Planetary Protection Policies
Planetary protection concerns were first raised in 1956 in a discussion of the nascent field of space law at the International Astronautical Federation’s 7th Congress, held in Rome.1 Planetary protection became an issue for the scientific community in December 1957 when Joshua Lederberg wrote a letter to the National Academy of Sciences. He was concerned that it would not be possible to test the panspermia hypothesis for the origin of life if the Moon became contaminated with terrestrial organic matter as a result of spacecraft missions.
The Moon was soon found to be barren, however, and interest in panspermia waned.2 Nevertheless, the importance of preserving extraterrestrial environments from terrestrial biological and organic contamination was generally recognized and ultimately enshrined in resolutions issued by the Committee on Space Research (COSPAR) of the International Council for Science and in the provisions of the United Nations Outer Space Treaty.3
As a signatory to the Outer Space Treaty, the United States is obliged to “. . . pursue studies of outer space, including the moon and other celestial bodies . . . so as to avoid their harmful contamination. . . .”4 To abide by the treaty’s imperatives, NASA has developed detailed planetary protection procedures in accordance with general requirements outlined in recommendations in various reports provided by the Space Studies Board. NASA implementation plans are then submitted to COSPAR for that organization’s approval in its de facto role as the international court of scientific opinion with respect to the Outer Space Treaty’s noncontamination policies.
It is generally acknowledged that minimizing the risk of forward contamination is motivated by two different imperatives.5The firt is preservation of the scientific integrity of the planetary body under study. That is, terrestrial organisms introduced into an extraterrestrial environment may cause false positives in life-detection experiments and may generally impede the study of indigenous life, if it exists. The second imperative is to preserve and protect indigenous organisms from possible harm by introduced terrestrial life.
For much of the history of the implementation of planetary protection regulations, the protection of future scientific experiments has been assigned the most weight in determining planetary protection requirements. Indeed, planetary protection policies have centered on the concept of a period of biological exploration, during which particular planetary bodies are accorded protection from contamination so that studies of their biological potential can proceed unhindered by terrestrial contamination. Following the expiration of this exploration period, contamination controls are then relaxed or abandoned altogether.
Over the past 15 years, however, ethical and philosophical papers have been published on the rights of alien beings, no matter how simple those beings. Combined with the emergence of environmental and animal-rights groups, this is a potential area for future debate. Indeed, a 1992 report of the National Research Council’s (NRC’s) Space Studies Board, Biological Contamination of Mars: Issues and Recommendations,6 recognized these issues and emphasized the need to encourage public discussion and dissemination of information concerning the steps taken to prevent planetary cross-contamination.
Mars has been the focus of the search for extraterrestrial life for most of the last 40 years. As such, the development and implementation of planetary protection policies have evolved in close concert with the evolution of our understanding of the martian environment and its biological potential. Before the Viking missions of the mid-1970s, the severity of the martian environment was not completely known. The subsequent improvement in understanding is reflected in the fact that the value adopted for the probability of growth of imported terrestrial microbes on Mars (Pg) used in the probabilistic approach to contamination control applicable at that time fell from 1 in 1964 to 10-10 in 1978.
The clarification of the biological potential of the martian surface had a major impact on planetary protection policies for Mars and, by extension, other solar system bodies. NASA’s current planetary protection requirements and those for Mars, in particular, derive from a policy adopted at COSPAR’s 25th General Assembly, held in Graz, Austria, in 1984,7 as refined in a 1992 NRC report on the topic.8 The key feature of COSPAR’s 1984 policy was the abandonment of the quantitative, statistical approach used in the Viking era and the adoption of a simpler, more straightforward methodology based on the type of mission (e.g., flyby, orbiter, lander, or sample return) and the degree to which the mission ’s destination is of interest to the process of chemical evolution.
The 1992 NRC report refined the COSPAR approach by drawing a distinction between Mars missions that carry instruments designed to search for evidence of life and those that do not carry them. Since terrestrial organisms are unlikely to grow on the martian surface, the report argued, they do not pose a significant contamination hazard. They could, however, confound the results from life-detection experiments. Thus, the report recommended that landers carrying instrumentation for in situ investigation of extant martian life “should be subject to at least Viking-level sterilization procedures” (see Box 1.1).9 Orbiters and landers without biological experiments, on the other hand, “should be subject to at least Viking-level presterilization procedures—such as clean-room assembly and cleaning of all components —for bioload reduction, but such spacecraft need not be sterilized. ”10 The NRC’s recommended distinction between Mars landers with and without life-seeking experiments was later codified and adopted by COSPAR.11 ,12
NASA’s implementation of these policies, described in Planetary Protection Provisions for Robotic Extraterrestrial Missions, involves adherence to the following procedures:13
Spacecraft that fly by or enter orbit around Mars are subject to planetary protection requirements designed to control contamination and to reduce the risk that spacecraft or its boosters will impact the planet. This is achieved by assembling the spacecraft in clean rooms rated at Class 100,000 or better (i.e., less than one particle in the size range 1 mm to 0.001 µm for every 100,000 cubic feet of air) and by ensuring that the probability of impact by the launch vehicle and the flyby spacecraft does not exceed 10 -4and 10-2, respectively. The lifetime of an orbiter must be such that it remains in orbit for a period in excess of 20 years from launch, and the probability of impact for the next 30 years must be no more than 0.05. If the lifetime requirements cannot be met, then the surface microbial bioburden must meet the Viking presterilization limit. Following bioassay, such spacecraft must be protected against recontamination.
