Planetary protection is a guiding principle in the design of an interplanetary mission, aiming to prevent biological contamination of both the target celestial body and the Earth. Planetary protection reflects both the unknown nature of the space environment and the desire of the scientific community to preserve the pristine nature of celestial bodies until they can be studied in detail.
There are two types of interplanetary contamination. Forward contamination is the transfer of viable organisms from Earth to another celestial body. A major goal of planetary protection is to preserve the planetary record of natural processes by preventing introduction of Earth-originated life. Back contamination is the transfer of extraterrestrial organisms, if such exist, back to the Earth's biosphere.
- 1 History
- 2 COSPAR recommendations and categories
- 3 Target categories
- 4 The Coleman-Sagan equation
- 5 Containment and quarantine for restricted Category V sample return
- 6 Decontamination procedures
- 7 Impact prevention
- 8 Controversies
- 9 Proposal for extension of protection to non-biological considerations
- 10 Proposal to extend planetary protection
- 11 See also
- 12 References
- 13 General references
- 14 External links
The potential problem of lunar and planetary contamination was first raised at the International Astronautical Federation VIIth Congress in Rome in 1956.
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.” This lead to creation of the ad hoc Committee on Contamination by Extraterrestrial Exploration (CETEX), which met for a year and recommended that interplanetary spacecraft be sterilized, and stated, “The need for sterilization is only temporary. Mars and possibly Venus need to remain uncontaminated only until study by manned ships becomes possible”
In 1959 planetary protection was transferred to the newly formed Committee on Space Research (COSPAR). COSPAR in 1964 issued Resolution 26
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.
In 1967, the US, Russia and the UK ratified the United Nations Outer Space Treaty. The legal basis for planetary protection lies in Article IX of this treaty:
"Article IX: ... States Parties to the Treaty shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose...
This treaty has since been signed by almost all nations including all the current space faring nations.
For forward contamination, the phrase to be interpreted is "harmful contamination", which could have varying meanings. However, an unofficial legal review concluded that Article IX must mean that “any contamination which would result in harm to a state’s experiments or programs is to be avoided”, and this is how it has come to be interpreted.
The 1979 Moon Treaty governing the activities of states on the Moon and other celestial bodies has more extended treatment of the subject of contamination, however it has not been ratified by any space faring nation to date:
5.3. In carrying out activities under this Agreement, States Parties shall promptly inform the Secretary-General, as well as the public and the international scientific community, of any phenomena they discover in outer space, including the moon, which could endanger human life or health, as well as of any indication of organic life....
7.1. In exploring and using the moon, States Parties shall take measures to prevent the disruption of the existing balance of its environment, whether by introducing adverse changes in that environment, by its harmful contamination through the introduction of extra-environmental matter or otherwise. States Parties shall also take measures to avoid harmfully affecting the environment of the earth through the introduction of extraterrestrial matter or otherwise.
COSPAR recommendations and categories
The Committee on Space Research (COSPAR) meets every two years, in a gathering of 2000 to 3000 scientists, and one of its tasks is to develop recommendations for avoiding interplanetary contamination. Its legal basis is Article IX of the Outer Space Treaty  (see history below for details).
Its recommendations depend on the type of space mission and the celestial body explored. COSPAR categorizes the missions into 5 groups:
- Category I: Any mission to locations not of direct interest for chemical evolution or the origin of life. such as the Sun, or Mercury
- Category II: Any mission to locations of significant interest for chemical evolution and the origin of life, but only a remote chance that spacecraft borne contamination could compromise investigations. Examples include the Moon, Venus, and comets.
- Category III: Flyby and orbiter missions to locations of significant interest for chemical evolution and/or origin of life, and with a significant chance that contamination could compromise investigations e.g., Mars, Europa, Enceladus
- Category IV: Lander or probe missions to the same locations as Category III.
- Missions to Mars in category IV are subclassified further:
- Category IVa. Landers that do not search for Martian life - uses the Viking lander pre-sterilization requirements, 300,000 spores per spacecraft and 300 spores per square meter.
- Category IVb. Landers that search for Martian life. Adds stringent extra requirements to prevent contamination of samples.
- Category IVc. Any component that accesses a Martian special region (see below) must be sterilized to at least to the Viking post-sterilization biological burden levels (30 spores total per spacecraft).
- Category V: This is further divided into unrestricted and restricted sample return.
- Unrestricted Category V: samples from locations judged by scientific opinion to have no indigenous lifeforms. No special requirements
- Restricted Category V: (where scientific opinion is unsure) the requirements include: absolute prohibition of destructive impact upon return, containment of all returned hardware which directly contacted the target body, and containment of any unsterilized sample returned to Earth.
After receiving the mission category a certain level of biological burden is allowed for the mission. In general this is expressed as a 'probability of contamination', required to be less than one chance in 10,000 of forward contamination per mission, but in the case of Mars Category IV missions (above) the requirement has been translated into a count of Bacillus spores per surface area, as an easy to use assay method.
