Planetary Protection and Missions Between Earth and Mars
Planetary Protection and Missions Between Earth and Mars
Journal of Astrobiology and Space Science Reviews, 1, 242--245, 2019
(reprinted with permission from the publisher)

Planetary Protection and Missions Between Earth and Mars
Robert J.C. McLean, Ph.D.
Department of Biology, Texas State University, San Marcos, TX


The protocols to be used for human missions to Mars require continuous re-appraisal in view of a rapidly changing science base. This includes re-appraisal of the physical conditions on Mars, Earth-Mars interactions, and the properties of bacteria, viruses and of infective processes. Public confidence in the safety of manned Mars missions is contingent on transparency on such matters and involvement of the global community in decision-making.

Key Words: Mars exploration, planetary protection, Earth-Mars cross infection

1. Planetary Protection

The paper by Rummel et al. (2019), published in this volume, describes a number of microbiological issues that need to be addressed during the planning of a robotic or especially a human mission to Mars. In the context of planetary protection policies developed by COSPAR, the authors have presented the important issues related to preventing Earth contamination of Mars (forward contamination) and Martian contamination of Earth (back contamination). The highest safety priority issue, even surpassing astronaut safety, is protection of the planet Earth. On one-way trips, Earth protection is not an issue. It does become a major issue should equipment, Martian samples or especially humans return from Mars.

Although fictional, The Andromeda Strain (Crichton, 1969) introduced the general public to the concept of a potential plague of extraterrestrial origin. A number of highly lethal viruses and other pathogenic microorganisms occur on Earth. In order to protect the public as well as laboratory scientists, the US Centers for Disease Control (CDC) and National Institutes of Health have established criteria whereby microorganisms can be studied. Based on their respective risk levels, microorganisms are classified from the least dangerous, Biosafety Level-1 (BSL-1), to most dangerous (BSL-4) (Centers for Disease Control and Prevention, 2009). The criteria are summarized in Table 1 (adapted from Centers for Disease Control and Prevention, 2009):

In the case of BSL-4 labs, which are equipped to handle the most dangerous microorganisms, all protocols and personnel are highly regulated in order to safeguard lab personnel and the public against accidental exposure and infection. Personnel must change clothing prior to entering the facility and shower upon leaving. They must wear a full body suit with supplied air under positive pressure and the laboratories are under relative negative pressure when compared to ambient atmospheric pressure. Initial risk assessments of new organisms are typically made on the basis of incomplete data. A recent example involved the newly described Severe Acute Respiratory Syndrome (SARS) virus in which the virus was shown to be lethal as well as capable of being transmitted readily by air (aerosol transmission). Here, this virus was initially characterized as the highest risk (BSL4) (Weingartl et al., 2004). With increasing knowledge and treatment, this virus is now classified at the second highest risk category (BSL3) (Centers for Disease Control and Prevention, 2009). Of relevance to a human mission to Mars, the health of the astronauts during the mission would provide a strong early indicator of potential infectious disease risk.

One other aspect of extraterrestrial organisms that is worth addressing is one of microbial ecology. There are a number of bacteria that associate naturally with humans. While some certainly are associated with diseases, the majority are not. In terms of cell mass, human cells predominate over other organisms. However, in terms of cell numbers, microbiologists estimate that approximately 90% of the cells in an average human are bacterial. Microorganisms are found in many environments in humans, but are particularly numerous in the intestinal tract and colon, wherein they contribute to the health and function of these organs. One adverse effect of some antibiotics such as clindamycin is the reduction of normal flora, overgrowth of an antibiotic resistant species, Clostridium difficile, and onset of diarrhea which is sometimes accompanied by a fever. Once the antibiotics are no longer administered the normal flora recover and suppress the growth of C. difficile (Chang et al., 1978).

Our knowledge in this area is really incomplete, in that many components of the human microflora (human microbiome) have not been identified. DNA-based and other approaches have shown the human microbiome to be quite complex in composition. Interestingly, culture based studies in which bacteria are grown in the lab, show the intestinal flora to be resilient (cf Chang et al., 1978)). However, many bacteria cannot be readily grown in the lab and so much current information is obtained by DNA-based approaches. Antibiotic effects on intestinal flora are much more pervasive than can be shown with culture based techniques (Dethlefsen et al., 2008). In the context of planetary and human protection, Martian effects on microbial ecology including the human microbiome need to be addressed.

Overall, Rummel et al (2010)have outlined important issues that need to be addressed in the context of planetary protection. Aside from protection against a potential plagues and diseases, effects of forward and reverse contamination must be considered in the context of the microbial ecology of Mars and Earth.


Centers for Disease Control and Prevention, (2009). Biosafety in Microbiological and Biomedical Laboratories. US Department of Health and Human Services, Bethesda, MD.

Chang, T.-W., Bartlett, J.G., Gorbach, S.L., Onderdonk, A.B., (1978). Clindamycin-induced enterocolitis in hamsters as a model of pseudomembranous colitis in patients. Infection and Immunity 20, 526-529.

Crichton, M., (1969). The Andromeda Strain. Alfred A Knopf Inc, New York.

Dethlefsen, L., Huse, S., Sogin, M.L., Relman, D.A., (2008). The pervasive effects of an antibiotic on the human gut microbiota, as revealed by deep 16S rRNA sequencing. PLOS Biology 6, e280.

Rummel, J. D., Race, M. S. Conley, C. A. Liskowsky, D. R. (2019). The integration of planetary protection requirements and medical support on a mission to Mars, Journal of Astrobiology and Space Science Reviews, 19, 248-252.