Since we’re building our prototype in our conference room and my garage, we won’t have to be concerned about this – A Rare New Bacterium Thrives In Spacecraft Cleanrooms:
Just because we haven’t found this weird little guy elsewhere doesn’t mean it doesn’t exist in nature. “We find a lot of bugs in clean rooms because we are looking so hard to find them there,” says Parag Vaishampayan, a microbiologist at NASA’s Jet Propulsion Laboratory who described the microbe in the International Journal of Systematic and Evolutionary Microbiology. “The same bug might be in the soil outside the clean room but we wouldn’t necessarily identify it there because it would be hidden by the overwhelming numbers of other bugs.” He says Tersicoccus phoenicis could live in caves or deserts, where it could survive on almost no nutrients, as in the cleanrooms.
This is an interesting point…how can we ensure planetary isolation if we can’t ever be sure that we’ve killed all the hitchhikers, because we either make incorrect or incomplete assumptions about what will kill them all or (in the extreme case) our knowledge/assumptions about what actually constitutes life blinds us to their presence? If your methods that should kill all bacteria don’t do so, and lead you to discover oddball extremophile-like species previously hidden from you, how do you know that when you’ve successfully eliminated those something else won’t appear (the competition having been removed), or that there isn’t something you simply can’t see?
Those who have read Niven/Pournelle/Barnes’ Legacy of Heorot will readily recognize some potential problems here.
Which is not to advocate for absolute isolation – rather, we may have to accommodate ourselves to the reality that contamination will occur no matter what we do so long as we interact with other planets. It’s certainly going to occur if we send settlers, but will occur with any probes as well. If we discover signs of life on Mars, for example, we may never know with certainty that they aren’t the result of cross-contamination (barring some truly bizarre differences), the knowledge that something could however improbably have survived spacecraft sterilization inducing a form of Observer’s Paradox.