Compliance with planetary protection requirements is mandatory for NASA missions, as per NASA Policy Directive (NPD) 8020.7G: 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 8020.12D: 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 the spacecraft);
- Avoiding impact of Mars over a time period of 50 years with a probability of < 1×10-2 for spacecraft the 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; and
- 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 to meeting these conditions, which are both mission and target body dependent. Consultations with the Planetary Protection Officer both before and during mission development will help in ensuring compliance with the planetary protection requirements.
Brief Summary of Requirements for Orbiters and Flyby Spacecraft
As noted above, requirements for such missions include limits on the probability of impact with Mars (and potential contamination of icy satellites), and 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 (during storage, pre-flight preparations, and during flight). 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. Additional concerns include the end of mission, where 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.
Brief Summary of Requirements for Landers and Rovers
For spacecraft intended to land on target bodies of biological interest, the planetary protection requirements also include limits on the spacecraft’s biological burden. The stringency of these limits depends upon the mission objectives, planned spacecraft operations, and specific target body. By judicious assembly, landers and rovers may be designed so that only components of the assembled exploration vehicle are exposed to the surface of a planet. In such cases, only the exposed components of the spacecraft are required to meet the most stringent cleanliness requirements. Dependent upon the the planned mission operations, a sub-system approach may also be used, where only the components that contact the planetary surface are required to meet the most stringent cleanliness requirements. A full-system approach, or partial 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. The current bioburden constraints for Category IVa-c missions to Mars are derived from quantitative studies performed on the surface, encapsulated, and mated materials of the Viking spacecraft. For icy satellites, the current limit on the probably of inadvertent contamination of a liquid water body is less than 1×10-4, which will be refined in future years.