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CHAPTER 26 THE OBJECTIVES AND TECHNOLOGY OF SPACECRAFT STERILIZATION LAWRENCE B. HALL Man's ability and intent to travel in space is increasing with irresistible force. The Apollo project will take man to the Moon. Instrumented flights will be the forerunners of manned flights to the planets. A most urgent, perhaps the most urgent, question to be answered by instrumented flights relates to the existence of life on the planets. An affirmative answer to this question has biological, medical and even religious implications that far transcend the results to be obtained by mere geographical and physical exploration of the planets. The major efforts of engineers and allied scientists who have made the opportunities of space flight possible must shortly turn from the struggle to obtain a successful flight to maximizing the use of a flight as a tool to obtain valuable scientific return. The first such missions, in which the requirements for successful scientific return may be more exacting than those of the flight itself, will be found in the first flights to Mars. In these flights, the necessity for preserving the planet as an uncontaminated subject for life detection experiments must take precedence, even at the expense of missing one or more launch opportunities. Other opportunities will follow, the geology and geography of the planet will remain for future examination, but any flight that infects Mars with terrestrial life will compromise forever a scientific opportunity of almost unequalled propor- tions. There will be no second chance. 463
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464 AVOIDING THE CONTAMINATION OF MARS Terrestrial life has survived and multiplied in simulated Martian environ- ments. Should it do so on Mars itself, it could result not only in competition with any Martian life, but in drastic changes in the geochemical and atmos- pheric characteristics of the planet. To avoid such a disaster, certainly the first, and probably many succeeding landers on Mars, must be sterile— completely devoid of life. Since the space environment will not in itself kill all life on board, the lander must leave the Earth in a sterile condition. Macroscopic life can be readily detected and kept or removed from the spacecraft, but the detection and removal of microscopic and submicro- scopic life is a much more difficult task. The National Aeronautics and Space Administration is approaching this problem in four stages: 1. Develop flight hardware that will withstand the sterilizing agent employed without significant loss of reliability; 2. Reduce the biological loading of the lander to a low level during manufacture and assembly; 3. Achieve surface and internal sterilization of the completely assem- bled lander; 4. Protect the spacecraft from recontamination during testing and launch. The development of hardware that will withstand sterilization is a major problem in itself. Sterilizing agents have been evaluated not only for their ability to kill both microbial life on surfaces and that sealed inside com- ponents, but for the agents' effect on spacecraft reliability as well. Of the available agents, only radiation and heat will penetrate to the interiors. Radiation is expensive, hazardous, complex and damages many materials more than does heat. Heat, therefore, has been selected as the primary method and will be used, except in specific cases where radiation may prove to be critically less detrimental to reliability. A qualification heat cycle has been established for materials and com- ponents at 145° C for 36 hours, repeated for three cycles. Following this heat application, the component must be proven to be capable of operating within established tolerances for a lifetime compatible with mission require- ments. Some spacecraft components are subjected to even higher tempera- tures during manufacture and test. A number of components are already being made of materials and in configurations that make them resistant to the maximum temperatures required. Other components, of which batteries and tape recorders are examples, cannot at present withstand the qualifi- cation cycles without subsequent failure. The problem of batteries has narrowed down largely to one of separators. The problem with recorders is largely one of the plastics used for belts and tapes. Based on results to date, no reason has been found to believe that a full complement of heat- sterilizable hardware cannot be available when needed. Every effort is
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Objectives and Technology of Spacecraft Sterilization 465 being made to improve the state-of-the-art to a point where spacecraft can not only withstand sterilization temperatures, but will be even more reliable than present state-of-the-art hardware that is not heated. Control of the quantity of biological loading on the spacecraft is neces- sary for the success of sterilization since the greater the number of organ- isms on the spacecraft, the longer heat must be applied, or the higher must be the temperature. Viable organisms can be kept from the spacecraft by the use of modern clean room methods. Engineers and mechanics under- standably are not anxious to work under surgical constraints, but recent data suggest that good vertical laminar flow clean room practice may limit the biological contamination to acceptable levels and concurrently increase reliability. In another decontamination technique now under study, components may be manufactured under normal factory conditions without special regard to contamination. Then before the component is used, it would be decontaminated, i.e., the biological population would be drastically re- duced, though not necessarily brought to zero, by the application of a modified heat cycle. Components decontaminated in this way and made into more complex assemblies would have to be handled under clean room conditions or the assembly must again be decontaminated. The adverse effect upon reliability of the application of extra heat cycles will vary with the materials being treated. Until these adverse effects can be fully evalu- ated, heat decontamination will not be considered as an operational concept. Those familiar with operational problems will point out that, despite clean assembly, the spacecraft will be heavily contaminated during neces- sary tests in facilities that cannot be kept up to clean room standards. This contamination, however, will be on surface only, not in interiors. It can therefore be reached by gaseous decontaminants such as ethylene oxide that will be used to reduce the viable contamination to a low level. By the use of clean techniques and decontamination by heat or ethylene oxide, it should be possible to bring a spacecraft to the point of sterilization with not more than 10s organisms on board. Sterility of any object is a concept that implies the complete absence of life. The presence of life, or the lack of sterility, may be proven, but the absence of life, or sterility, cannot be proven, for the one viable organism that negates sterility may remain undetected. Certification of sterility, based on experience with the process used, knowledge of the kinetics of the thermal death of organisms, and computation of the probability that no life exists on the spacecraft may be combined to produce the closest possible approach to sterility. The actual goal established for Mars landers is a probability of less than one in ten thousand, or 10~4, that a single organism will be on board.
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466 AVOIDING THE CONTAMINATION OF MARS Laboratory studies of the kinetics of heat kill of resistant organisms show that at 135° C the number of organisms can be reduced one logarithm (or 90 per cent of those remaining) for every two hours of exposure. The reduction in microbial count needed is the logarithm of the maximum number of organisms on the spacecraft—108, plus the logarithm of the reciprocal of the probability of a survivor—104, or a total of twelve loga- rithms of reduction in microbial count. Thus a total of twelve logarithms of reduction in count has been accepted as a safe value which can be achieved by a heat treatment of 135° C for 24 hours. This is the heat cycle that has been adopted and is being used in all studies. However, other heat treatments are under study at temperatures as low as 105° C for periods of time of 170 hours or longer. Such cycles, when developed to the extent that full reliance may be placed upon them to produce sterility, would provide the reliability engineer with a choice of heat treatments so that the cycle selected can be the least damaging to the reliability of the particular equipment involved. The possibility of error in the definition of an adequate heat cycle rests largely upon the possibility of radically increased heat resistance for organisms encased in solids. This potential problem is under investigation. Proposals have been made to insert into the sterilized lander, by aseptic techniques, heat-sensitive components that have been sterilized by other means. While this is recognized as a possibility, present National Aero- nautics and Space Administration policy will not permit its use, unless evidence can be developed proving the technique to have reliability com- parable with heat sterilization. Once sterilized, the lander must be protected from recontamination until it reaches outer space. Ideally, the spacecraft should be sterilized inside a hermetically-sealed canister and remain therein until the canister is opened in outer space. Other concepts would permit the spacecraft to be heated outside the canister. It would be encapsulated after sterilization by me- chanics working through glove ports set in the wall of the oven or by remote control actuators.