Reference Work Entry

Encyclopedia of Astrobiology

pp 325-325

Coleman–Sagan Equation


The Coleman Sagan equation is used for establishing the probability of contaminating another planetary body by Earth microorganisms, and was first published by M. Coleman and C. Sagan in 1965.

This formula can be written as Pc = N0RPSPIPRPg.

In planetary protection, the formulation is often used by determining the initial number of microorganisms (N0) that could be present on or in a spacecraft, and multiplying this number by an appropriately-selected set of factors representing the probability this number could be reduced. The first reduction in proportion (R) is depending on various parameters, including conditions to which the spacecraft is exposed both before and after launch. Then, while present on the spacecraft, the microorganisms have to reach the surface of the planet. The value PI describes the probability of the spacecraft to hit the planet. This value is ranging from 10−5 for a satellite up to one for a landing probe. The probability for a microbe to be re ...

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