensitive environments on Earth, such as the international treaty governing Antarctica.11 Like the Outer Space Treaty, the Antarctic Treaty calls for peaceful use for humanity, freedom of scientific investigations, and international cooperation. The Antarctic Treaty also specifically calls for the preservation and conservation of living resources in Antarctica. Examination of the administrative oversight and controls imposed on research and activities in polar areas, such as the designation of special regions, requirements for waste disposal and cleanup, and reversibility of human actions, may be useful in developing a framework for addressing concerns related to forward contamination by human missions. The committee does not, however, take a position on whether human missions to Mars will or will not necessarily broadly contaminate the martian surface with terrestrial microorganisms—a topic that will require extensive study and possibly research and development (R&D). Implementation of Planetary Protection Translating planetary protection into actual practice involves a complex mix of intertwined policies, elements of science and engineering, uncertainties, and implementation protocols. For past robotic missions to Mars, forward contamination controls have included the requirement to reduce the biological contamination of the spacecraft, constraints on spacecraft operating procedures, and inventories of organic constituents of the spacecraft and organic samples, along with documentation of spacecraft operations, impact potential, and the location of landing or impact points on the planetary surface. Upon request, the SSB has provided advice to NASA about planetary protection (see Appendix B). In 1978, the SSB produced Recommendations on Quarantine Policy for Mars, Jupiter, Saturn, Uranus, Neptune, and Titan, which considered planetary protection for several bodies (NRC, 1978). In 1992, the SSB issued Biological Contamination of Mars: Issues and Recommendations (NRC, 1992), while other reports have considered sample return and back contamination. SSB reports have provided scientific input that has been used to update COSPAR policies as well as practices that must be implemented to meet those policies. COSPAR maintains and issues policy guidance on planetary protection to the international space science community. NASA and other national space agencies adhere to COSPAR planetary protection policy to avoid the contamination of extraterrestrial bodies. NASA’s Planetary Protection Office provides the implementation requirements for planetary protection for NASA’s planetary exploration program, including the Mars Exploration Program. The methods and practices of planetary protection developed during the 1970s for the Viking mission set the standard operating procedures that are still in use today. Current planetary protection practices and their historical development are discussed in detail in Chapter 2. Important terminology in the planetary protection lexicon is defined in Box 1.1. 11 The Antarctic Treaty, December 1, 1959. For text, see Rauf et al. (2000), pp. 132-135. For a historical account of the treaty, see NRC (1993).
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Preventing the Forward Contamination of Mars BOX 1.1 Planetary Protection Terminology The practices of detecting, cleaning, and reducing the bioburden presented by microorganisms on spacecraft are described with a particular set of terms that is used in discussing planetary protection. Some of these practices and terms stem from the Viking Lander program, during which researchers conducted extensive studies on spacecraft cleaning methods and spacecraft sterilization that have continued to serve as the basis for planetary protection requirements (see Chapter 2). Other terms refer to probabilistic approaches that have been used in the past to establish requirements for the cleanliness of spacecraft. Assay: an experimental analysis, usually involving sampling techniques, used to derive data on which to base an estimate of the number or kind of microorganisms associated with an item of interest. Bioburden: level of microbial contamination (total number of microbes or microbial density) in or on an item of interest. Bioburden reduction (also known as microbial reduction): reduction by any qualified process (temperature, chemical, radiative, or combinations thereof) of the number of organisms on spacecraft or components to a specified level. Committee on Space Research (COSPAR): the international body responsible for formulating policies in accordance with the Outer Space Treaty; it is a committee of the International Council of Scientific Unions (ICSU; now called the International Council for Science). Dry-heat cycle (also known as baking): the only NASA-certified method for reduction on an entire spacecraft and the preferred method for bringing spacecraft to sterile or near-sterile conditions; involves a heat cycle using prescribed temperature, pressure, gas, and humidity conditions for a specified length of time. Encapsulated (embedded) bioburden: bioburden buried inside nonmetallic spacecraft material. Forward contamination: contamination by biological or other organic material carried on outbound spacecraft to celestial bodies that may jeopardize the conduct of scientific investigations of possible extraterrestrial life forms, both extinct and extant. PAST DELIVERY OF MICROORGANISMS TO MARS The committee considered the implications for planetary protection requirements of past natural and mission-associated delivery of Earth microorganisms to Mars. An extreme viewpoint would be that because some past missions have already likely delivered significant quantities of microorganisms to Mars, and because Mars experiences substantial windblown transport of dust, there is no longer any point in continuing planetary protection practices. All past missions that have landed or crashed on Mars (even the rigorously heat-sterilized Viking missions) have virtually certainly delivered some viable microorganisms to the martian surface. Table 1.1 displays the outcome of all missions of all nationalities sent from Earth to Mars; it also notes which of these missions failed and which crashed onto the martian surface. Soviet planetary protection measures were judged by the U.S. planetary protection officer in 1972 to “approximate compliance with COSPAR constraints,” assuming that the Soviet space
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Preventing the Forward Contamination of Mars Mated bioburden: microbial burden associated with spacecraft surfaces that have been joined with fasteners rather than adhesives (which embed bioburden when surfaces are joined) during the spacecraft assembly process. Period of biological exploration: a period referred to as either a defined number of years or the time to completion of a series of robotic missions to, or experiments on, Mars, during which strict planetary protection practices must be followed to protect the planet for the conduct of scientific investigations, including the search for life. Probability of contamination (Pc): the probability that a mission will contaminate a planet, calculated according to a formulaic approach based on measurements of bioburden at launch, combined with microorganisms’ likely survival in space, release onto the planet, and growth in the new environment. Probability of growth (Pg): the probability that a terrestrial microbe on a spacecraft delivered to an extraterrestrial body will grow and reproduce in that environment. Special region: a specially designated region on Mars—currently defined by COSPAR as “a region within which terrestrial organisms are likely to propagate, or a region which is interpreted to have a high potential for the existence of extant martian life forms.” Currently applied to regions where liquid water is present or may occur. Viking pre-sterilization: treatment to a level of cleanliness based on cleaning or sterilizing, or both, the spacecraft and its component parts in a manner such that the density of culturable microbial spores is less than 300/m2 on the spacecraft surface and the total number on the launched spacecraft is not greater than 3 × 105. Viking post-sterilization: treatment to a level of cleanliness of an assembled spacecraft accomplished by using a final sterilization process (e.g., dry heat) to reduce the Viking pre-sterilization levels of microbes by 4 orders of magnitude, resulting in a total of no more than 30 culturable microbial spores on the surface of the launched spacecraft. program “did, or will, carry out the measures described.”12 Neither the U.S. Pathfinder or Mars Polar Lander, nor the Mars Exploration Rover (MER) missions were subject to the dry-heat sterilization of the Viking missions. Missions that have crashed on the martian surface, such as the Mars Polar Lander mission launched in 1999, are likely not only to have exposed the martian environment to some interior surfaces, but also to have released some of their embedded bioburden, due to ruptures in spacecraft materials. The likelihood of past delivery of spacecraft microbial material to Mars does not vitiate ongoing planetary protection measures. The prospects for the forward contamination of Mars (which requires microorganism survival 12 Lawrence B. Hall, “Analysis of the Planetary Quarantine Effort in the U.S.S.R.,” memorandum to Associate Administrator for Space Science [date obscured; believed to be January 1972, the date of an associated memorandum] and enclosures.
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Preventing the Forward Contamination of Mars TABLE 1.1 History of Successes and Failures of Mars Missions Mission Outcome Mission Type Country Launch Date Success Failure Comments Unnamed Mars Mission Flyby USSR 10-24-62 X Interplanetary stage failure; main engine turbopump exploded Mars 1 Flyby USSR 11-01-62 X Lost in space due to antenna pointing problem Sputnik 24 Flyby USSR 11-04-62 X Launch sequence failure Mariner 3 Flyby USA 11-05-64 X Launch sequence failure Mariner 4 Flyby USA 11-28-64 X Zond 2 Flyby USSR 11-30-64 X Flew by Mars without returning any data Mariner 6 Flyby USA 02-24-69 X Mariner 7 Flyby USA 03-27-69 X Unnamed Mars Mission Orbiter USSR 03-27-69 X Proton third-stage failure Unnamed Mars Mission Orbiter USSR 