For restricted Category V missions, sterilization of the returned samples would destroy much of their science value. For proposals to fulfill the containment requirements, see #Containment and quarantine below. Of course, Category V missions also have to fulfill the requirements of Category IV to protect the target body from forward contamination.
Mars special regions
A special region is a region classified by COSPAR within which terrestrial organisms could readily propagate, or one thought to have an elevated potential for existence of Martian life forms. This is understood to apply to any region on Mars where liquid water occurs, or can occasionally occur, based on the current understanding of requirements for life.
If a hard landing risks biological contamination of a special region, then the whole lander system must be sterilized to COSPAR category IVc.
Some targets are easily categorized. Others are assigned provisional categories by COSPAR, pending future discoveries and research.
The 2009 COSPAR Workshop on Planetary Protection for Outer Planet Satellites and Small Solar System Bodies covered this in some detail. Most of these assessments are from that report, with some future refinements. This workshop also gave more precise definitions for some of the categories:
- Io, Sun, Mercury, Undifferentiated metamorphosed asteroids
… where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration”. In this case we define “remote chance” as “the absence of niches (places where terrestrial microorganisms could proliferate) and/or a very low likelihood of transfer to those places.” 
- Callisto, Comets, Asteroids of category P, D and C, Venus, KBOs (< 1/2 size of Pluto)
Provisional Category II
- Ganymede, Titan, Triton, the Pluto-Charon system, and other large KBOs (> 1/2 size of Pluto), Ceres,
Provisionally they assigned these objects to Category II. However, they state that more research is needed, because there is a remote possibility that the tidal interactions of Pluto and Charon could maintain some water reservoir below the surface. Similar considerations apply to the other larger KBOs
Triton they thought was insufficiently well understood at present to say it is definitely devoid of liquid water. The only close up observations to date are those of Voyager 2.
In a detailed discussion of Titan, scientists concluded that there was no danger of contamination of its surface, except short term adding of negligible amounts of organics, but Titan could have a below surface water reservoir that communicates with the surface, and if so this could be contaminated.
In the case of Ganymede, the question is, given that its surface shows pervasive signs of resurfacing, is there any communication with its subsurface ocean? They found no known mechanism by which this could happen, and Galileo found no evidence of cryovolcanism.
Initially they assigned it as Priority B minus, meaning that precursor missions are needed to assess its category before any surface missions. However after further discussion they provisionally assigned it to Category II, so no precursor missions required, depending on future research.
If there is cryovolcanism on Ganymede or Titan, the undersurface reservoir is thought to be 50 – 150 km below the surface. They were unable to find a process that could transfer the surface melted water back down through 50 km of ice to the under surface sea. This is why they assigned both Ganymede and Titan a reasonably firm provisional Category II but pending results of future research.
They recommended more generally, that icy bodies that show signs of recent resurfacing need further discussion and might need to be assigned to a new category depending on future research.
This approach has been applied, for instance, to missions to Ceres. The planetary protection Category is subject for review during the mission of the Ceres orbiter depending on the results found.
Category III / IV
“…where there is a significant chance that contamination carried by a spacecraft could jeopardize future exploration.” We define “significant chance” as “the presence of niches (places where terrestrial microorganisms could proliferate) and the likelihood of transfer to those places.” 
- Mars because of possible surface habitats
- Europa because of its subsurface ocean
- Enceladus because of evidence of water plumes.
In the category V for sample return the conclusions so far are:
- Restricted Category V: Mars, Europa
- Unrestricted Category V: Venus, Moon
If there has been no activity for 3 billion years, it will not be possible to destroy the surface by terrestrial contamination, so can be treated as Category I. Otherwise, the category may need to be reassessed.
The Coleman-Sagan equation
The aim of the current regulations is to keep the number of microorganisms low enough so that the probability of contamination of Mars (and other targets) is acceptable. It is not an objective to make the probability of contamination zero.
The aim is to keep the probability of contamination of 1 chance in 10,000 of contamination per mission flown. This figure is obtained typically by multiplying together the number of microorganisms on the spacecraft, the probability of growth on the target body, and a series of bioload reduction factors.
In detail the method used is the Coleman-Sagan equation.
- = the number of microorganisms on the spacecraft initially
- = Reduction due to conditions on spacecraft before and after launch
- = Probability that microorganisms on the spacecraft reach the surface of the planet
- = Probability that spacecraft will hit the planet - this is 1 for a lander
- = Probability of microorganism to be released in the environment when on the ground, usually set to 1 for crashlanding.
- = Probability of growth. For targets with liquid water this is set to 1 for sake of the calculation.
Then the requirement is
The is a number chosen by Sagan et al., somewhat arbitrarily. Sagan and Coleman assumed that about 60 missions to the Mars surface would occur before the exobiology of Mars is thoroughly understood, 54 of those successful, and 30 flybys or orbiters, and the number was chosen to endure a probability to keep the planet free from contamination of at least 99.9% over the duration of the exploration period.
The Coleman Sagan equation has been criticised because the individual parameters are often not known to better than a magnitude or so. For example, the thickness of the surface ice of Europa is unknown, and may be thin in places, which can give rise to a high level of uncertainty in the equation. It has also been criticised because of the inherent assumption made of an end to the protection period and future human exploration. In the case of Europa, this would only protect it with reasonable probability for the duration of the period of exploration.
Greenberg has suggested an alternative, to use the natural contamination standard - that our missions to Europa should not have a higher chance of contaminating it than the chance of contamination by meteorites from Earth.
As long as the probability of people infecting other planets with terrestrial microbes is substantially smaller than the probability that such contamination happens naturally, exploration activities would, in our view, be doing no harm. We call this concept the natural contamination standard.
Another approach for Europa is the use of binary decision trees which is favoured by the Committee on Planetary Protection Standards for Icy Bodies in the Outer Solar System under the auspices of the Space Studies Board. This goes through a series of seven steps, leading to a final decision on whether to go ahead with the mission or not.
Recommendation: Approaches to achieving planetary protection should not rely on the multiplication of bioload estimates and probabilities to calculate the likelihood of contaminating solar system bodies with terrestrial organisms unless scientific data unequivocally define the values, statistical variation, and mutual independence of every factor used in the equation.
Recommendation: Approaches to achieving planetary protection for missions to icy solar system bodies should employ a series of binary decisions that consider one factor at a time to determine the appropriate level of planetary protection procedures to use.
Containment and quarantine for restricted Category V sample return
In the case of restricted Category V missions, Earth is protected through quarantine of sample and astronauts, and sample containment.
In the case of a Mars sample return, Missions would be designed so that no part of the capsule that encounters the Mars surface is exposed to the Earth environment.
One way to do that is to enclose the sample container within a larger outer container from Earth, in the vacuum of space. The integrity of any seals is essential and the system must also be monitored to check for the possibility of micro-meteorite damage during return to Earth. 
The recommendation of the ESF report is that 
“No uncontained Mars materials, including space craft surfaces that have been exposed to the Mars environment should be returned to Earth unless sterilised"
..."For unsterilised samples returned to Earth, a programme of life detection and biohazard testing, or a proven sterilisation process, shall be undertaken as an absolute precondition for the controlled distribution of any portion of the sample.”
No restricted category V returns have been carried out in recent times.
During the Apollo program the sample returns were regulated through the Extra-Terrestrial Exposure Law. This was rescinded in 1991. So new legislation would need to be enacted. The Apollo era quarantine procedures are of interest as the only attempt to date of a return to Earth of a sample that, at the time, was thought to have a remote possibility of including extraterrestrial life.
Samples and astronauts were quarantined in the Lunar Receiving Laboratory. The methods used would be considered inadequate for containment by modern standards. Also the lunar receiving laboratory would be judged a failure by its own design criteria as the sample return didn't contain the lunar material, with two failure points during the Apollo 11 return mission, at the splashdown and at the facility itself.
However the Lunar Receiving Laboratory was built quickly with only two years from start to finish, a time period now considered inadequate. Lessons learnt from it can help with design of any Mars sample return receiving facility.
Design criteria for a proposed Mars Sample Return Facility, and for the return mission, have been developed by the American National Research Council, and the European Space Foundation. They concluded that it could be based on biohazard 4 containment but with more stringent requirements to contain unknown microorganisms possibly as small as or smaller than the smallest Earth microorganisms known, the ultramicrobacteria. The ESF study also recommended that it should be designed to contain the smaller gene transfer agents if possible, as these could potentially transfer DNA from martian microorganisms to terrestrial microorganisms if they have a shared evolutionary ancestry. It also needs to double as a clean room facility to protect the samples from terrestrial contamination that could confuse the sensitive life detection tests that would be used on the samples.
Before a sample return, new quarantine laws would be required. Environmental assessment would also be required, and various other domestic and international laws would need to be negotiated not present in the Apollo era.
For all spacecraft missions requiring decontamination, the starting point is clean room assembly in class 100 cleanrooms. These are rooms with less than 100 particles of size 0.5 µm or larger per cubic foot. Engineers wear cleanroom suits with only their eyes exposed. Components are sterilized individually before assembly, as far as possible, and they clean surfaces frequently with alcohol wipes during assembly.
For Category IVa missions (Mars landers that do not search for Martian life), the aim is to reduce the bioburden to 300,000 bacterial spores on any surface from which the spores could get into the Martian environment. Any heat tolerant components are heat sterilized to 114°C. Sensitive electronics such as the core box of the rover including the computer, are sealed and vented through high-efficiency filters to keep any microbes inside.
