When Neil Armstrong, Buzz Aldrin, and Michael Collins returned from the Moon in the summer of 1969, they spent three weeks isolated in quarantine to make sure that they hadn’t brought back any microbial lifeforms from the Moon, which could prove harmful to Earth life. Later, once the Moon had been unequivocally proved to be a dead world, future Apollo missions were allowed to skip quarantine. Elsewhere in the solar system, however, NASA still has to take planetary biosecurity seriously, because life could be out there. If we bring it back to Earth, it could be a danger to us and our ecosystems. Conversely, microbial Earth life could invade a fragile alien ecosystem, destroying a newly discovered lifeform before we have the chance to study it. Imagine discovering life on Mars, only to realize that it was life we had brought there with us.

To prevent these scenarios, planetary protection strategies are employed by NASA and other space agencies worldwide to minimize the risk of interplanetary cross-contamination. Mars rovers, for example, are all meticulously decontaminated before launch, ensuring that no Earth life makes its way to the surface of Mars.

For the moment, these practices are a safeguard against a purely hypothetical risk – no one knows if life exists beyond Earth. But if it does, we need to be ready for the consequences.

Technicians and engineers in clean-room garb monitor the first drive test of NASA’s Curiosity rover, on July 23, 2010. Credit: NASA/JPL-Caltech.

How do you prepare for something that might not exist? By examining something that does. Invasive species are a major problem worldwide. Human trade and travel imports species – often by accident – from one corner of the world to another. The effects can be devastating, wiping out local flora and fauna, reducing biodiversity, and forever altering ecosystems. What lessons can we learn from these very real challenges, to help us prepare for the possibility of an interplanetary equivalent?

A paper published in BioScience on November 17th by Anthony Ricciardi, Phillip Cassey, Stefan Leuko, and Andrew Woolnough examines this question and lays out several takeaways from the battle against invasive species here on Earth that apply to space exploration. Three highlights from the paper stand out:

Insular systems are the most vulnerable: Remote islands, small isolated lakes, or remote habitats are always hit hardest by invasive species. New predators can throw off the balance of these isolated ecosystems, destroying them quickly and ruthlessly. On a planetary scale, this means that we are more of a threat to small microbial ecosystems that might exist on Mars, Europa, or Titan, for example, then they are to us. Earth’s interconnected biodiversity is a safeguard that might protect us from an invasive alien species. But it is all the more important that we take care not to destroy any isolated lifeform we might find elsewhere in the Solar System, because it may not take much.Invasions are often caused by the least secure human activities, so fix the weak links in our practices: Planetary protection practices (like disinfecting the Mars rovers) kill off less dangerous microbes, but the hardiest one can survive. It is these extremophiles who pose the most risk to alien ecosystems. In our attempt to sterilize the rovers, we put microbes through a life-or-death test. Any that survive sterilization face another test on route, being exposed to deep space radiation. Only the strongest microbes will ever make it to Mars. If we’re not careful, our incomplete attempts to clean the rovers might actually enhance an organism’s tolerance and invasion potential. This is bad news, and means we need to get better at detecting microbes to make sure our sterilization efforts are as rigorous as possible and eliminate even these super-resilient microbes.Early detection and rapid response are crucial: When invasive species reach a new
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