Planetary Protection

Clean room assembly of a Viking component

Planetary protection is the term given to the practice of protecting solar system bodies (i.e., planets, moons, comets, and asteroids) from contamination by Earth life, and protecting Earth from possible life forms that may be returned from other solar system bodies. Planetary protection is essential for several important reasons: to preserve our ability to study other worlds as they exist in their natural states; to avoid contamination that would obscure our ability to find life elsewhere—if it exists; and to ensure that we take prudent precautions to protect Earth's biosphere in case it does.

NASA uses a variety of methods to measure, control and reduce spacecraft microbial contamination for planetary protection purposes. Assembly of spacecraft hardware is carefully controlled and often takes place in clean-room facilities using, aseptic techniques in order to meet planetary protection requirements. Dry heat microbial reduction techniques first used on the Viking spacecraft are still used today. Measurement techniques are cultivation-based microbial assays using well characterized biological methods.

Requirements for Protecting Life on Other Bodies

Preparing a Viking Lander for dry heat sterilization

Planetary protection requirements for each mission and target body are determined based on the scientific advice of the Space Studies Board and on NASA or international policy guidelines. Each mission is categorized according to the type of encounter it will have (e.g., flyby, orbiter, or lander) and the nature of its destination (e.g., a planet, moon, comet, or asteroid). If the target body has the potential to provide clues about life or prebiotic chemical evolution, a spacecraft going there must meet a higher level of cleanliness, and some operating restrictions will be imposed. Spacecraft going to target bodies with the potential to support Earth life must undergo stringent cleaning and sterilization processes, and greater operating restrictions.

Mission Design and Cleanliness

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

Mars Rover Opportunity, with solar arrays partially deployed, in JPL's Spacecraft Assembly Facility's cleanroom.

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.