The first and most important step in complying with NASA planetary protection policy is avoiding unintended encounters with solar system objects. Careful mission design and planning are essential to meeting this requirement. For example, at the end of an orbiter mission the spacecraft may be placed into a long-term orbit so that radiation and other elements of the local space environment can eliminate any Earth microbes that might be onboard. After navigation considerations are taken into account, missions must meet stringent cleanliness requirements. Spacecraft and their components must be cleaned very carefully, and sometimes sterilized. After cleaning, spacecraft are tested to ensure that cleanliness requirements have been met and can be maintained until launch.
Orbiters and Flyby Spacecraft
As noted above, requirements for such missions may include limits on the probability of impact with the target body, and orbital lifetime constraints for orbiter missions. If the probability that the spacecraft will impact the surface of its target body is small, cleanliness requirements may be reduced. However, if the spacecraft cannot meet these requirements, then constraints are placed on its total biological burden. These constraints may require decontamination procedures, the effectiveness of which is measured by a series of verification assays. Furthermore, after cleaning, procedures need to be implemented that assure prevention of recontamination. For orbiters and flyby spacecraft to target bodies of lesser biological interest, the requirements may only include an effort to minimize inadvertent impact and, should impact occur, documentation of the location and status of the final disposition of the hardware.
Landers and Rovers
For spacecraft intended to land on target bodies of biological interest, requirements include limits on the spacecraft’s biological burden. How stringent these limits are depends on the spacecraft’s planned operations and the specific target body. Landers and rovers can be designed so that only some parts are exposed to the surface of a planet. In such cases, only exposed spacecraft parts have to meet the most stringent cleanliness requirements. Sterilization of the entire spacecraft may be required for landers and rovers with life detection experiments, and for those landing in or moving to a region where terrestrial microorganisms may survive and grow, or where indigenous life may be present. For other landers and rovers, the requirements would be for decontamination and partial sterilization of the landed hardware.