For more sensitive missions such as Category IVc (to Mars special regions), a far higher level of sterilization is required. This needs to be similar to implemented on the Viking landers, which were sterilized for a surface which, at the time, was thought to be potentially hospitable to life similar to special regions on Mars today.
In microbiology, it is usually impossible to prove that there are no microorganisms left viable, since many microorganisms are either not yet studied, or not cultivable. Instead, sterilization is done using a series of tenfold reductions of the numbers of microorganisms present. After a sufficient number of ten-fold reductions, the chance that there any microorganisms left will be extremely low.
The two Viking Mars landers were sterilized using dry heat sterilization. After preliminary cleaning to reduce the bioburden to similar levels to present day Category IVa spacecraft, the Viking spacecraft were heat-treated for 30 hours at 125°C (five hours at 125°C was considered enough to reduce the population tenfold even for enclosed parts of the spacecraft, so this was enough for a million-fold reduction of the originally low population).
Modern materials however are often not designed to handle such temperatures, especially since modern spacecraft often use "commercial off the shelf" components. Problems encountered include nanoscale features only a few atoms thick, plastic packaging, and conductive epoxy attachment methods. Also many instrument sensors cannott be exposed to high temperature, and high temperature can interfere with critical alignments of instruments.
As a result, new methods are needed to sterilize a modern spacecraft to the higher categories such as Category IVc for Mars, similar to Viking. Methods under evaluation, or already approved, include:
- Vapour phase hydrogen peroxide - effective, but can affect finishes, lubricants and materials that use aromatic rings and sulfur bonds. This has been established, reviewed, and a NASA/ESA specification for use of VHP has been approved by the Planetary Protection Officer, but it has not yet been formally published.
- Ethylene oxide - this is widely used in the medical industry, and can be used for materials not compatible with hydrogen peroxide. Is under consideration for missions such as ExoMars.
- Gamma radiation and electron beams have been suggested as a method of sterilization, as they are used extensively in the medical industry. They need to be tested for compatibility with spacecraft materials and hardware geometries, and is not yet ready for review.
Some other methods are of interest as they can sterilize the spacecraft after arrival on the planet.
- Supercritical carbon dioxide snow (Mars) - is most effective against traces of organic compounds rather than whole microorganisms. Has the advantage though that it eliminates the organic traces - while other methods though they kill the microorganisms, leave organic traces that can confuse life detection instruments. Is under study by JPL and ESA.
- Passive sterilization through UV radiation (Mars). Highly effective against many microorganisms, but not all, as a Bacillus strain found in spacecraft assembly facilities is particularly resistant to UV radiation. Is also complicated by possible shadowing by dust and spacecraft hardware.
- Passive sterilization through particle fluxes (Europa). Plans for missions to Europa take credit for reductions due to this.
Bioburden detection and assessment
The spore count is used as an indirect measure of the number of microorganisms present. Typically 99% of microorganisms by species will be non-spore forming and able to survive in dormant states, and so the actual number of viable dormant microorganisms remaining on the sterilized spacecraft is expected to be many times the number of spore-forming microorganisms.
One new spore method approved is the "Rapid Spore Assay". This is based on commercial rapid assay systems, detects spores directly and not just viable microorganisms and gives results in 5 hours instead of 72 hours.
It is also long been recognized that spacecraft cleaning rooms harbour polyextremophiles as the only microbes able to survive in them. For example, in a recent study, microbes from swabs of Curioisty rover were subjected to desiccation, UV exposure, cold and pH extremes. Nearly 11% of the 377 strains survived more than one of these severe conditions.
This does not mean that these microbes have contaminated Mars. This is just the first stage of the process of bioburden reduction. To contaminate Mars they also have to survive the low temperature, vacuum, UV and ionizing radiation during the months long journey to Mars, and then have to encounter a habitat on Mars and start reproducing there. Whether this has happened or not is a matter of probability. The aim of planetary protection is to make this probability as low as possible. The currently accepted target probability of contamination per mission is to reduce it to less than 0.01%, though in the special case of Mars, scientists also rely on the hostile conditions on Mars to take the place of the final stage of heat treatment decimal reduction used for Viking. But with current technology scientists cannot reduce probabilities to zero.
- Adenosine triphosphate (ATP) detection - this is a key element in cellular metabolism. This method is able to detect non cultivable organisms. It can also be triggered by non viable biological material so can give a "false positive".
- Limulus Amebocyte Lysate assay - detects lipopolysaccharides (LPS). This compound is only present in Gram-negative bacteria. The standard assay analyses spores from microbes that are primarily Gram-positive, making it difficult to relate the two methods.
This particularly applies to orbital missions, Category III, as they are sterilized to a lower standard than missions to the surface. It is also relevant to landers, as an impact gives more opportunity for forward contamination, and impact could be on an unplanned target, such as a special region on Mars.
The requirement for an orbital mission is that it needs to remain in orbit for at least 20 years after arrival at Mars with probability of at least 99% and for 50 years with probability at least 95%. This requirement can be dropped if the mission is sterilized to Viking sterilization standard.
In the Viking era (1970s), the requirement was given as a single figure, that any orbital mission should have a probability of less than 0.003% probability of impact during the current exploratory phase of exploration of Mars.
For both landers and orbiters, the technique of trajectory biasing is used during approach to the target. The spacecraft trajectory is designed so that if communications are lost, it will miss the target.
Issues with impact prevention
Despite these measures[which?], there has been one notable failure of this element[which?] of the planetary protection policy, the Mars Climate Orbiter which was sterilized only to Category III, and crashed on Mars[when?] due to a mix-up of imperial and metric units. The office of planetary protection stated that it is likely that it burnt up in the atmosphere, but if it survived to the ground, then it could cause forward contamination.
Mars Observer is another Category III mission with potential planetary contamination. Communications were lost three days before its orbital insertion maneuver. It seems most likely it did not succeed in entering into orbit around Mars,[when?] and simply continued past on a heliocentric orbit. If it did succeed in following its automatic programming, and attempted the manoeuvre, however, there is a chance it crashed on Mars.
Alberto G. Fairén and Dirk Schulze-Makuch published an article in Nature recommending that planetary protection measures need to be scaled down. They gave as their main reason for this, that exchange of meteorites between Earth and Mars means that any life on Earth that could survive on Mars has already got there and vice versa.
Rebuttal by NRC
The meteorite argument was examined by the NRC in the context of back contamination. It is thought that all the Martian meteorites originate in relatively few impacts every few million years on Mars. The impactors would be kilometers in diameter and the craters they form on Mars tens of kilometers in diameter. Models of impacts on Mars are consistent with these findings.
Earth receives a steady stream of meteorites from Mars, but they come from relatively few original impactors, and transfer was more likely in the early Solar System. Also some life forms viable on both Mars and on Earth might be unable to survive transfer on a meteorite, and there is so far no direct evidence of any transfer of life from Mars to Earth in this way.
The NRC concluded that though transfer is possible, the evidence from meteorite exchange does not eliminate the need for back contamination protection methods.
Impacts on Earth able to send microorganisms to Mars are also infrequent. Impactors of 10 km across or larger can send debris to Mars through the Earth's atmosphere but these occur rarely, and were more common in the early Solar System.
Proposal to end planetary protection for Mars
In a paper "The Over Protection of Mars" in astrobiology magazine, in 2013, Alberto Fairén and Dirk Schulze-Makuch suggested that we no longer need to protect Mars, using Zubrin's meteorite transfer argument essentially. This was rebutted in a follow up article "Appropriate Protection of Mars", in the same journal by the planetary protection officer Catherine Conley.
Critique of Category V containment measures
The scientific consensus is that the potential for large-scale effects, either through pathogenesis or ecological disruption, is extremely small. Nevertheless, returned samples from Mars will be treated as potentially biohazardous until scientists can determine that the returned samples are safe. The goal is to reduce the probability of release of a Mars particle to less than one in a million.
The International Committee Against Mars Sample Return agree with the assessment of low probability of large-scale effects, but consider the proposed containment measures insufficient, given the possible severity of the worst-case scenario. They come to this conclusion partly as a result of considerations of human error and the novelty of the mission proposal. Consequently, they advocate much more in situ research before undertaking a Mars Sample Return (MSR).
Proposal for extension of protection to non-biological considerations
Recommendations of the workshop include:
Recommendation 3 COSPAR should add a separate and parallel policy to
provide guidance on requirements/best practices for protection of non-living/nonlife-related aspects of Outer Space and celestial bodies
Ideas for doing this suggested since the workshop include protected special regions, or "Planetary Parks" to keep regions of our solar system pristine for future scientific investigation, and also for ethical reasons.
Proposal to extend planetary protection
Astrobiologist Christopher McKay has argued that until we have better understanding of Mars, our explorations should be biologically reversible. For instance if all the microorganisms introduced to Mars so far remain dormant within the spacecraft, they could in principle be removed in the future, leaving Mars completely free of contamination from modern Earth lifeforms.
In the 2010 workshop one of the recommendations for future consideration was to extend the period for contamination prevention to the maximum viable lifetime of dormant microorganisms introduced to the planet.
"'Recommendation 4.' COSPAR should consider that the appropriate protection of potential indigenous extraterrestrial life shall include avoiding the harmful contamination of any habitable environment —whether extant or foreseeable— within the maximum potential time of viability of any terrestrial organisms (including microbial spores) that may be introduced into that environment by human or robotic activity."
In the case of Europa, a similar idea has been suggested, that it is not enough to keep it free from contamination during our current exploration period. It might be that Europa is of sufficient scientific interest that we have a duty to keep it pristine for future generations to study as well. This was the majority view of the 2000 task force examining Europa, though there was a minority view of the same task force that such strong protection measures are not required.
"One consequence of this view is that Europa must be protected from contamination for an open-ended period, until it can be demonstrated that no ocean exists or that no organisms are present. Thus, we need to be concerned that over a time scale on the order of 10 million to 100 million years (an approximate age for the surface of Europa), any contaminating material is likely to be carried into the deep ice crust or into the underlying ocean."
- List of microorganisms tested in outer space
- Mars 2020 rover mission
- Assessment of Planetary Protection and Contamination Control Technologies for Future Planetary Science Missions, Jet Propulsion Laboratory, January 24, 2011
3.1.1 Microbial Reduction Methodologies:
"This protocol was defined in concert with Viking, the first mission to face the most stringent planetary protection requirements; its implementation remains the gold standard today."
- "Planetary protection policy overview and application to future missions". Advances in Space Research 9 (6): 181–184. 1989. Bibcode:1989AdSpR...9..181T. doi:10.1016/0273-1177(89)90161-0. Retrieved 2012-09-11.
- Portree, David S.F. (2 October 2013). "Spraying Bugs on Mars (1964)". Wired (magazine). Retrieved 3 October 2013.
- NASA Office of Planetary Protection. "Planetary Protection History". Retrieved 2013-07-13.
- Preventing the Forward Contamination of Mars ( 2006 ) - Page 12
- Preventing the Forward Contamination of Mars
- Preventing the Forward Contamination of Mars - p12 quotes from COSPAR 1964 Resolution 26
- Full text of the Outer Space Treaty Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies - See Article IX
- Centre National d’Etudes Spatiales (CNES) (2008). "Planetary protection treaties and recommendations". Retrieved 2012-09-11.
- Preventing the Forward Contamination of Mars ( 2006 ) - Page 13
- COSPAR scientific assemblies
- COSPAR PLANETARY PROTECTION POLICY (20 October 2002; As Amended to 24 March 2011)
- Planetary Protection Standards for Icy Bodies in the Outer Solar System - about the Committee on Planetary Protection Standards for Icy Bodies in the Outer Solar System
- Carl Sagan and Sidney Coleman Decontamination Standards for Martian Exploration Programs, Chapter 28 from Biology and the Exploration of Mars: Report of a Study edited by Colin Stephenson Pittendrigh, Wolf Vishniac, J. P. T. Pearman, National Academies, 1966 - Life on other planets
- Keeping it clean
- COSPAR Workshop on Planetary Protection for Outer Planet Satellites and Small Solar System Bodies European Space Policy Institute (ESPI), 15–17 April 2009
- COSPAR power point type presentation, gives good overview of the detailed category decisions
- Assessment of Planetary Protection Requirements for Venus Missions -- Letter Report
- COSPAR Final
- COSPAR Workshop on Planetary Protection for Titan and Ganymede
- Catharine Conley Planetary Protection for the Dawn Mission, NASA HQ, Jan 2013
- edited by Muriel Gargaud, Ricardo Amils, Henderson James Cleaves, Michel Viso, Daniele Pinti Encyclopedia of Astrobiology, Volume 1 page 325
- Richard Greenberg, Richard J. Greenberg Unmasking Europa: the search for life on Jupiter's ocean moon ISBN 0387479368
- Paul Gilster Europa: Thin Ice and Contamination
- B. Randall Tufts, Richard Greenberg Infecting Other World, American Scientist, July 2001
- Europa the Ocean Moon, Search for an Alien Biosphere, chapter 21.5.2 Standards and Risks
- Committee on Planetary Protection Standards for Icy Bodies in the Outer Solar System; Space Studies Board; Division on Engineering and Physical Sciences; National Research Council Assessment of Planetary Protection Requirements for Spacecraft Missions to Icy Solar System Bodies ( 2012 ) / 2 Binary Decision Trees
- Designing a Box to Return Samples From Mars Astrobio.net, 2013
- Office of Planetary Protection: Mars Sample Quarantine Protocol Workshop
- Mars sample return mission concept study (for decadal review 2010)
- Proof of concept of a Bio-Containment System for Mars Sample Return Mission
- Mars Sample Return backward contamination – Strategic advice and requirements- forward and section 1.2
- Richard S. Johnston, John A. Mason, Bennie C. Wooley, Gary W. McCollum, Bernard J. Mieszkuc BIOMEDICAL RESULTS OF APOLLO, SECTION V, CHAPTER 1, THE LUNAR QUARANTINE PROGRAM
- Nancy Atkinson How to Handle Moon Rocks and Lunar Bugs: A Personal History of Apollo’s Lunar Receiving Lab, Universe Today, July 2009. See quote from: McLane who lead the group that designed and built the Lunar Receiving Facility:
"The best that I hear now is that the techniques of isolation we used wouldn’t be adequate for a sample coming back from Mars, so somebody else has a big job on their hands."
- The Quarantine and Certification of Martian Samples - Chapter 7: Lessons Learned from the Quarantine of Apollo Lunar Samples, Committee on Planetary and Lunar Exploration, Space Studies Board
- Assessment of Planetary Protection Requirements for Mars Sample Return Missions (Report). National Research Council. 2009. http://www.nap.edu/openbook.php?record_id=12576.
- European Science Foundation - Mars Sample Return backward contamination - strategic advice July, 2012, ISBN 978-2-918428-67-1
- M. S. Race Planetary Protection, Legal Ambiguity, and the Decision Making Process for Mars Sample Return Adv. Space Res. vol 18 no 1/2 pp (1/2)345-(1/2)350 1996
- In-situ Exploration and Sample Return: Planetary Protection Technologies JPL - Mars Exploration Rovers
- Office of Planetary Protection (August 28, 2012). "Office of Planetary Protection - Methods and Implementation". NASA. Retrieved 2012-09-11.
- Benton C. Clark (2004). "Temperature–time issues in bioburden control for planetary protection". Advances in Space Research 34 (11): 2314–2319. Bibcode:2004AdSpR..34.2314C. doi:10.1016/j.asr.2003.06.037.
- Assessment of Planetary Protection and Contamination Control Technologies for Future Planetary Science Missions see Section 3.1.2 Bio-burden Detection and Assessment. January 24, JPL, 2011
- Fei Chen, Terri Mckay, James Andy Spry, Anthony Colozza, Salvador Distefano, Robert Cataldo Planetary Protection Concerns During Pre-Launch Radioisotope Power System Final Integration Activities - includes the draft specification of VHP sterilization and details of how it would be implemented. Proceedings of Nuclear and Emerging Technologies for Space 2013. Albuquerque, NM, February 25–28, 2013 Paper 6766
- Microbial characterization of the Mars Odyssey spacecraft and its encapsulation facility, Environ Microbiol. 2003 Oct;5(10):977-85., "Several spore-forming isolates were resistant to gamma-radiation, UV, H2O2 and desiccation, and one Acinetobacter radioresistens isolate and several Aureobasidium, isolated directly from the spacecraft, survived various conditions."
- http://www.ncbi.nlm.nih.gov/pubmed/20446872 Recurrent isolation of extremotolerant bacteria from the clean room where Phoenix spacecraft components were assembled] Astrobiology. 2010 Apr;10(3):325-35. doi: 10.1089/ast.2009.0396 "Extremotolerant bacteria that could potentially survive conditions experienced en route to Mars or on the planet's surface were isolated with a series of cultivation-based assays that promoted the growth of a variety of organisms, including spore formers, mesophilic heterotrophs, anaerobes, thermophiles, psychrophiles, alkaliphiles, and bacteria resistant to UVC radiation and hydrogen peroxide exposure".
- Webster, Guy (6 November 2013). "Rare New Microbe Found in Two Distant Clean Rooms". NASA. Retrieved 6 November 2013.
- Madhusoodanan, Jyoti (19 May 2014). "Microbial stowaways to Mars identified". Nature (journal). doi:10.1038/nature.2014.15249. Retrieved 23 May 2014.
- A. Debus (2004). "Estimation and assessment of Mars contamination". Advances in Space Research 35 (9): 1648–1653. Bibcode:2005AdSpR..35.1648D. doi:10.1016/j.asr.2005.04.084. PMID 16175730.
- Preventing the Forward Contamination of Mars ( 2006 ) Page 27 (footnote to page 26) of chapter 2 Policies and Practices in Planetary Protection
- Preventing the Forward Contamination of Mars ( 2006 ) Page 22 of chapter 2 Policies and Practices in Planetary Protection
- Mars Climate Orbiter page at 
- Alberto G. Fairén & Dirk Schulze-Makuch The Over Protection of Mars Nature Geoscience 6, 510–511 (2013) doi:10.1038/ngeo1866
- Robert Zubrin "Contamination From Mars: No Threat", The Planetary Report July/Aug. 2000, P.4–5
- transcription of a tele-conference interview with ROBERT ZUBRIN conducted on March 30, 2001 by the class members of STS497 I, "Space Colonization"; Instructor: Dr. Chris Churchill
- O. Eugster, G. F. Herzog, K. Marti, M. W. Caffee Irradiation Records, Cosmic-Ray Exposure Ages, and Transfer Times of Meteorites, see section 4.5 Martian Meteorites LPI, 2006
- L.E. NYQUIST1, D.D. BOGARD1, C.-Y. SHIH2, A. GRESHAKE3, D. STÖFFLER AGES AND GEOLOGIC HISTORIES OF MARTIAN METEORITES 2001
- Tony Irving Martian Meteorites - has graphs of ejection ages - site maintained by Tony Irving for up to date information on Martian meteorites
- "5: The Potential for Large-Scale Effects"". Assessment of Planetary Protection Requirements for Mars Sample Return Missions (Report). National Research Council. 2009. p. 48. http://www.nap.edu/openbook.php?record_id=12576&page=48. "Despite suggestions to the contrary, it is simply not possible, on the basis of current knowledge, to determine whether viable martian life forms have already been delivered to the Earth. Certainly in the modern era there is no evidence for large-scale or other negative effects that are attributable to the frequent deliveries to Earth of essentially unaltered Martian rocks. However the possibility that such effects occurred in the distant past cannot be discounted. Thus it is not appropriate to argue that the existence of martian microbes on Earth negates the need to treat as potentially hazardous any samples returned from Mars via robotic spacecraft."
- The Overprotection of Mars?, astrobio.net, Andrew Williams - Nov 18, 2013 - has links to the papers
- http://mepag.nasa.gov/reports/iMARS_FinalReport.pdf Preliminary Planning for an International Mars Sample Return Mission Report of the International Mars Architecture for the Return of Samples (iMARS) Working Group June 1, 2008
- European Science Foundation - Mars Sample Return backward contamination - Strategic advice and requirements July, 2012, ISBN 978-2-918428-67-1 - see Back Planetary Protection section. (for more details of the document see abstract )
- Joshua Lederberg Parasites Face a Perpetual Dilemma Volume 65, Number 2, 1999 / American Society for Microbiology News 77.
- http://planetaryprotection.nasa.gov/summary/msr Mars Sample Return: Issues and Recommendations. Task Group on Issues in Sample Return. National Academies Press, Washington, DC (1997).
- International Committee Against Mars Sample Return
- Barry E. DiGregorio The dilemma of Mars sample return August 2001 Vol. 31, No. 8, pp 18–27.
- Rummel, J., Race, M., and Horneck, G. eds. 2011. COSPAR Workshop on Ethical Considerations for Planetary Protection in Space Exploration COSPAR, Paris, 51 pp.
- Ethical Considerations for Planetary Protection in Space Exploration: A Workshop, Astrobiology. 2012 November; 12(11): 1017–1023.
- 'Planetary Parks' Could Protect Space Wilderness by Leonard David, SPACE.com’s Space Insider Columnist, January 17, 2013
- Christopher P. McKay Planetary Ecosynthesis on Mars: Restoration Ecology and Environmental Ethics NASA Ames Research Center
- Christopher P. McKay Biologically Reversible Exploration Science 6 February 2009: Vol. 323 no. 5915 p. 718 doi:10.1126/science.1167987
- Preventing the forward contamination of Europa - Executive Summary page 2 National Academies Press
- Sagan, C., and S. Coleman (1965). "Spacecraft sterilization standards and contamination of Mars". Journal of Astronautics and Aeronautics 3 (5): 22–27.
- L. I. Tennen (2006). "Evolution of the planetary protection policy: conflict of science and jurisprudence". Advances in Space Research 34 (11): 2354–2362. Bibcode:2004AdSpR..34.2354T. doi:10.1016/j.asr.2004.01.018.
- L. Perek (2006). "Planetary protection: lessons learned". Advances in Space Research 34 (11): 2354–2362. Bibcode:2004AdSpR..34.2368P. doi:10.1016/j.asr.2003.02.066.
- J. D. Rummel, P. D. Stabekis, D. L. Devincenzi, J. B. Barengoltz (2002). "COSPAR's planetary protection policy: A consolidated draft". Advances in Space Research 30 (6): 1567–1571. Bibcode:2002AdSpR..30.1567R. doi:10.1016/S0273-1177(02)00479-9.
- D. L. DeVincenzi, P. Stabekis and J. Barengoltz (1996). "Refinement of planetary protection policy for Mars missions". Advances in Space Research 18 (1–2): 311–316. Bibcode:1996AdSpR..18..311D. doi:10.1016/0273-1177(95)00821-U. PMID 11538978.
- J. Barengoltz and P. D. Stabekis (1983). "U.S. planetary protection program: Implementation highlights". Advances in Space Research 3 (8): 5–12. Bibcode:1983AdSpR...3....5B. doi:10.1016/0273-1177(83)90166-7.
- L. P. Daspit, J. A. Stern, E. M. Cortright (1975). "Viking heat sterilization—Progress and problems". Acta Astronautica 2 (7–8): 649–666. doi:10.1016/0094-5765(75)90007-7.
- No bugs please, this is a clean planet! (ESA article)
- COSPAR planetary protection policy, July 2008 (COSPAR article)
- NASA Planetary Protection Website
- JPL Develops High-Speed Test to Improve Pathogen Decontamination at JPL.
- Geoethics in Planetary and Space Exploration
- Catharine Conley: NASA & international planetary protection policy, methodology & applications, The Space Show, October 2012
- GYRE.org Mars Robot may have destroyed evidence of life
- St. Louis Post-Dispatch Scientists fear Interplanetary Contamination in new Mars missions
- Wired Magazine Water Bears Are Headed for a Martian Moon