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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 microbial life from Earth to another celestial body. A major goal of planetary protection is to preserve the planetary record of natural processes by preventing human-caused microbial introductions. Back contamination would be the introduction of hypothetical microbial extraterrestrial organisms into Earth's biosphere.

Resilience of life in space
It is expected that the harsh environments encountered throughout the rest of the Solar System do not seem to support complex terrestrial life; however, certain extremophiles or even ordinary bacteria may be resilient enough to survive space travel to possibly contaminate a sterile planet or planetoid.

For example, the camera mounted onboard the Surveyor 3 lunar lander may have been contaminated with Streptococcus mitis before launch. The camera was returned to Earth by the Apollo 12 mission, and viable spores of Streptococcus mitis were reported within the camera. However, the laboratory protocol at the time was not as stringent as current standards. Therefore, it is possible that the contamination occurred after the camera was returned to Earth.

In addition, scientists reported in 2012 that lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).

COSPAR recommendations
The Committee on Space Research (COSPAR) translates these considerations into recommendations for avoiding interplanetary contamination. Recommendations are tailored to the type of space mission—from planetary flybys to probe landings—and celestial body explored. COSPAR categorizes the missions into 5 groups:
 * Category I: Any mission to the Sun, Mercury, other locations not of interest for studying prebiotic chemistry or the origin and evolution of life.
 * Category II: Any mission to the Earth's Moon, Venus, comets, Jupiter, Pluto/Charon, Kuiper Belt Objects, other locations of interest for studying prebiotic chemistry and the origin of life but for which there is an insignificant probability of contamination with Earth life.
 * Category III: Flyby and orbiter missions to locations with the potential to host life and for which there is a possibility of contamination by Earth life; e.g., Mars, Europa, Titan or Enceladus.
 * Category IV: Lander or probe missions to locations with the potential to host life and for which there is a possibility of contamination by Earth life; e.g., Mars, Europa, Titan or Enceladus. Missions to Mars in category IV are subclassified further:
 * Category IVa. Landers that do not search for Martian life - same as Viking pre-sterilization levels, 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.
 * Category V: Any earth return mission. Missions returning samples from locations with the potential to support life are considered 'Restricted Earth Return' and returned samples must be contained at levels more stringent than Biosafety level 4. Samples from locations judged unlikely to support life are considered 'Unrestricted Earth Return' and merit no constraints for planetary protection purposes.

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', but in the case of Mars this has been translated into a metric for the number of Bacillus spores per surface area and present in total on or within the spacecraft: 300 spores per m² free surface, but not more than 300,000 spores in total (category IVa). These amounts are ten thousand times less if the lander is in category IVc (a maximum of 30 spores total). Any sample-return vehicle must then be designed such that the sample is returned in highly reliable containers with measures in place to dispose of any parts of the vehicle which could have been contaminated before re-entry into the Earth's atmosphere.

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.

History
In 1967, most of the world's nations ratified the United Nations Outer Space Treaty. The policy of protecting pristine celestial environments is accepted with virtual unanimity, and has been incorporated into positive international law. The treaty's planetary protection provisions stipulate that nations 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." The United Nations incorporated these provisions in its 1979 Moon Treaty governing the activities of states on the Moon and other celestial bodies, and in the Vienna Declaration of 1999.

Investigation on the microbial diversity found on spacecraft and in assembly halls used by Europe and U.S.A. were initially limited to cultivable microorganisms, and later, to surface sampling with swabs for nucleic acids screening through the PCR method. While using both the swabbing and cultivation approach for the detection of microorganisms on a surface, two impairing factors have to be accounted for: First, not all bacteria present on a surface will be taken up by the swab and second, not all the bacteria taken up will be released again from the swab. This has led to an underestimation —by a factor of 3— of microorganisms transported by spacecraft. The fact today is that Mars orbital environment includes orbiters and perhaps debris, and that its atmosphere and its surface include terrestrial compounds and dormant microorganisms.

The following are some historical and proposed efforts to avoid interplanetary contamination:
 * The Galileo spacecraft was deliberately crashed into Jupiter at the end of the mission in order to avoid contaminating any of the moons of Jupiter.


 * In order to prevent back contamination of the Earth from hypothetical lunar microbes, the astronauts and samples from the early manned Apollo missions that landed on the Moon (11, 12, & 14) were quarantined in the Lunar Receiving Laboratory. Note that immediately after the splashdown of Apollo 11, the crane aboard the aircraft-carrier to be used to retrieve the sealed command module was found to be unsafe. In addition, the Apollo 11 crew were becoming seasick in the rough sea conditions. Consequently, U.S. Navy frogmen opened the command modules hatch, allowing the astronauts--and any lunar microbes, had they been aboard--to exit the spacecraft.


 * Although there are no concrete plans to return samples from Mars, the potential scientific value of the proposed Mars Sample Return (MSR) mission remains a high priority for the NASA Decadal Survey. Although it is unknown whether Mars actually harbor life, in order to avoid back contamination from Mars, any returned samples will be treated as potentially biohazardous, and will be kept in a Biosafety Level 4 receiving facility until scientists determine the samples are safe.

Process
The risk of forward contamination by terrestrial micro-organisms depends on their ability to survive the voyage and on the environmental conditions they find on arrival. The spacecraft must be sterilized before leaving Earth in order to minimize the risk of depositing Earth-originating biological material at the destination. Heat energy, administered in the form of an elevated temperature heat soak over a specific interval of time, is a well-known method for inactivating organisms. Clean room assembly and microbial reduction through heat, chemicals or radiation are the basic techniques used to accomplish microbial control when this is necessary for a mission. NASA currently has only one approved method – dry heat microbial reduction. This technique was used on the Viking Mars landers, which were built and launched in the 1970s. Advanced materials, electronics, and other heat-sensitive equipment being used on spacecraft today could be damaged by such high-temperature treatment, however. Consequently, NASA researchers are developing an alternative sterilization method, a low-temperature, vapor-phase, hydrogen peroxide-based sterilization process. The certification process to support this goal is lengthy and requires substantial fundamental research and method standardization. Two methods being considered for near-term submission to NASA for use on spacecraft are Limulus Amebocyte Lysate assay, and Adenosine Triphosphate assay.

However, it is should be noted that as long as spacecraft missions are sent to other extraterrestrial bodies, it is not possible to keep them perfectly clean. Recent experiments demonstrate that various organisms can easily survive the vacuum conditions of outer space, for instance the Expose E experiments on the ISS.