S210: Keeping planets clean -- NASA's approach to preventing biocontamination of space

We explore the solar system to search for life, but investigations could easily be compromised by Earth life carried on the spacecraft. Preventing this is one of the goals of planetary protection.  In the early stages of planetary exploration, the international community developed policies and procedures to implement planetary protection, that started with an identification of acceptable risk and derived the necessary level of cleanliness of a spacecraft to prevent contamination at a specified level of confidence.  Within the US, the Viking missions to Mars are a premiere example of such a 'bioburden controlled' implementation approach.  An overall acceptable risk of contaminating Mars with Earth life was established, based on determining the 'probability of contamination' using a Coleman-Sagan formulation for summing individual probabilities.  Sub-allocations of this overall probability were distributed to the US, the USSR, and other nations, by the International Council for Science, advisory to the UN.  Of the US allocation for a probability of contaminating Mars, each Viking spacecraft was assigned a fraction of that probability.  Statistical approaches were developed by the Viking project to estimate the total microbial 'bioburden' carried on spacecraft, based on data from the subset of surfaces sampled, that provided an upper bound on the number of organisms present.  Heat-sterilization of the spacecraft was  subsequently used to reduce the ‘bioburden’ to levels that met the pre-determined probability of contamination.  Such analytical techniques may have broader relevance for determining how clean is clean enough.