The same microbes that grow on cheese can also grow inside a Mars simulation chamber.
That might not sound like a big deal. But what if those microbes somehow made it to the real Mars?
We've launched tons of spacecraft, probes, and rovers to the red planet, all potentially carrying a few microbes from Earth.
Accidentally contaminating Mars could have big consequences. It might trick us into thinking we've discovered life on Mars when it's actually just Earth-grown stowaways.
But in some sense we've already contaminated it.
"Every mission to Mars makes it dirtier," Catharine Conley, a planetary protection officer at NASA, told Business Insider.
Why NASA has a celestial sheriff
Conley probably has the coolest job title at NASA, but it comes with a huge responsibility.
A planetary protection officer is charged with making sure we don't contaminate the rest of the universe with life from Earth. And, even more importantly, the reverse — to make sure we don't bring any alien microbes home.
The job was created because of an international law called the Outer Space Treaty of 1967. Specifically, Article IX is what a planetary protection officer worries about. Mostly just this one sentence:
"States Parties to the Treaty shall pursue studies of outer space, including the moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose."
That's why you couldn't just shove the body of your astronaut friend out of the airlock if he or she died during space travel. There's a chance (a very small chance) that the body could drift to an alien world and colonize it with Earth microbes.
Even unmanned space exploration missions have to address contamination risk, but it's pretty impossible to completely sterilize spacecraft and space probes before they leave Earth. So the creators of the Outer Space Treaty decided that risk was worth what we could learn from exploring the universe.
They settled on a 1-in-10,000 risk. That means any space exploration mission must have less than a 1-in-10,000 chance of contaminating an alien world.
"It's a moderate level," Conley said. "It's not extremely careful, but it's not extremely lax."
Problem #1: Bringing Earth to alien worlds
Mars has bared the brunt of human meddling. It's unlikely any spacecraft that landed there was 100% sterile.
NASA sends its spaceships and rovers through a microbe-killing heat blast, but recent evidence shows that some particularly hardy microbes can survive.
The good news is that Mars (and most of the rest of the solar system) seems pretty hostile to life. So anything we've carried there isn't going to flourish even if it survived the journey.
NASA's Viking missions pretty much confirmed that at least the surface of Mars isn't conducive to life. It'll most likely kill off any bacteria that land on Martian soil. So now anything that goes to Mars has about as many microbes as a human hand, since the risk of contamination is so low.
There are, however, a few promising potential homes for life in our solar system, including Jupiter's moon Europa and Saturn's moon Enceladus. Any mission that comes close to either of those moons is treated with a lot of caution, Conley said.
For example, the European Space Agency's (ESA) upcoming mission to Jupiter needs special attention, since that probe will fly so close to Europa, Conley said.
ESA wants to make a couple of very tight passes around Europa, but only if its spacecraft, named the Jupiter Icey moons Explorer (JUICE), proves it has excellent control — crash-landing on Europa simply isn't an option.
To mitigate that risk, ESA will first command JUICE to make a few test orbits around Jupiter. If all seems well, they'll program the spacecraft to fly by Europa a few times, gathering a bunch of data in the process, then return back into orbit around Jupiter.
So what would happen if we did contaminate somewhere in our solar system with Earth microbes — and ones that could survive on an alien world? A clean-up mission would be too expensive. The bottom line is that we "recognize that there is a risk of failure," Conley said. We take the risk, Conley said, because what we gain from space exploration is worth it.
We're exploring as carefully and as smartly as we know how, but eventually places like Mars — despite being so hostile to life — might end up too contaminated for certain types of microbe-hunting scientific experiments.
Problem #2: Bringing aliens to Earth
Bringing Earth microbes to an alien world would be bad, but the reverse scenario is a much bigger concern, Conley said. We don't want to accidentally bring something nasty, like the fictional Andromeda Strain, to humanity's only haven.
So making sure we don't contaminate Earth with extra-terrestrial microbes is the highest priority of a planetary protection officer.
As space agencies plan more and more missions out into the Solar System, it's becoming a serious concern.
For example, if Mars astronauts got infected with something while on Mars, should we bring them back to Earth? The short answer is no, Conley said during a talk at the recent Humans to Mars Summit in Washington, D.C.
The Apollo program, in fact, set up a quarantine facility to receive the first astronauts who returned from the moon, since they were covered in moon dust. Moon rock samples were cautiously sent to biosafety labs. We soon learned the moon didn't harbor any life, so later missions didn't use the same kind of rigorous screening.
But when we bring people and samples back from still unexplored worlds like Mars, we'll need to exercise the same kind of caution. And we have a lot of policy and protocol to establish before that happens.
The United Nations, ethicists, and lawyers are already meeting to discuss the best way to handle future manned missions.
