With the rise of space exploration, the question of interplanetary contamination naturally arose, and scientists as well as institutions had to look into the problem in order to develop protocols to avoid any risk. Indeed, it is not impossible that a spacecraft (probe, manned spacecraft, etc.) launched from the Earth biologically contaminates another celestial body or, on the contrary, can introduce biological elements which are foreign to it on the Earth.
Currently, questions of interplanetary contamination are the subject of recommendations prescribed both by the Space Treaty and by both the COSPAR (Committee for Research on Space). In general, they concern only microbial biological contamination. Non-biological contamination (lunar regolith for example) is also addressed but to a lesser extent.
There are two types:direct contamination, which consists of the contamination of a celestial body by terrestrial organisms. And indirect (or return) contamination, which consists of the introduction of foreign organisms on Earth upon return from a robotic or manned space mission. Although contamination by multicellular organisms is unlikely to occur, requirements exist for certain elements such as lichen.
The question of interplanetary contamination arises because even if no extraterrestrial life has yet been discovered, several places in the Solar System could harbor it, such as Mars, Enceladus, Europe or even Titan. It is well known that some extremophile organisms can withstand drastic environmental conditions and could thus easily survive space travel, resulting in either of two types of contamination.
Such an event could then have serious consequences. Indeed, the introduction of terrestrial microorganisms on a foreign body would therefore distort any attempt at scientific analyzes of a potential life. Because it would become impossible for researchers to know whether or not these organisms are native to the planet in question. Furthermore, the chemical elements produced by these terrestrial organisms would confuse the search for biosignatures of past or present extraterrestrial life.
In most cases, extremophiles and archaea cannot be cultured in the laboratory because of the necessary experimental conditions. These bacteria are possibly only known thanks to their genetic sequencing and many of them have never been sequenced. It would then become difficult for scientists to distinguish the DNA of an extraterrestrial bacterium from that of a bacterium introduced from Earth.
The risk of introducing invasive species should also be considered, although it is much less likely. It mainly concerns multicellular organisms whose involuntary introduction remains unlikely. However, the consequence could be a competition, even an annihilation of the endemic species of the planet. Nevertheless, some scientists believe that these species would in any case be better adapted than the introduced ones, and that the danger would therefore be minimal.
Direct contamination is subject to strict international rules. Any space object sent into space from Earth must be decontaminated by several methods generally applied simultaneously:thermal sterilization, chemical sterilization, oxidation, irradiation and UV. These mandatory standards were formalized in the early 1960s, notably with recommendations NMI-4-4-1, NASA Unmanned Spacecraft Decontamination Policy, adopted in 1963.
However, these decontamination rules are not universal, they are dealt with on a case-by-case basis depending on the intended destination. For example, the planet Mercury is not the subject of any particular recommendation, the possibility of extraterrestrial life being ruled out due to the environmental conditions prevailing there. Regarding the Moon, no sterilization is also necessary, only documentation is mandatory. As for Mars, any object sent (especially rovers) must be strictly decontaminated.
In the case of manned missions, the process is much more complex. A human cannot be decontaminated in the same way as a robot. However, the risk of contamination is much greater since humans are natural hosts of many types of microorganisms. And these cannot be eliminated because most of them contribute to the proper functioning of our body (especially the microbiome). Only isolation and quarantine are possible, but difficult to implement.
Robotic planetary exploration remains the best way to avoid any risk of contamination. A human crew can remain in orbit in order to control exploration robots on the surface of the planet. Thus, the mission benefits from all the advantages of efficient exploration while considerably reducing the risk of introducing microorganisms.
The planet Mars is the main destination currently being recommended by the scientific community. In the case of a manned mission or a mission to collect and return samples, if the risk of indirect contamination is low, it is however not excluded. Although the return of Mars samples to Earth is not yet on the agenda, it is considered to be of great scientific interest by the European Space Foundation report.
If NASA had already established rules regarding indirect contamination during the return of lunar samples by the Apollo 11 mission, these were relatively weak, the possibility of life on the Moon being considered extremely unlikely. But since then, these standards have been updated and incorporated into a more drastic regulatory framework.
The first recommendation when returning samples is to break the chain of contact between the sample container and the source planet. The container could be hermetically sealed and introduced into a larger container in the vacuum of space, before returning to Earth. In addition, the sample container should be properly developed (structure, alloy, etc.) in order to withstand a crash on Earth if the capsule's parachute were to malfunction.
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The second recommendation is the creation by each country organizing this kind of mission of at least one P4 type laboratory (pathogen 4); i.e. laboratories intended for the manipulation and study of extremely virulent pathogenic microorganisms, for which no vaccine or effective treatment exists. This is for example the case of the French P4 laboratory Jean Mérieux, located in the city of Lyon, which handles viruses such as ebola, the Lassa or Marburg virus, and bacterial agents such as smallpox or the plague.
Such a laboratory should be specially designed to contain organisms still unknown on Earth. For this, strict construction rules are provided. Considering that the size of extraterrestrial microorganisms is unpredictable, a filtration system trapping any particle of at least 0.01 µm is mandatory.
This size is recommended in order to be able to trap gene transfer agents (GTAs), which are DNA segments that certain bacteria exchange with each other in the case of horizontal gene transfers. In addition, such a filtration diameter also makes it possible to trap ultramicrobacteria, the size of which is generally less than 0.3 µm.
Finally, a clean room with all the pressure, air renewal and sterilization parameters must be provided. Successive decontamination airlocks must be present at each entrance to the analysis rooms, as well as secure access with double validation (biometric fingerprints, retinal scanners, etc.). In the case of the Mars Sample Return Receiving Facility (MSRRF) laboratory envisaged by NASA for the return of Martian samples, a construction period of 7 to 10 years is estimated.
Many exobiologists propose to study and analyze samples taken directly from the surface of the planet, rather than considering return missions that are costly, complex to set up and risky in terms of contamination. To do this, they recommend equipping rovers with much more sensitive and sophisticated analysis instruments, allowing them to study rocks, soil and potential biosignatures.
These in-situ robotic missions could be conducted from orbit of the planet by a human crew. At the 2012 Exploration Telerobotics Symposium, many experts agreed on the enormous benefits of remote sample manipulation. Telerobotics allows the analysis of samples directly on the surface of the planet while benefiting from the scientific knowledge of the crew in orbit, and while neutralizing latency times between robots and humans.
