The transfer of material that is not native to a planet has been happening over the history of the Solar System, with meteorite delivery being a common example of this interchange. With the development of rocket launchers capable of injecting objects into interplanetary trajectories, mankind joined Nature in being able to alter another planet's composition. Generally spacecraft and their associated hardware are designed and assembled so as to minimize the amount of debris that they carry. This chapter examines the problems associated with the unintentional delivery of living or dead organic matter to celestial bodies; so-called 'forward contamination'. The topic is often referred to by the phrase planetary protection, and its scope includes not only the possible contamination of planetary bodies, but also the potential introduction to the Earth of materal from a non-terrestrial biosphere. Furthermore, the threat that planetary protection seeks to minimize is not restricted to the introduction of non-native organisms to another planetary body. Non-living material, such as DNA fragments and other complex biorelevant molecules might trigger false-positives from equipment designed to detect extant or extinct life.
A practical definition of a living entity might be that the agent processes matter and energy in such a way that it can reproduce, and in doing so prosper in the face of environmental stresses. If the environment of the organism changes too radically then the organism may be killed or rendered dormant. Techniques that are intended to kill microbes may, if applied without sufficient vigour, only make the organism dormant. Many bacteria take on a spore-form in such stressful times, they become water-deficient and develop protective coats which make them able to withstand a wider range of stresses than when in their active form (Nicholson et al., 2000). Spores can be revived and grown to form colonies that may be counted visually, allowing the lethality of the killing process to be measured. In such a way the effectiveness of a sterilizing technique can be quantified by noting the conditions needed to kill off a given fraction of the original microbial population. A process, such as heating the sample to a given temperature, causes the death of all but 10% of an original microbial population after a period of time termed the 'D10' (decadal) value. If nine-tenths of an initial number of organisms are killed and if the remaining biota are unexceptional, a further 90% of that surviving population will die if the process is applied a second time. In Figure 14.1(a) three idealized plots are shown for the fractions of three hypothetical microbial groups that survive being exposed to different temperatures.
The survival curves suggest that a constant fraction of a given population is killed per unit of time. In Figure 14.1(b) the D10 durations of each curve are plotted (each curve in Figure 14.1(a) is associated with a point in Figure 14.1(b), with the wider error bar associated with Ta reflecting the wider variation in the durations needed to cause a 10-fold reduction in that population.
Plots of D10 with respect to some measure of a sterilizing process' vigour (such as temperature, T) tend to follow an Arrhenius-like rate relationship such as
D10 r V kT
Here E represents a deactivation energy, and values for spores are generally around 105 J mol-1. Although the word 'sterile' is usually assumed to imply the total absence of any viable biota, the preceding suggests that no object can be proved to host no viable biota, only that none were detected. Determining the
actual bioload of a component frequently means reviving and culturing spores that have passed through a sterilization process. More sophisticated techniques that rely on reactions between chemical marker molecules and the organism can be used to detect individual living biota, but it cannot be assumed that the detection of contamination on an article of hardware can be performed with complete accuracy. Many bacteria are difficult to cultivate and spores may need differing revival conditions.
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