Contaminarea extraterestră (engleză back-contamination) este o introducere de organisme ipotetice microbiene extraterestre în biosfera Pământului, de exemplu cu ocazia reîntoarcerii misiunilor spațiale. Se presupune că un astfel de contact ar fi perturbator sau cel puțin ar putea avea consecințe puțin controlabile de către ființele umane. Amenințarea contaminării extraterestre cu organisme microbiene originare de pe Lună (ipotetice) a fost principalul motiv pentru adoptarea procedurilor de carantină în cadrul programului Apollo, până la finalizarea lui Apollo 14. Astronauții și mostrele lunare aduse pe Pământ au trebuit să stea în carantină la întoarcerea pe Terra în clădirea numită Lunar Receiving Laboratory (LRL).
Această contaminare la înapoierea pe Pământ poate fi ușor înțeleasă greșit. Probabilitatea ca o ființă umană sau orice alt animal să dobândească literalmente un virus extraterestru este efectiv zero, pentru că virusurile au gazde specifice. Totuși, microbii extratereștri, dacă ar exista, ar putea acționa patogenic asupra noastră: sporii ar putea utiliza un organism ca gazdă, în timp ce ingestia de bacterii în orice formă ar putea produce substanțe chimice toxice. Când ființele umane ingeră alimente contaminate, de exemplu, aceste alimente nu aduc în organism vreun virus capabil de a produce gripa, dar cu toate astea experiența ar putea fi letală – din cauza compușilor toxici rezultanți.
În plus, există posibilitatea ca un microb extraterestru ipotetic să metabolizeze agresiv unele resurse ale Pământului sau să modifice condițiile atmosferice sau ale circuitului apei.[necesită citare]
Back contamination is the introduction of extraterrestrial organisms and other forms of contamination into Earth’s biosphere, it also covers infection of humans and human habitats in space and on other celestial bodies by extraterrestrial organisms, if such exist.
The main focus is on microbial life and on potentially invasive species. Non biological forms of contamination have also been considered including e.g. contamination of sensitive deposits (such as lunar polar ice deposits) of scientific interest by rocket exhausts. In the case of back contamination, multicellular life is thought unlikely but not been ruled out, and in case of forward contamination, then again, forward contamination by multicellular life (e.g. lichens) becomes a consideration if you have human missions though unlikely for robotic missions.
Current space missions are governed by the Outer Space Treaty and the COSPAR guidelines for planetary protection. Forward contamination is prevented primarily by sterilizing the spacecraft. In the case of backward contamination, however, the aim of the mission is to return biological material to Earth if such exists, and sterilization of the samples would make them of much less interest. So back contamination would be prevented mainly by containment, and breaking the chain of contact between the planet and Earth. It would also require quarantine procedures for the materials and for anyone who comes into contact with them.
Since the Moon is now generally considered to be free from life, the most likely source of contamination is from Mars either during a Mars sample return or as a result of colonization of Mars.
There are no immediate plans for a Mars sample return, but it remains a high priority for NASA and the ESA, because of its great potential biological interest. The European Space Foundation report cites many advantages of a Mars sample return. In particular, it would permit extensive analysis with any of the equipment available on Earth, without the size and weight constraints for instruments sent to Mars on rovers. These analyses could also be carried out without the communication delays for experiments carried out on Martian rovers. It would also make it possible to repeat experiments in multiple laboratories with different instruments to confirm key results.
Carl Sagan was first to raise and publicise back contamination issues that might follow from a Mars sample return. In Cosmic Connection (1973) he writes:
Precisely because Mars is an environment of great potential biological interest, it is possible that on Mars there are pathogens, organisms which, if transported to the terrestrial environment, might do enormous biological damage.
He makes the analogy of the plot twist in The War of the Worlds, by H.G. Wells, where Earth pathogens made the Martian invaders of Earth sick and then they die. Perhaps, he suggests, the same could happen to us on Earth if we return samples containing micro-organisms from Mars. On the one hand, he points out, this possibility seems unlikely because of the lack of contact between the two planets and because pathogens adapt to their host. But on the other hand, the lack of contact also means that we would have never evolved any defences against any pathogens.
Also, pathogens when they adapt to a host normally evolve to be less rather than more lethal. Also, some pathogens such as Legionnaire’s disease attack humans using essentially the same mechanism they use to infect other microbes (in this case amoeba) so a disease of microbes on Mars could become a pathogen of animals on Earth.
