Mated bioburden: microbial burden associated with spacecraft surfaces that have been joined with fasteners rather than adhesives (which embed bioburden when surfaces are joined) during the spacecraft assembly process.

Period of biological exploration: a period referred to as either a defined number of years or the time to completion of a series of robotic missions to, or experiments on, Mars, during which strict planetary protection practices must be followed to protect the planet for the conduct of scientific investigations, including the search for life.

Probability of contamination (Pc): the probability that a mission will contaminate a planet, calculated according to a formulaic approach based on measurements of bioburden at launch, combined with microorganisms’ likely survival in space, release onto the planet, and growth in the new environment.

Probability of growth (Pg): the probability that a terrestrial microbe on a spacecraft delivered to an extraterrestrial body will grow and reproduce in that environment.

Special region: a specially designated region on Mars—currently defined by COSPAR as “a region within which terrestrial organisms are likely to propagate, or a region which is interpreted to have a high potential for the existence of extant martian life forms.” Currently applied to regions where liquid water is present or may occur.

Viking pre-sterilization: treatment to a level of cleanliness based on cleaning or sterilizing, or both, the spacecraft and its component parts in a manner such that the density of culturable microbial spores is less than 300/m2 on the spacecraft surface and the total number on the launched spacecraft is not greater than 3 × 105.

Viking post-sterilization: treatment to a level of cleanliness of an assembled spacecraft accomplished by using a final sterilization process (e.g., dry heat) to reduce the Viking pre-sterilization levels of microbes by 4 orders of magnitude, resulting in a total of no more than 30 culturable microbial spores on the surface of the launched spacecraft.

program “did, or will, carry out the measures described.”12 Neither the U.S. Pathfinder or Mars Polar Lander, nor the Mars Exploration Rover (MER) missions were subject to the dry-heat sterilization of the Viking missions. Missions that have crashed on the martian surface, such as the Mars Polar Lander mission launched in 1999, are likely not only to have exposed the martian environment to some interior surfaces, but also to have released some of their embedded bioburden, due to ruptures in spacecraft materials.

The likelihood of past delivery of spacecraft microbial material to Mars does not vitiate ongoing planetary protection measures. The prospects for the forward contamination of Mars (which requires microorganism survival

12  

Lawrence B. Hall, “Analysis of the Planetary Quarantine Effort in the U.S.S.R.,” memorandum to Associate Administrator for Space Science [date obscured; believed to be January 1972, the date of an associated memorandum] and enclosures.



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