Spacecraft that land on Mars but are not equipped with life-detection experiments are subject to planetary protection requirements designed to control the lander’s bioburden and to prevent accidental impact by hardware not intended to land. The total probability of any accidental impacts by any hardware other than the lander must be no more than 10-4. Bioburden control involves assembly in a Class 100,000 or better clean room, periodic microbiological assays, and maintenance of hardware cleanliness. Bioburden reduction to the Viking presterilization level is required. The mission team is also required to inventory, document, and archive samples of organic compounds used in the construction of the lander and associated hardware that might accidentally impact the planet. Finally, the locations of landing sites and impact points must be determined as accurately as possible, and the condition of the hardware at each site must be estimated to assist in determining the potential spread of organic compounds.
Spacecraft that land on Mars and are equipped with life-detection experiments are subject to all of the requirements outlined above and must, in addition, undergo a Viking-level sterilization process.
Although the bioburden reduction employed for all types of landers may be measured by any microbiological assay, it is incumbent on the project to prove the equivalency between its assay and that employed on Viking. Moreover, no allowance can be made for burden reduction in flight or associated with surface conditions on Mars.
The central question to be addressed in this report concerns the degree to which Europa can be incorporated into the planetary protection framework developed in light of 40 years of experience with the exploration of Mars. In other words, is our current knowledge of Europa and its ability to sustain terrestrial organisms analogous to our understanding of martian conditions before the Viking missions or after them? In an attempt to answer this question, Chapter 2 focuses on current understanding of Europa and Chapter 3 discusses the limits of terrestrial life.
BOX 1.1 Viking’s Approach to Bioload Reduction
Viking employed a twofold approach to controlling the population of terrestrial organisms that might find their way to Mars. First there was a careful cleaning of the spacecraft, and then the bioload was reduced still further by heat sterilization.
The Viking landers were assembled in Class 100,000 clean rooms. During assembly, thousands of microbial assays were conducted, and these established that the average spore burden per square meter was less than 300 and the total burden of spores on the lander’s surface (i.e., the exposed exterior and those parts of the interior communicating directly with the exterior) was less than 300,000.14 The spore-forming microbe Bacillus subtilis was used as the indicator organism in the microbiological assays on the basis of its enhanced resistance to heat, desiccation, and radiation.
Once the landers had been assembled and sealed inside their bioshields, their bioload was further reduced by dry heating. The landers were heated at a humidity of 1.3 mg/liter such that at the coldest point a temperature of 111.7 °C was maintained for some 30 hours. In other words, much of the lander was subject to a higher temperature for a longer period of time. Once sterilized, the landers were no longer accessible for additional microbial assays. Thus the efficacy of the sterilization procedure was estimated indirectly on the basis of the known heat-survival characteristics of B. subtilis and was credited with reducing the lander’s bioburden by a factor of 104.
1 A.G. Haley, “Space Law and Metalaw—A Synoptic View,” Proceedings of the VIIth International Astronautical Congress, Rome, Italy, 1956.
2 For a recent discussion of panspermia see, for example, Paul Davies, “Interplanetary Infestations,” Sky & Telescope, September 1999, page 33.
3 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, January 1967.
4 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, January 1967.
5 See, for example, L.B. Hall and R.G. Lyle, “Foundations of Planetary Quarantine,” L.B. Hall (ed.), Planetary Quarantine, Gordon and Breach, New York, N.Y., 1971, page 5.
6 Space Studies Board, National Research Council, Biological Contamination of Mars: Issues and Recommendations, National Academy Press, Washington, D.C., 1992.
7 D.L. DeVincenzi and P.D. Stabekis, “Revised Planetary Protection Policy for Solar System Exploration, ” Advances in Space Research 4: 291, 1984.
8 Space Studies Board , National Research Council, Biological Contamination of Mars: Issues and Recommendations, National Academy Press, Washington, D.C., 1992.
9 Space Studies Board, National Research Council, Biological Contamination of Mars: Issues and Recommendations, National Academy Press, Washington, D.C., 1992, page 47.
10 Space Studies Board, National Research Council, Biological Contamination of Mars: Issues and Recommendations, National Academy Press, Washington, D.C., 1992, page 47.
11 D.L. DeVincenzi, P. Stabekis, and J. Barengoltz, “Refinement of Planetary Protection Policy for Mars Missions,” Advances in Space Research 18: 311, 1996.
12 COSPAR, Decision No. 1/94, COSPAR Information Bulletin 131, 1994, page 30.
13 National Aeronautics and Space Administration, Office of Space Science , Planetary Protection Provisions for Robotic Extraterrestrial Missions, NPG 8020.12B, Washington, D.C., 1999.
14 Viking '75 Project, Pre-launch Analysis of Probability of Planetary Contamination, Volumes II-A and II-B, M75-155-01 and M75-155-02, Jet Propulsion Laboratory, Pasadena, Calif., 1975.