Ledeberg wrote 
“Whether a microorganism from Mars exists and could attack us is more conjectural. If so, it might be a zoonosis to beat all others. On the one hand, how could microbes from Mars be pathogenic for hosts on Earth when so many subtle adaptations are needed for any new organisms to come into a host and cause disease? On the other hand, microorganisms make little besides proteins and carbohydrates, and the human or other mammalian immune systems typically respond to peptides or carbohydrates produced by invading pathogens. Thus, although the hypothetical parasite from Mars is not adapted to live in a host from Earth, our immune systems are not equipped to cope with totally alien parasites: a conceptual impasse”
This possibility has been confirmed in all the later studies, as the worst-case scenario. Other possibilities have also been raised such as micro-organisms that have harmful effects on crops, or that disrupt natural cycles, and pathogens that infect other micro-organisms.
As a result, the possibility of new human pathogens, or environmental disruption due to back contamination is considered to be of extremely low probability but can’t yet be ruled out completely.
Later in Cosmos (1980) Carl Sagan wrote:
Perhaps Martian samples can be safely returned to Earth. But I would want to be very sure before considering a returned-sample mission.
The PPO and NASA and ESA view is similar to this. The findings were that with present day technology, Martian samples can be safely returned to Earth provided the right precautions are taken.
The risks of environmental disruption resulting from the inadvertent contamination of Earth with putative martian microbes are still considered to be low. But since the risk cannot be demonstrated to be zero, due care and caution must be exercised in handling any martian materials returned to Earth.
Understanding of pathogenesis and the nature of biological epidemics has expanded significantly in recent years.2,3 However, the potential for large-scale pathogenic effects arising from the release of small quantities of pristine martian samples is still regarded as being very low. Significant changes have been made in requirements for containing both known pathogens and novel, or unknown, biological materials, and there have been major improvements in containment design, laboratory practices, and operational oversight.4,5,6 Numerous reports for planning a Mars sample return mission have acknowledged that biocontainment requirements and planetary protection controls will be integrated as essential elements for handling and testing returned samples.7,8,9,10
As reviewed in Chapter 3, extreme environments on Earth have not yet yielded any examples of life forms that are pathogenic in humans. However, it is worth noting in this context that interesting evolutionary connections between alpha proteobacteria and human pathogens have recently been demonstrated for natural hydrothermal environments on Earth,11 suggesting that evolutionary distances between nonpathogenic and pathogenic organisms may be quite small in some instances. It follows that, since the potential risks of pathogenesis cannot be reduced to zero,12 a conservative approach to planetary protection will be essential, with rigorous requirements for sample containment and testing protocols.
New discoveries in environmental microbiology continue to expand understanding of the taxonomic and metabolic diversity of the microbial world, yet much remains unknown.13 It is worth noting, however, that extreme environments on Earth have not yet yielded any examples of life forms that are disruptive to ecosystem functions. The risks of environmental disruption resulting from the inadvertent contamination of Earth with putative martian microbes are still considered to be low. But since the risk cannot be demonstrated to be zero, due care and caution must be exercised in handling any martian materials returned to Earth. The demand for a conservative approach to both containment and test protocols remains appropriate.
Toxicity and Other Potential Effects
Although negative effects from nonreplicating biological materials (e.g., toxins and other metabolic by-products) are possible, they are unlikely to be responsible for large-scale pathogenic effects.14 Nonetheless, they are important as potential biohazards that must be considered when designing protection for the workers who will handle returned martian materials. Operationally, the committee anticipates that existing regulatory frameworks (e.g., that of the Occupational Safety and Health Administration and the Centers for Disease Control and Prevention), coupled with rigorous laboratory biosafety controls, will be incorporated into future discussions of handling and testing protocols and other operations used in the analysis of returned martian materials.
Dissenting views on back contamination issues
The International Committee Against Mars Sample Return maintains that it is not possible to return samples to Earth safely at this stage. Their main reason for saying this is the novelty of the containment procedures required combined with the possibility of human error and mission design mistakes, either during the return flight or after return of the samples. They urge more in situ studies on Mars first, and preliminary biohazard testing in space before the samples are returned to Earth.
At the other extreme, Robert Zubrin (Mars surface colonization advocate and director of the Mars Society) maintains that the risk of back contamination has no scientific validity. He supports this using an argument based on the possibility of transfer of life from Earth to Mars on meteorites.