SPACECRAFT STERILIZATION

The search for extraterrestrial life with unmanned space probes requires the total sterilization of the landing capsule and its contents. Scientists agree that terrestrial organisms released on other planets would interfere with exobiological explorations (refs. [ref.36]-[ref.43]). Any flight that infects a planet with terrestrial life will compromise a scientific opportunity of almost unequaled proportions. Studies on microbiological survival in simulated deep-space conditions (low temperature, high ultraviolet flux, and low dose levels of ionizing radiation) indicate that these conditions will not sterilize contaminated spacecraft (refs. [ref.44]-[ref.48]). Furthermore, many terrestrial sporeformers and some vegetative bacteria, especially those with anaerobic growth capabilities, readily survive in simulated Martian environments (refs. [ref.49]-[ref.54]). It has been estimated that a single micro-organism with a replication time of 30 days could, in 8 years of such replication, equal in number the bacterial population of the Earth. This potential could result not only in competition with any Martian life, but in drastic changes in the geochemical and atmospheric characteristics of the planet. To avoid such a disaster, certainly the first, and probably many succeeding landers on Mars, must be sterile—devoid of terrestrial life ([ref.55]). Since the space environment will not in itself kill all life aboard, the lander must leave the Earth in a sterile condition.

The sterility of an object 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. Many industrial products which must be guaranteed as sterile cannot be tested for sterility in a nondestructive manner. A similar situation exists in determining the sterility of a spacecraft. Certification of sterility—based on experience with the sterilizing process used, knowledge of the kinetics of the death of micro-organisms, and computation of the probability of a survivor from assays for sterility—is the only accurate approach to defining the sterility of such treated items.

Macroscopic life can be readily detected and kept from or removed from the spacecraft, but the detection and removal of microscopic and submicroscopic life is an extremely difficult task. The destruction of micro-organisms can be achieved by various chemical and physical procedures. Sterilizing agents have been evaluated not only for their ability to kill microbial life on surfaces and sealed inside components, but also for the agents' effects on spacecraft reliability as well (refs. [ref.56]-[ref.59]). Of the available agents, only heat and radiation will penetrate solid materials. Radiation is expensive, hazardous, difficult to control, and apparently damages more materials than does heat. Heat, therefore, has been selected as the primary method of spacecraft sterilization and will be used, except in specific instances where radiation may prove to be less detrimental to the reliability of critical parts ([ref.60]).

The sterilization of spacecraft is a difficult problem if flight reliability is not to be impaired. The development of heat-resistant parts will enable the design and manufacture of a heat-sterilizable spacecraft. Without careful microbiological monitoring of manufacture and assembly procedures, many bacteria could be trapped in parts and subassemblies. To permit sterilization at the lowest temperature-time regimen that will insure kill of all organisms, the microbiological load inside all parts and subassemblies must be held to a minimum.

The role of industrial clean rooms in reducing the biological load on spacecraft is currently being defined. NASA-supported studies indicate that biological contamination in industrial clean rooms for extended time periods is about 1 logarithm less (tenfold reduction), compared with conditions in a well-operated microbiological laboratory ([ref.61]). With the use of clean-room techniques and periodic decontamination by low heat cycles or ethylene oxide treatment, it should be possible to bring a spacecraft to the point of sterilization with about 10⁶ organisms on board ([ref.60]).

The sterilization goal established for Mars landers is a probability of less than 1 in 10 000 (10^-4) that a single viable organism will be present on the spacecraft. Laboratory studies of the kinetics of dry-heat kill of resistant organisms show that at 135° C the number of bacterial spores can be reduced 1 logarithm (90 percent) for every 2 hours of exposure (refs. [ref.58] and [ref.62]). The reduction in microbial count needed is the logarithm of the maximum number on the spacecraft (10⁶) plus the logarithm of the reciprocal of the probability of a survivor (10⁴), or a total of 10 logarithms of reduction in microbial count. Thus, with an additional 2 logarithms added as a safety factor, a total of 12 logarithms of reduction in count has been accepted as a safe value which can be achieved by a dry-heat treatment of 135° C for 24 hours. This is the heat cycle that is currently under study and being developed for use in spacecraft sterilization ([ref.60]). However, other heat treatments at temperatures as low as 105° C for periods of 300 hours or longer are under study ([ref.63]).

Based on results to date, it is reasonable to believe that a full complement of heat-sterilizable hardware will be available when needed for planetary exploration. Every effort is 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 the present state-of-the-art hardware that is not heated.

[chapter 3]

Environmental Biology