04-02-69 X Proton first-stage failure Mariner 8 Orbiter USA 05-08-71 X Launch sequence failure Kosmos 419 Orbiter USSR 05-10-71 X Upper stage failure Mars 2 Orbiter/Lander USSR 05-19-71 X Successful entry, descent, landing (EDL), but crash landed without returning any data Mars 3 Orbiter/Lander USSR 05-28-71 X Descent module instruments transmitted for 20 seconds after landing; then ceased transmitting Mariner 9 Orbiter USA 05-30-71 X Mars 4 Orbiter/Lander USSR 07-21-73 X Flew past Mars due to orbit rocket failure Mars 5 Orbiter/Lander USSR 07-25-73 X Entered orbit but failed several days later Mars 6 Orbiter/Lander USSR 08-05-73 X Successful EDL but failed with terminal rocket ignition Mars 7 Orbiter/Lander USSR 08-09-73 X Failed due to pre-Mars separation of orbiter and lander Viking Orbiter 1 Orbiter USA 08-20-75 X Viking Lander 1 Lander USA 08-20-75 X Viking Orbiter 2 Orbiter USA 09-09-75 X Viking Lander 2 Lander USA 09-09-75 X Mars Observer Orbiter USA 09-25-92 X Probable failure of propellant line due to hypergolic propellant contamination Mars Global Surveyor Orbiter USA 11-07-96 X Mars ’96 Orbiter/Landers/ Penetrators USSR 11-17-96 X Launch sequence failure Mars Pathfinder Lander/Rover USA 12-04-96 X Mars Climate Orbiter Orbiter USA 12-11-98 X Disintegrated in the atmosphere due to navigation error Mars Polar Lander Lander USA 01-03-99 X Crashed due to premature shutdown of retrorocket engines Mars DS-2 (renamed Amundsen) Penetrator USA 01-03-99 X Unknown EDL failures; surface communication system malfunction suspected Mars DS-2 (renamed Scott) Penetrator USA 01-03-99 X Unknown EDL failures; surface communication system malfunction suspected
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Preventing the Forward Contamination of Mars Mission Outcome Mission Type Country Launch Date Success Failure Comments Mars Odyssey Orbiter USA 04-07-01 X Mars Express Orbiter ESA 06-02-03 X Beagle 2 Lander UK 06-02-03 X Unknown EDL failure; excessive impact velocity suspected MER (Spirit) Lander/Rover USA 06-10-03 X MER (Opportunity) Lander/Rover USA 07-07-03 X Total Successes/Failures 15 21 Overall Success Rate 42% NASA Successes/Failures 13 7 NASA Success Rate 65% NASA Lander Successes/Failures 5 3 NASA Lander Success Rate 63% SOURCES: Data for missions launched before 1992 were taken from NASA (1991), Siddiqi (2002), and <http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1971-049A>, accessed on November 15, 2005. Data for all subsequent missions were obtained from NASA Web sites. and growth) are inherently probabilistic. Even if each previous mission did have some probability of having contaminated Mars, those probabilities were likely small (see Chapters 4 and 5), so that care with subsequent missions is still important for keeping at a low level the probability of contaminating Mars summed over all missions. The committee illustrates this concept with an analogy: even if the campfires of a dozen campers have previously posed the risk of a forest fire, it is still important that subsequent campers extinguish their campfires properly.13 Unless a previous mission has delivered microbes to an environment in which they can reproduce and geographically expand via the martian subsurface, existing experimental evidence for the survival of microorganisms at the surface of Mars suggests that the contamination resulting from these missions is likely to be at most local. More than 30 research papers have been published reporting experimental results for microbial survival under simulated martian conditions, with inconsistent results. Only recently have such experiments been conducted in a Mars simulation chamber that permitted good simulation of the pressure, temperature, atmospheric composition, and ultraviolet (UV)-visible-infrared light environment at Mars (Schuerger et al., 2003). These experiments showed that B. subtilis spores were rapidly (timescales of hours at most) killed even when partly shielded against UV light by being covered with simulated martian dust particles up to 50 microns in diameter. Viable spores were significantly reduced after an 8-h period even when covered by a 0.5-mm contiguous dust layer. Experiments with the dessication-tolerant, endolithic cyanobacterium Chroococcidiopsis sp. 029 (Cockell et al., 2005) showed survival for this organism, when exposed to martian-simulated UV, about 10 times higher than that previously reported for B. subtilis, but there was still a 99 percent loss of cell viability after 5 minutes. However, if protected by 1 mm of rock, Chroococcidiopsis sp. could survive and potentially grow, if water and nutrient requirements for growth were met. It appears likely that most microorganisms exposed to the martian UV environment and unable to gain access to the martian subsurface will rapidly die. Moreover, because windblown dust particles on Mars have diameters in the range of 1 to 2 microns, transport via dust particles is also likely to lead to rapid death, and so windblown transport of microorganisms on Mars seems unlikely to contaminate distant parts of Mars. 13 The committee called this the “Smokey the Bear” argument for ongoing planetary protection.
