the authors of the 2000 Europa report calculated NXs, or the number of organisms estimated to survive and grow in the target environment, summed across each physiological class, where
NX0 = Number of viable cells on the spacecraft before launch,
F1 = Total number of cells relative to cultured cells,
F2 = Bioburden reduction treatment fraction,
F3 = Cruise survival fraction,
F4 = Radiation survival fraction,
F5 = Probability of landing at an active site,
F6 = Burial fraction,
F7 = Probability that an organism survives and proliferates,
F7a = Survivability in exposure environments,
F7b = Availability of nutrients,
F7c = Suitability of energy sources, and
F7d = Suitability of environment for active growth.
Figure 2.1 shows how the Coleman-Sagan factors are mapped to the different phases of a planetary mission.
The example calculation in the 2000 Europa report shows that the value of NX (summed across all four physiological classes) had a combined probability of 3.8 × 10–5, i.e., below COSPAR requirements of 10–4. This approach, which seeks to identify conditions that constrain the sum of NXs below 10–4, identifies multiple factors that could influence contamination of solar system objects, but only if each factor represents an independent process and their values and variances are known.
The committee departs from the conclusions of the 2000 Europa report by claiming that not all bioload reduction factors are independent, and with the possible exception of F5 (probability of landing at an active site) current knowledge makes it impossible to confidently assign values for these factors within orders of magnitude of their true value. Multiplication of uncertain overestimates of bioload reduction factors can lead to unsubstantiated, low estimates of likely contamination. Alternatively, underestimates of bioload reduction coupled with overestimates of bioload on the spacecraft and the flawed assumption that any organism delivered to the target body will grow (Pg = 1), would impose unnecessary and possibly unachievable planetary protection demands. The vast majority of different terrestrial microbes have specific requirements for growth that rarely occur in nature or in manipulated laboratory environments. The assumption that Pg = 1 in any environment inclusive of icy bodies is conservative. However, the expectation that all microbes can grow anywhere is not supported by available scientific data.
In the example calculation for the NRC’s 2000 Europa report, the bio-reduction factors F3 (cruise survival fraction) and F4 (radiation survival fraction) have a combined bio-load reduction of 10–6 to 10–11 for the different physiological classes. Yet F3 and F4 represent highly correlated, non-independent mechanisms of sensitivity to radiation and vacuum. A significant fraction of the organisms lost due to the combination of ultrahigh vacuum
FIGURE 2.1 Mapping the Coleman-Sagan factors to the different phases of a planetary mission. The initial cell counts and cleaning are performed during spacecraft assembly. The survival fraction to irradiation and deep-space conditions corresponds to interplanetary cruise; and the characteristics of the planetary destination, either in orbit or within the planetary environment, dictate the remaining factors.