once Pc is fixed by policy, a given estimate of Pg then places a limit that the intervening product of factors must meet for that Pc to be achieved. In particular, estimates of Pg drive the requirements for bioburden reduction R.

Current NASA practices for estimating bioburden (be it N0 or N0R, the bioburden present after bioburden reduction measures) on spacecraft are by proxy (as measured by colony-forming units after heat shock at 80°C followed by incubation for 72 h; see Appendix C). This procedure has served as a cornerstone for estimating microbial burden on spacecraft and will continue to do so in the near term. However, recent advances in microbial ecology reveal two significant limitations to these spore-based estimates of numbers of bacteria on the spacecraft. As detailed in Chapter 5, molecular diversity surveys demonstrate that cultivation techniques fail to recover as many as 99 percent of the microbes in a microbial population (Pace, 1997). By logical extension, the technologies on which current NASA estimates of bioburden are based will not detect the majority of heat-resistant organisms on the spacecraft. More important, spore-based estimates tell little about the cellular physiology or genomic diversity of organisms on the spacecraft surface or within enclosed components, both of which directly influence Pg and provide valuable new information on possible strategies for reducing bioburden (see Chapter 6). The heat treatment protocol means that those colony-forming units that are found are likely due to spore-forming organisms capable of surviving heat shock. However, this spore proxy can record only what grows within a few days under a given set of laboratory conditions; it does not consider what might grow under different environmental conditions or over protracted periods. That is, it does not survey many organisms that may occur in the clean-room environment but about which little is known. The use of proxies in estimating spacecraft contamination therefore brings with it inherent risks.

Reasonably robust models could estimate levels of bioburden reduction during flight (PS) by considering the presence of radiation-tolerant and heat-tolerant microorganisms, if these data were available. Estimates of PI may be derived from actual crash data, and engineering models may assign values to PR. In contrast, there is little scientific basis for estimates of Pg, and estimates of Pg have in fact varied by as much as 10 orders of magnitude during the period from 1964 to 1978 (Klein, 1991) (see Appendix D).

The Viking program, consisting of two orbiters and two landers that were launched in the mid-1970s, provided for the first time significant in situ scientific data on the martian environment. Following analysis of the Viking results, NASA asked the National Research Council (NRC) to evaluate Pg comprehensively, based on available knowledge of planetary conditions and the limits of life known at the time. The NRC report Recommendations on Quarantine Policy for Mars, Jupiter, Saturn, Uranus, Neptune, and Titan (1978) addressed planetary protection policy for exploratory missions to solar system locations that had been launched or planned for launch between 1974 and 1994. In that report Pc was stipulated as less than 1 × 10–3 for each planet, and Pg values were set separately for three different regions on Mars based on a “comparison between the known physical and chemical limits to terrestrial growth and the known and inferred conditions [on Mars]” (NRC, 1978, p. 4). The Pg values for Mars were set at <10–10, 10–8, and 10–7 for above- and below-surface subpolar areas and the polar caps, respectively. Although the report stipulated quantitative values for Pg, the values were arrived at subjectively and became a matter for debate.6 In fact, the 1992 NRC report noted, “It is clear that considerable uncertainty has been engendered by the probabilistic approach to planetary protection. This concern has been restated over the years by virtually every group that has analyzed the problem, and indeed by NASA” (NRC, 1992, p. 44). The debate over the probabilistic approach to planetary protection continued in the years following the 1978 report and set the stage for a subsequent significant overhaul of planetary protection policy.7


As noted in NRC 1992: “Although the [1978] committee expressed a reluctance in recommending a particular value for Pg, they argued that while the Pg for Mars is exceedingly low, the probability is not zero” (p. 44). The 1978 report had stated: “And yet a numerical value for Pg is required in order to determine what procedures are needed to reduce the microbial burden on future spacecraft to Mars to levels that fulfill current COSPAR quarantine policy. Reluctantly, then, we recommend for these purposes, and these purposes alone, that NASA adopt a value of Pg less than 10–10 for the subpolar region …” (italics in original).


No Mars missions were ever flown whose forward contamination polices were based on the recommendations of the 1978 NRC report.

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