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Preventing the Forward Contamination of Mars In addition, the committee considered that Mars and Earth likely exchange meteorites in a size range sufficiently large to protect microorganisms against exposure to solar UV and some cosmic rays during travel in interplanetary space, but small enough to allow soft landings after deceleration in the atmosphere.14 A careful though necessarily speculative treatment of this problem suggests that ~1011 to 1012 viable bacteria may have been delivered to Mars in such Earth-originating meteorites (Mileikowsky et al., 2000). Some extremely small fraction of such meteorites is expected to complete the Earth-Mars trajectory within years, but most will take ~105 to 106 years for the journey. While the estimates of Mileikowsky et al. (2000) for total viable bacteria delivered to Mars endeavor to account for mortality over these long timescales, the estimates necessarily represent extrapolations of 4 to 5 orders of magnitude beyond actual data (obtained from the Long Duration Exposure Facility or European Retrievable Carrier) for survival of microorganisms in the space environment. Moreover, delivery by meteorite to one or another spot on the martian surface is not the same as delivery as a result of a spacecraft landing intended to access martian regions where liquid water is especially likely to exist. The search for liquid water is in fact a high priority in current Mars exploration (see Chapter 3). Despite past mission-associated and natural delivery of microorganisms to Mars, the committee concluded that the challenge of planetary protection cannot be put aside in upcoming Mars exploration. The remainder of this report first gauges the magnitude of that challenge and then recommends how NASA should address it. ISSUES IN AND ORGANIZATION OF THIS REPORT Chapter 2 presents a detailed account of current planetary protection policies and policy implementation, establishing a baseline of current practices against which the requirements of future exploration can be measured. There are four broad reasons that current policies should be reconsidered and updated: (1) an extensive planned series of missions to Mars, (2) new information about the surface of Mars relative to life, (3) new findings regarding microorganisms on Earth, and (4) advances in technologies relevant to life detection and bioburden reduction that make improved approaches possible. Chapter 3 describes the planned Mars exploration strategy now envisioned for the coming decades. These missions will target regions (including subsurface regions) of Mars that are especially likely to harbor liquid water, an approach that emphasizes the growing challenges that the “rolling wave” of upcoming missions could pose for planetary protection. Based largely on Mars exploration missions to date, scientific understanding of Mars and the prospects for liquid water and potentially habitable environments there is evolving rapidly. Questions regarding the environments and conditions in which liquid water may be present on Mars, which are discussed in Chapter 4, are central to the prospects for and potential habitats of life on Mars, as well as the prospects for forward contamination. Prospects for the forward contamination of Mars depend both on the nature of the Mars environment and on the conditions in which Earth microorganisms can survive and grow. Chapter 5 describes the rapid growth in knowledge of the limits of life on Earth, with emphasis on the state of scientific research on microbial survival in Earth environments that in some ways approximate those that may exist on Mars. Chapter 6 describes the new molecular technologies that now permit a far greater understanding of the numbers and nature of microorganisms present on spacecraft bound for Mars. Many of these technologies have been proposed recently for use in detecting life in Antarctica, one proposed Earth analog to Mars (NRC, 2003). The application of modern techniques to assaying spacecraft bioburden would permit planetary protection measures to focus on the small number of microorganisms of greatest concern. New methods can also be applied to limit the viable bioburden on spacecraft before launch. These methods are crucial for evaluating the prospects for the future 14 The committee notes, however, that the “contamination” of concern in planetary protection (see Chapter 1) refers strictly to contaminants carried on spacecraft. The natural exchange between Earth and Mars of microorganisms in meteorites, whatever its magnitude, does not constitute “contamination” from the point of view of planetary protection. [Editor’s note—This footnote was approved and added after release of the prepublication copy of this report.]
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Preventing the Forward Contamination of Mars application of either more rigorous or more selective bioburden reduction techniques. Such potential new methodologies for bioburden reduction are also discussed in Chapter 6. Many of the same molecular detection technologies that will permit development of a detailed understanding of spacecraft bioburden can also be used to conduct extremely sensitive searches for life on Mars. Some of these searches could be confounded by nonliving terrestrial contaminants, such as the remains of dead microorganisms that were carried on spacecraft. Chapter 7 discusses the need to detect and limit nonliving contaminants of concern. Finally, in Chapter 8, drawing on the information in Chapters 1 through 7, the committee presents its findings and recommendations. Chapter 9 concludes the report by providing a roadmap for the implementation of these recommendations. REFERENCES Barengoltz, J. 2004. Planning for project compliance. 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Representative terms from entire chapter: