they were shielded from solar ultraviolet light, as would be the case inside a spacecraft. Typical Earth-Mars spacecraft trajectories take less than 1 year.

Spacecraft assembled within highly controlled class-100,000 clean rooms have bacterial spore densities of ~103 spores per square meter on their surfaces (Barengoltz, 2004).3 Thus, it is virtually certain that, in the absence of special measures, a large number of still-viable microbes will be present on interior spacecraft surfaces at the time a spacecraft reaches Mars from Earth. The focus then shifts to whether there are environments on Mars in which such organisms might survive and reproduce, whether these environments will be accessed by the spacecraft, and the likelihood and implications of varying answers to these questions. Planetary protection policy addresses these issues and the measures that should be taken in response.


The idea of planetary protection emerged with the genesis of the space program. Scientific leaders were the early proponents of planetary protection,4 and in 1958 the U.S. National Academy of Sciences (NAS) passed a resolution stating, “The National Academy of Sciences of the United States of America urges that scientists plan lunar and planetary studies with great care and deep concern so that initial operations do not compromise and make impossible forever after critical scientific experiments.”5 The NAS resolution was brought to the International Council of Scientific Unions (ICSU, now known as the International Council for Science), which in 1958 created the ad hoc Committee on Contamination by Extraterrestrial Exploration (CETEX). CETEX met for about a year and provided the first guidance for planetary protection, including recommendations that interplanetary spacecraft be sterilized, and it further stated, “The need for sterilization is only temporary. Mars and possibly Venus need to remain uncontaminated only until study by manned ships becomes possible” (CETEX, 1959). CETEX also recommended that planetary protection be transferred to the newly formed multidisciplinary, international committee of the ICSU, the Committee on Space Research (COSPAR). COSPAR continues to serve as the international policy-making body on planetary protection, and it is a consultative body to the United Nations’ Committee on the Peaceful Uses of Outer Space (Cypser, 1993).

Acting on the advice of its Consultative Group on Potentially Harmful Effects of Space Experiments, COSPAR in 1964 issued Resolution 26 (COSPAR, 1964, p. 26), which

affirms that the search for extraterrestrial life is an important objective of space research, that the planet of Mars may offer the only feasible opportunity to conduct this search during the foreseeable future, that contamination of this planet would make such a search far more difficult and possibly even prevent for all time an unequivocal result, that all practical steps should be taken to ensure that Mars be not biologically contaminated until such time as this search can have been satisfactorily carried out, and that cooperation in proper scheduling of experiments and use of adequate spacecraft sterilization techniques is required on the part of all deep space probe launching authorities to avoid such contamination.


A class-100,000 clean room is defined as a clean room with 100,000 0.5-micron-diameter particles per cubic foot of atmosphere. Class-100,000 clean rooms typically require restricted access, positive pressurization, and perhaps other measures. Clean rooms with fewer atmospheric particles have more stringent requirements. Class-10,000 clean rooms are typical of hospital operating rooms; class-1,000 facilities are typical for making computer disk drives, and class-100 are typical for semiconductor and pharmaceutical manufacture. Class-10 and class-1 rooms also exist. Definitions of clean rooms are given in “Federal Standard 209E: Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones,” available at <>. Federal Standard 209E has been formally superseded by International Organization for Standardization (ISO) Standards in Metric Units; see “Cancellation of Fed-STD-209E,” available at <>. However, U.S. usage still often refers to the imperial unit definitions. For a discussion of clean-room levels and requirements, see, for example, <>.


Letter from Joshua Lederberg, University of Wisconsin, to Detlev Bronk, President, National Academy of Sciences, December 24, 1957, with enclosed memorandum entitled “Lunar Biology?”, National Academy of Sciences, Records Office, Washington, D.C.


National Academy of Sciences, resolution adopted by the Council of the NAS, February 8, 1958. Addendum to Minutes of the Meeting of the Council of the National Academy of Sciences, February 8, 1958.

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement