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 in the case of sample-return missions. 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. Back contamination is the transfer of potential extraterrestrial organisms back to the Earth's biosphere. Sterilizing human missions is impossible, as the microbiome of a single human typically hosts a hundred trillion (1014) microorganisms of thousands of species, which cannot be removed without killing the person.
History
The potential problem of lunar and planetary contamination was first raised at the International Astronautical Federation VIIth Congress in Rome in 1956.
In 1958Preventing the Forward Contamination of Mars (2006) - Page 12 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 led 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".Preventing the Forward Contamination of Mars
In 1959, planetary protection was transferred to the newly formed Committee on Space Research (COSPAR). COSPAR in 1964 issued Resolution 26 affirming that:
In 1967, the US, USSR, and UK ratified the United Nations Outer Space Treaty. The legal basis for planetary protection lies in Article IX of this treaty:
This treaty has since been signed and ratified by 104 nation-states. Another 24 have signed but not ratified. All the current space-faring nation-states, along with all current aspiring space-faring nation-states, have both signed and ratified the treaty.
The Outer Space Treaty has consistent and widespread international support, and as a result of this, together with the fact that it is based on the 1963 declaration which was adopted by consensus in the UN National Assembly, it has taken on the status of customary international law. The provisions of the Outer Space Treaty are therefore binding on all states, even those who have neither signed nor ratified it.
For forward contamination, the phrase to be interpreted is "harmful contamination". Two legal reviews came to differing interpretations of this clause (both reviews were unofficial). However the currently accepted interpretation is that "any contamination which would result in harm to a state's experiments or programs is to be avoided". NASA policy states explicitly that "the conduct of scientific investigations of possible extraterrestrial life forms, precursors, and remnants must not be jeopardized".Preventing the Forward Contamination of Mars, page 13 Summarizes this para in the book:
COSPAR recommendations and categories
The Committee on Space Research (COSPAR) meets every two years, in a gathering of 2000 to 3000 scientists,COSPAR scientific assemblies 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 PLANETARY PROTECTION POLICY (20 October 2002; As Amended to 24 March 2011) 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. No planetary protection requirements. 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. Requires simple documentation only, primarily to outline intended or potential impact targets, and an end of mission report of any inadvertent impact site if such occurred. Category III: Flyby and orbiter missions to locations of significant interest for chemical evolution or the origin of life, and with a significant chance that contamination could compromise investigations e.g., Mars, Europa, Enceladus. Requires more involved documentation than Category II. Other requirements, depending on the mission, may include trajectory biasing, clean room assembly, bioburden reduction, and if impact is a possibility, inventory of organics. Category IV: Lander or probe missions to the same locations as Category III. Measures to be applied depend on the target body and the planned operations. "Sterilization of the entire spacecraft may be required for landers and rovers with life-detection experiments, and for those landing in or moving to a region where terrestrial microorganisms may survive and grow, or where indigenous life may be present. For other landers and rovers, the requirements would be for decontamination and partial sterilization of the landed hardware." 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, a maximum of 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 of 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.
For Category IV missions, 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.
More extensive documentation is also required for Category IV. Other procedures required, depending on the mission, may include trajectory biasing, the use of clean rooms during spacecraft assembly and testing, bioload reduction, partial sterilization of the hardware having direct contact with the target body, a bioshield for that hardware, and, in rare cases, complete sterilization of the entire spacecraft.
For restricted Category V missions, the current recommendationMars Sample Return backward contamination – Strategic advice and requirements - foreword and section 1.2 is that no uncontained samples should be returned unless sterilized. Since sterilization of the returned samples would destroy much of their science value, current proposals involve containment and quarantine procedures. For details, see Containment and quarantine below. 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 where terrestrial organisms could readily propagate, or thought to have a high 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.
Target categories 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:COSPAR Workshop on Planetary Protection for Outer Planet Satellites and Small Solar System Bodies European Space Policy Institute (ESPI), 15–17 April 2009COSPAR power point type presentation, gives good overview of the detailed category decisions
Category I
Io, Sun, Mercury, undifferentiated metamorphosed asteroids
Category II
Callisto, comets, asteroids of category P, D, and C, Venus, Kuiper belt objects (KBO) < 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 is 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 the Galileo spacecraft 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 are 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 both Ganymede and Titan were assigned a reasonably firm provisional Category II, but pending results of future research.
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 (Dawn) depending on the results found.Catharine Conley Planetary Protection for the Dawn Mission, NASA HQ, Jan 2013
Category III / IV
Mars because of possible subsurface habitats. Europa because of its subsurface ocean. Enceladus because of evidence of water plumes.
Category V