Planetary Protection requirements are specific to the type of mission and explored planetary bodies. Accordingly, NASA assigns each mission a Planetary Protection Category based on the type of planned encounter (e.g., flyby, orbiter or lander) and the type of planetary bodies that are encountered and explored during the mission (e.g., planet, moon, comet or asteroid). In general, the Planetary Protection Categories (Categories I-V) are organized to reflect the degree a target (or encountered) planetary body may provide clues regarding life or prebiotic chemical evolution and can be further refined through planetary target and type of mission. When exploring a target body that may provide clues to the process of chemical evolution and/or the origin of life, spacecraft will have a higher level of cleanliness, with the mission providing detailed operating procedures. When exploring a target body that potentially supports Earth life, spacecraft will undergo stringent cleaning and sterilization processes and may be subject to certain operating restrictions. Planetary Protection requirements and mission categories are based on the scientific advice of the Space Studies Board and on NASA or international policy guidelines.
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 Planning
Compliance with Planetary Protection requirements is mandatory for NASA missions, per NPD 8020.7, Biological Contamination Control for Outbound and Inbound Planetary Spacecraft.
The first and most important step in complying with NASA Planetary Protection policy
is avoiding unintended encounters with solar system objects. As described in NPR 8020.12, Planetary Protection Provisions for Robotic Extraterrestrial Missions, missions
must meet a certain set of forward contamination criteria including
- Limiting the probability that a planetary body will be contaminated during the period of exploration
to no more than 1×10
-3 (unless otherwise specified), where the period of exploration shall extend
at least 50 years after a Category III or IV
mission arrives at its protected target (and no longer than the time point after which no organisms remain viable on
- Avoiding impact of Mars over a time period of 50 years with a probability of < 1×10
-2 for spacecraft that cross the orbit of Mars en route to other targets and < 1×10
-4 for all launch elements that leave Earth’s orbit
- Avoiding impact of target bodies, including orbital lifetime constraints
- Minimizing the probability of contamination using mission-dependent pre- and post-launch approaches,
such as cleanroom usage, aseptic assembly of spacecraft, partial sterilization of spacecraft
components and trajectory biasing.
Careful mission design and planning are essential elements when considering Planetary Protection
requirements, which are both mission and target body dependent. Consultations with the Planetary Protection Officer (PPO) during mission development is critical in ensuring
compliance with NASA policy.