If one rewinds 500 years, to when exploration of new worlds involved sailing the oceans, the discovery of the Americas introduced viruses which decimated the native population at that time [7]. That in itself was far from a unique event in history, of course. There have been many occurrences throughout history where travel between distant lands has resulted in the introduction of devastating plagues to one population or the other — not least the Black Death, which arrived in Europe from commercial travel with Asia in the 1300s [8]. Meanwhile, 2020 has reminded us how a novel virus can prove virtually unstoppable from spreading worldwide in a matter of months and reaching pandemic level, once introduced to our now interconnected world [9].

Indeed when the first astronauts returned from the Moon in the 60s, they had to undergo weeks of quarantine as a precaution against introducing a lunar pathogen to Earth [10]. We now know the Moon to be a sterile world, but this should not give us a false sense of security when visiting and returning from other worlds, which are far more likely to harbour microbial life. It is quite plausible to consider that any microbes which have evolved to survive in the harsh environments on other worlds could multiply out of control if introduced to a more fertile environment on Earth. The likelihood of any such foreign microbes being capable of becoming infectious pathogens to our species is difficult to measure, but one could still cause problems regardless, by undermining Earth’s ecosystem in competing with native microbial life as a runaway invasive species.

Fortunately, due to the vast distances involved in inter-planetary travel, returning astronauts would likely show symptoms of infection from any dangerous pathogen long before reaching home, as such a journey would be expected to take many months, even with more advanced propulsion technology than we use in space travel today. That is not to say they could not inadvertently return with microbial life on board — or even on the exterior of craft: Earth’s tardigrades, for example, have proven quite durable in journeys into outer space [11].

Undoubtedly, finding life on any other world — even if just primitive microbial life — would be hailed as an unprecedented scientific discovery. As that potential draws nearer, any such discovery should surely be met with due caution, rather than wild excitement. While not as dramatic as an invasion of Spiders from Mars, the discovery of microbial life on other worlds could prove to be a far more sobering prospect — and pose new ethical questions of whether to leave their ecosystems preserved intact, or take the risk blending them with ours.

[1] Strange chemical in clouds of Venus defies explanation. Could it be a sign of life?
https://www.space.com/venus-clouds-possible-life-chemical-discovery.html

[2] How Martian Microbes Could Survive in the Salty Puddles of the Red Planet https://www.space.com/salt-tolerant-microbes-life-on-mars.html

[3] If There’s Life on Saturn’s Moon Enceladus, It Might Look Like This https://www.wired.com/story/if-theres-life-on-saturns-moon-e…like-this/

[4] ‘Racing certainty’ there’s life on Europa, says leading UK space scientist
https://phys.org/news/2020-02-certainty-life-europa-mars-uk.html

[5] The dwarf planet Ceres might be home to an underground ocean of water
https://www.technologyreview.com/2020/08/11/1006447/ceres-la…alty-water

[6] ‘Possibility of life’: scientists map Saturn’s exotic moon Titan
https://www.reuters.com/article/us-space-titan-idUSKBN1XS2H2

[7] How Europeans brought sickness to the New World
https://www.sciencemag.org/news/2015/06/how-europeans-brought-sickness-new-world

[8] The Black Death: The Greatest Catastrophe Ever
https://www.historytoday.com/archive/black-death-greatest-catastrophe-ever

[9] Coronavirus: A timeline of how the deadly COVID-19 outbreak is evolving
https://www.pharmaceutical-technology.com/news/coronavirus-a…k-evolved/

[10] Apollo 11 Astronauts Spent 3 Weeks in Quarantine, Just in Case of Moon Plague
https://www.space.com/apollo-11-astronauts-quarantined-after-splashdown.html

[11] How do tardigrades survive in space?
https://cen.acs.org/biological-chemistry/biochemistry/tardig…ace/97/i41