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.
International Treaties and Organizations with Cognizance of Planetary Protection Activities
The 1967 United Nations Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Bodies states that all countries party 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.” Internationally, technical aspects of planetary protection are developed through deliberations by the Committee on Space Research (COSPAR), part of the International Council of Science (ICSU), which consults with the United Nations in this area. The COSPAR Panel on Planetary Protection develops and makes recommendations on planetary protection policy to COSPAR, which may adopt them as part of the official COSPAR Planetary Protection Policy.
NASA Planetary Protection Policy
NASA maintains a planetary protection policy and administers associated procedures to ensure compliance with it. The NASA policy generally follows the COSPAR policy. NASA’s Planetary Protection Officer oversees compliance with formal implementation requirements that are assigned to each mission. In accordance with the NASA policy, requirements are based on the most current scientific information available about the target bodies and about life on Earth. The Planetary Protection Officer requests recommendations on implementation requirements for missions to a specific solar system body, or class of bodies, from internal and external advisory committees—but most notably from the Space Studies Board of the National Research Council. In recent years the Space Studies Board has provided recommendations on planetary protection requirements for Mars, Europa, and sample return missions from a variety of small solar system bodies such as moons, comets, and asteroids. These recommendations will be reassessed as new information becomes available.
Requirements for Protecting Life on Other Bodies
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
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.
