The agency tries to keep humans from inadvertently populating other planets.
In 1967, the United States joined the United Kingdom and the Soviet Union in signing the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies. The agreement, more casually known as the "Outer Space Treaty," remains the closest thing the world has to "space law" -- and it stipulates, among other things, that as countries explore space they should avoid contaminating it with the microbial life of Earth. So while we may talk, with a mixture of fantasy and inevitability, about the colonization of other planets by humans, NASA takes great pains to avoid colonizing those bodies with life of a different variety: bacteria and spores that might hitchhike their way through the galaxy via American spacecraft.
But keeping space free of earthly critters is a difficult task. In fact, it's an effectively impossible one. Curiosity, for example, was not completely sterile at its launch; rather, the rover was built to ensure that it would "carry a total of no more than 300,000 bacterial spores on any surface from which the spores could get into the Martian environment."
In that way, Curiosity is like the Mars landers that preceded it: just a tiny bit dirty. "When we clean these things, it's virtually impossible to get them completely, totally, 100 percent clean, without any organic material at all," says Dave Lavery, NASA's program executive for solar system exploration. Instead, he told me, the agency enforces "allowable limits" -- a kind of controlled biological chaos -- that aims to mitigate, rather than eliminate, microbial life on its vehicles. The margins here are extraordinarily slim: When you're talking about microorganisms, 300,000 across the entire spacecraft is actually a remarkably low number. (A human adult, after all, can play host to as many as 200 trillion microorganisms. Trillion, with a T.)
To keep the Mars Science Laboratory mission within its 300,0000-critter range, the technicians who built Curiosity regularly cleaned the rover's surfaces -- and those of the spacecraft that delivered it to Mars -- by wiping them with an alcohol solution. They baked the mechanical components that could tolerate high heats to kill the microbes that remained. And they sealed off Curiosity's core box, which contains its main computer and other key electronics, to prevent any traveling microbes from escaping its confines.
Here, pictured in the video below, is the clean room at the Jet Propulsion Laboratory where Curiosity spent much of its pre-Martian existence. Note the bunny suits worn by the technicians, the better to ensure that human microbes wouldn't be transferred to NASA's now-rove-ready rover:
For its standard antibiotic regimen, Lavery told me, NASA has three main goals. First, of course, there's scientific accuracy -- since, for many of the agency's missions, the subtext if not the stated objective is to learn about the life that might exist beyond our atmosphere. "If we've taken Earth bugs with us," Lavery notes, "it defeats the entire purpose." Second, there's the Outer Space Treaty and the desire to be a good steward of space -- by avoiding contamination of the world beyond Earth's borders. Third, there's protecting Earth itself -- not just by preventing the passage of earthly life into space, but also by preventing any extraterrestrial life from coming back. (Hence those amazing photos of Armstrong, Aldrin, and Collins hanging out in their decontamination module after completing the Apollo 11 mission.)
Planetary protection has been one of the protocols that has unified NASA's missions since they started as missions in the first place. It's been a priority, Lavery notes, "since the very beginning of the space program."
And yet sterilization, just like other NASA protocols, varies significantly by mission. The particulars are determined by two broad considerations: where a mission is going and what kind of spacecraft it's using to get there. There's an overall cleanliness standard that's in place for every mission, Lavery notes -- no earth bugs being the general goal -- but beyond that, there's a procedural spectrum NASA employs to determine its approach to decontamination. For vehicles like the Voyager crafts, wandering the void of space with no planetary destination in mind, standards can be (relatively) less stringent. For a lander like Curiosity, however -- or like the lunar modules that brought human life to the moon during the Apollo missions -- the sterilization standards are stricter. Because, harsh as those environments may be to earthly life, large or small, there's a far greater chance that life would find a way to survive in those environments than elsewhere. We already know, for example, about the space-surviving skills of the tardigrade. And just recently, scientists discovered a species of bacteria able to survive in a lava tube, gleaning energy from a chemical reaction with the iron from basalt rock -- precisely the kind of rock abundant on Mars.