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Preventing the Forward Contamination of Mars
The 1964 COSPAR policy also incorporated the notion that “all practical steps should be taken to ensure that Mars be not biologically contaminated until such time as this search [for extraterrestrial life] can have been satisfactorily carried out …” (COSPAR, 1964, p. 26). To that end, COSPAR called for limiting the probability of an accidental planetary impact by unsterilized spacecraft as well as reducing spacecraft microbial bioburdens to specified levels. NASA policy incorporated the requirement that “outbound automated spacecraft and planetary exploration programs shall not, within [established] probabilities … transport terrestrial life to planets until it is determined that life does or does not exist on the planet and the character of existing life is explored” (NASA, 1967, p. 1). A finite period of time into the future by which the search for extraterrestrial life on Mars would be completed became known as the “period of biological exploration” (COSPAR, 1969).
During the 1960s and 1970s, the period of biological exploration was described in two different ways: (1) it was estimated as the time span it would take either to send a certain number of spacecraft to, or conduct a certain number of experiments on, Mars (Sagan and Coleman, 1965, 1966) and (2) it was translated into an absolute number of years, for example, the 20-year period from 1968 to 1988 (COSPAR, 1969).2 According to Hall (1968), studies at that time indicated that accidental impacts of spacecraft on the martian surface and premature entry of orbiting vehicles into the martian atmosphere represented principal forward contamination concerns. Concerns about non-nominal (accidental) impact and the requirement for orbital spacecraft to achieve on-orbit lifetimes of at least 50 years are still reflected in planetary protection policies today, although time spans are rolling time limits now that are reset for each mission.3
Probability of Contamination and Probability of Growth
Historically, the approach used in establishing planetary protection requirements for spacecraft sent to Mars was to require that the probability of contamination (Pc ) with terrestrial microorganisms—that is, the probability that Earth microorganisms introduced to Mars would then reproduce in situ on Mars—be below some threshold. One approach was to require that Pc multiplied by the total number of missions expected to be sent to Mars during the period of biological exploration would remain small compared with 1, that is, that the probability of contamination summed over all missions would remain small. Thus, Pc was set to 10–3 for all spacefaring nations, with different nations then being allotted fractions of this probability (COSPAR, 1969).
This approach depends on efforts to estimate Pg, the probability that Earth microorganisms will grow in the martian environment. The probability of contamination can be written as
where N0 is the total number of organisms present on the spacecraft prior to bioburden reduction steps, R is the bioburden reduction factor achieved by any pre-launch sterilization procedures, PS is the probability of surviving exposure to radiation, vacuum, temperature fluctuations, and so on during spaceflight and entry onto the planet’s surface, PI is the probability of impact on the surface (of interest for flybys or orbiters intended to avoid the surface, or for landers that risk non-nominal impacts with the surface), and PR is the probability of release of microbes into the environment.4 As written, Equation 2.1 assumes that Pc is small compared to unity.5 Therefore,
For example, “60 landers and 30 flyby and orbiter missions and a total of 1200 biological experiments on Mars” (Sagan and Coleman, 1965). Also, “the period of unmanned martian exploration shall be assumed not to extend beyond the year 2000, followed by manned exploration” and “the years 1966 to 2000 shall be considered the period for unmanned exploration … a total of 64 interplanetary flights toward Mars are expected” (Light et al., 1967).
That is, 20- and 50-year time periods of relevance to planetary protection requirements set for each mission upon mission launch; see NPR 8020.12C (NASA, 2005a), p. 63. Available at <planetaryprotection.nasa.gov/pp/index.htm>.
The particular form of Equation 2.1 varies from study to study, depending on which factors are collected into a single variable or broken out for individual assessment (e.g., PS, the probability of surviving spaceflight, is sometimes written as a product of PVT, the probability of surviving exposure to space vacuum and temperature, and PUV, the probability of surviving exposure to ultraviolet light, during the voyage). For alternate conventions in writing Equation 2.1 see, for example, Klein (1991) and NRC (1978, 1992). The formulation given here is perhaps closest to that used by Stabekis, as described in P.D. Stabekis, Lessons learned from Viking, presentation to the Committee on Preventing the Forward Contamination of Mars, February 27, 2004.
For an early exact formulation, see Sagan and Coleman (1965, 1966).