BOX 1.1
Planetary Protection Terminology

The practices of detecting, cleaning, and reducing the bioburden presented by microorganisms on spacecraft are described with a particular set of terms that is used in discussing planetary protection. Some of these practices and terms stem from the Viking Lander program, during which researchers conducted extensive studies on spacecraft cleaning methods and spacecraft sterilization that have continued to serve as the basis for planetary protection requirements (see Chapter 2). Other terms refer to probabilistic approaches that have been used in the past to establish requirements for the cleanliness of spacecraft.

Assay: an experimental analysis, usually involving sampling techniques, used to derive data on which to base an estimate of the number or kind of microorganisms associated with an item of interest.

Bioburden: level of microbial contamination (total number of microbes or microbial density) in or on an item of interest.

Bioburden reduction (also known as microbial reduction): reduction by any qualified process (temperature, chemical, radiative, or combinations thereof) of the number of organisms on spacecraft or components to a specified level.

Committee on Space Research (COSPAR): the international body responsible for formulating policies in accordance with the Outer Space Treaty; it is a committee of the International Council of Scientific Unions (ICSU; now called the International Council for Science).

Dry-heat cycle (also known as baking): the only NASA-certified method for reduction on an entire spacecraft and the preferred method for bringing spacecraft to sterile or near-sterile conditions; involves a heat cycle using prescribed temperature, pressure, gas, and humidity conditions for a specified length of time.

Encapsulated (embedded) bioburden: bioburden buried inside nonmetallic spacecraft material.

Forward contamination: contamination by biological or other organic material carried on outbound spacecraft to celestial bodies that may jeopardize the conduct of scientific investigations of possible extraterrestrial life forms, both extinct and extant.


The committee considered the implications for planetary protection requirements of past natural and mission-associated delivery of Earth microorganisms to Mars. An extreme viewpoint would be that because some past missions have already likely delivered significant quantities of microorganisms to Mars, and because Mars experiences substantial windblown transport of dust, there is no longer any point in continuing planetary protection practices.

All past missions that have landed or crashed on Mars (even the rigorously heat-sterilized Viking missions) have virtually certainly delivered some viable microorganisms to the martian surface. Table 1.1 displays the outcome of all missions of all nationalities sent from Earth to Mars; it also notes which of these missions failed and which crashed onto the martian surface. Soviet planetary protection measures were judged by the U.S. planetary protection officer in 1972 to “approximate compliance with COSPAR constraints,” assuming that the Soviet space

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