Encyclopedia of Astrobiology

2011 Edition
| Editors: Muriel Gargaud, Ricardo Amils, José Cernicharo Quintanilla, Henderson James (Jim) CleavesII, William M. Irvine, Daniele L. Pinti, Michel Viso

Coleman–Sagan Equation

  • Catharine A. Conley
Reference work entry
DOI: https://doi.org/10.1007/978-3-642-11274-4_317


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 10−5for a satellite up to one for a landing probe. The probability for a microbe to be...

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References and Further Reading

  1. Sagan C, Coleman S (1965) Spacecraft sterilization standards and contamination of Mars. Astron Aeron 3(5):1–22Google Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 2011

Authors and Affiliations

  1. 1.NASA HeadquartersWashingtonUSA