National Aeronautics and Space Administration Planetary Protection Office

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Mission Design and Requirements

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 criteria that include:

  • Avoiding impact of Mars over a time period of 50 years with a probability of < 0.01 for spacecraft the cross the orbit of Mars en route to other targets, and < 0.0001 for all launch elements that leave Earth’s orbit;
  • Avoiding impact of target bodies, including orbital lifetime constraints; and
  • Minimizing contamination of target bodies through approaches that include the use of aseptic spacecraft assembly techniques and reducing the biological burden of spacecraft and spacecraft components.

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, 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.