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CHAPTER 27 SPACECRAFT STERILIZATION N. H. HOROWITZ The chance that a spacecraft will contaminate Mars with terrestrial microorganisms is a function not only of the number of bacteria carried to the planet, but also of their location in the spacecraft. Bacteria on exposed surfaces will obviously have a better chance of infecting the planet than those which are sequestered within electronic or other components. For- tunately, exposed bacteria are also accessible to bactericidal agents such as ethylene oxide and can easily be destroyed. Sequestered cells, however, can be killed only by heat or other drastic treatments the use of which would seriously jeopardize the mission. This note is a preliminary attempt to assess the magnitude of the risk that would be involved if interplanetary spacecraft were subjected only to ethylene oxide sterilization. THE BACTERIAL LOAD Data on the extent of internal contamination of electronic components have been published by Phillips and Hoffmann (Science 132: 991, 1960). These authors determined the number of items with internal contamination among 150 assorted electronic components: transistors, capacitors, re- sistors, etc. The surfaces were first sterilized with ethylene oxide, and each component was then pulverized aseptically and used to inoculate a flask 467
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468 AVOIDING THE CONTAMINATION OF MARS of broth. After incubation, the flasks were scored for growth or no growth. In this way, it was determined that 21, or 14 per cent, of the items were contaminated. The remainder carried no detectable microorganisms. If the plausible, but unproven, assumption is made that bacteria are dis- tributed in a Poisson distribution in electronic components, it becomes possible to estimate the average number of bacteria per component. This number is 0.15, an intuitively obvious result. If it is assumed that the average weight of the components was 3 grams, then there is one bacterium per 20 grams of electronic apparatus, or 1000 bacteria per 20 kilograms. RELEASE OF SEQUESTERED BACTERIA Bacteria carried to Mars inside electronic components do not constitute a contamination hazard so long as the components remain intact. If the spacecraft is broken up, as in a crash landing, then there is a certain proba- bility of release (P). An estimate of P as a function of the extent of fragmentation has been made, based on the following model: Bacteria are assumed to be spherical cells of diameter 1 p. Spacecraft components are assumed to be fragmented into -spherical particles of radius r. A bacterial cell is considered to be released if it comes to lie within 5 /i of the surface of any fragment; thus, for example, a bacterium contained anywhere in a 10 fi diameter particle is considered released. The probability that a bacterium will lie within 5 /t of the surface of a spherical particle is equal to the volume of the spherical shell of thickness 5 fi, divided by the volume of the particle : 4/3 a-/* — 4/3 u(r— 5)3 4/3 where r is the radius of the particle. Some representative values of P are shown in the following table: r (in microns) P 5 1.000 10 0.875 50 0.37 100 0.14 150 0.1 200 0.07 500 0.03 1000 0.015 1500 0.011
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Spacecraft Sterilization 469 It can be seen that fragmentation to millimeter size liberates only a few per cent of the sequestered bacteria; fragmentation to the micron range is necessary to free more than 50 per cent. CONCLUSIONS AND RECOMMENDATIONS The limited data at hand suggest that electronic components are very clean from a bacteriological viewpoint. Furthermore, the organisms which are the most dangerous from a cosmo-ecological point of view are those which, being located on exposed surfaces, are most easily killed by gaseous sterilants such as ethylene oxide. Organisms that are located within com- ponents can be released only by considerable fragmentation of the compo- nents. These considerations suggest the possibility that reasonable sterility levels might be attained without terminal heat-sterilization of spacecraft. In order to make a judgment of this possibility, further information is urgently needed on the following points: 1. More extensive studies like those of Phillips and Hoffmann, covering the complete range of spacecraft materials and components, should be carried out. 2. A determination is needed of actual numbers of bacteria in con- taminated components, so that the statistical distribution can be calculated. It is possible that a Poisson distribution describes the situation for some types of components, but not others. 3. Also important is knowledge of the kinds of microorganisms preva- lent in spacecraft materials and components. Is the population a typical sample of soil or air organisms, or does it show special features? Particular attention should be paid to the proportions of aerobes vs. anaerobes, and nutritionally exacting vs. nutritionally unexacting species. 4. Manufacturing processes should be examined with a view toward suggesting changes that would reduce the bacterial contamination to mini- mal levels. 5. Data should be obtained on how spacecraft components fragment under impact. Possible methods of minimizing fragmentation in the event of hard landings should be studied. 6. A soft-landed spacecraft on Mars will eventually be broken up by weathering, although this may be very slow in the dry, anaerobic Martian environment. The diurnal temperature change is very great, however, and the effect of this variable on the integrity of spacecraft materials should be studied under simulated Martian conditions.