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CHAPTER 29 THE SPECIAL PROBLEM OF ENCAPSULATED CONTAMINANTS A. H. BROWN Viable contaminants lodged within the solid materials which make up a spacecraft pose a special operational problem when spacecraft sterilization is required. On the one hand, encapsulation of a living organism will pre- vent it from contaminating a virgin planetary surface as long as the organ- ism does not escape from confinement. To assess the risk of contamination presented by encapulated organisms we must estimate their chances of escape. On the other hand, encapsulation per se introduces a large uncer- tainty into our estimates of viability for, although we have evidence that viability is enhanced by encapsulation, we are not yet able to put this on a thoroughly quantitative basis. The principal difficulty lies with the assay methods which are now available. Thus encapsulation is responsible for two effects which alter the risk in opposite directions. If we could be sure that resistance to sterilization procedures would not be enhanced by encapsulation, we might relax the rigor of sterilization requirements to some degree because surface contaminants are more readily accessible to sterilizing agents. Moreover, ensured confinement of some of the spacecraft contaminants should effectively reduce the total hazard associated with their presence as long as the numbers remain relatively very small. We should then be most concerned with finding ways to reduce the probability of fragmentation of spacecraft components in the event of a hard landing, so as to reduce the likelihood of escape. If we could be 482

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Special Problem of Encapsulated Contaminants 483 convinced that our efforts to avoid planetary contamination would be re- warded effectively by progress in preserving structural integrity in the face of all contingencies, we should be well advised to introduce design require- ments compatible with this philosophy into the basic planning and devel- opment of planetary mission landing capsules. To do so would, of course, impose restrictions on the design of scientific experiments to be included in the mission and at present it is difficult to be certain how serious such restrictions might become. Great strides have been made in fragmentation design, and to construct a "fragmentation proof spacecraft perhaps is not very far beyond the present state of the art. Nevertheless, our provisional conclusion has been that we cannot depend on such measures to provide the required protec- tion; the interiors of spacecraft components must be sterilized as well as the surfaces. When surface-sterilized solids (metals, plastics, circuit components) are crushed, ground up, and cultured, microorganisms are found to grow in many instances. The number that can be retrieved from a given solid varies greatly and we simply have no reliable assay method for determining the number of viable organisms present in any given solid component. Research on this topic is badly needed. Until we have a satisfactory, quan- titative, assay method for encapsulated components we cannot expect to measure with desired precision the resistance of encapsulated organisms toward sterilization methods under consideration. For a variety of reasons sterilization of the entire spacecraft by dry heat soak would be the method of choice if it can be accomplished. This of course depends on the designers' ability to employ none but heat resistant components. We are optimistic that this can be done. If not, then we must resort to clean assembly of sterile components. Possibly, most, of the spacecraft can be heat sterilized and the few thermally sensitive com- ponents can be inserted by sterile techniques. Such components might be sterilized by other methods. In our judgment this is a possible, but not very desirable, alternative to a 100 per cent heat sterilizable spacecraft. The difficulties with using sterile insertion methods for many spacecraft components could be very serious and the risk of accidental breach of sterility correspondingly large. If sterilization methods other than heat soak are required, penetrating ionizing radiation may be efficacious. However, for many types of com- ponents the dose of radiation needed to destroy all internal contaminants would impair function and reliability to a prohibitive degree. For only some components, it is likely that radiation will be the method of choice. As we now assess the special hazard imposed by internal contaminants, we endorse strongly the policy of starting with components having the I

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484 AVOIDING THE CONTAMINATION OF MARS lowest achievable levels of contamination and of maintaining rigorous standards of cleanliness throughout spacecraft assembly. This will require an extensive program of component selection by biological criteria and an unusually ambitious program of training, supervision, and testing by a "sterility control group." During July of 1964 a conference was held on the Hazards of Planetary Contamination Due to Microbiological Contamination in the Interior of Spacecraft Components. Salient points arising from this Conference are summarized above, although the Report of the Conference (title as above, Space Science Board, 15 February 1965) should be consulted for the detailed arguments. Perhaps the most significant recommendation made by the Conference was that research should be accelerated on the develop- ment of reliable assay methods for encapsulated contaminants. Only by use of such methods can design decisions be placed on a quantitative experimental instead of a conjectural basis.

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APPENDIXES

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