NASA's planetary protection office classifies missions into five different categories, depending on the threat of forward or back contamination.

NASA's Approach to Planetary Protection

Considering a single person has more bacteria on her body than there are people in the United States and considering a single NASA rocket or probe is a hands-on project for thousands of workers, it might seem like a fool's errand to try to decontaminate a spacecraft [source: Hurst and Reynolds]. Then again, skeptics scoffed at the idea of sending humans to the moon and returning them safely. To tackle these complex scenarios, NASA planners do what they always do: they break the problem down and make sure each small piece has an adequate solution.

For planetary protection, this meticulous process begins by defining the mission in terms of the target body (let's say Mars), the type of encounter (land and operate an unmanned rover named Curiosity) and the specific goals (figure out whether Mars could have supported life by doing lots of chemical analysis on Martian samples).

Because each type of mission presents unique contamination challenges, the Planetary Protection Officer determines specific requirements based on the current scientific knowledge and input from advisory bodies. He or she passes these requirements to the engineers and planners, who must incorporate them as they build, test and develop mission components. In NASA's current policy, the officer will classify a mission into one of five categories, each with its own planetary protection requirements (see table).

Up next, we'll see how NASA battles all those contamination risks.