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 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. Provided below are examples of how consultation with the PPO leads to mission-specific implementation of NASA policy.
As presented in the December 2015 Planetary Protection Subcommittee meeting, and pending final approval, the Mars 2020 project will be designated a Category V Restricted Earth Return mission, while the outbound leg will be required to comply to Category IVb constraints at the sub-system level. Accordingly, certain clarifications to NPR 8020.12D will apply to Mars 2020 (M2020). Examples clarifications include:
Sections 184.108.40.206c & 220.127.116.11c
- Due to the presence of a radiothermal generator (RTG) used to power the M2020 rover, the M2020 project shall ensure that candidate landing sites exclude the following from the post-parachute-opening 3-sigma landing ellipse:
- Locations with ice or hydrated minerals at depths of <5 meters (based on MSL impact calculations), for which exposure to an RTG could cause liquid to be liberated sufficient to mobilize a particle of <50 nm in size,
- Special Regions as formally defined in NPR 8020.12D Section 18.104.22.168 or as modified by mutual agreement prior to launch, pending evaluation of the definition rendered by the 2014-15 MEPAG/SSB/ESF evaluations, and subject to review by the NASA Planetary Protection Subcommittee,
- Transient Special Regions created by the presence of an RTG on the rover are included in these constraints: their absence shall be demonstrated by test and analysis.
- In addition to the standard reviews, the final candidate landing sites shall undergo an independent review, organized by the PPO, as part of the pre-launch landing site selection process and prior to the preparation and presentation of landing site options to the Science Mission Directorate Associate Administrator.
- Later access to previously identified locations, via either vertical or horizontal mobility of rover elements, shall be prohibited.
- Clarifications to the Special Regions include a lower temperature limit of -28˚C, reevaluation of the 500 year timescale for limits, and a focus on regions “including dark streaks”.
- Caching hardware involved in acquisition, delivery, and storage of samples intended for Earth return shall be
- Cleaned to a level of <300 heat resistant ‘spores’/m2,
- Enclosed in a physical biobarrier,
- And subjected to a validated bioburden reduction process the achieves at least 4-orders of magnitude of microbial reduction.
- The physical biobarrier that prevents recontamination shall not be opened until operations at Mars.
- Example acceptable contamination limits while conducting operations on Mars include:
- <1 viable Earth organism per cached sample,
- 100 ppb TOC baseline (Tier 1 & 2 compounds),
- And 40 ppb TOC threshold (Tier 1 & 2 compounds).
- The M2020 project shall ensure that Mars samples intended for possible future return are not contaminated by terrestrial organic compounds or viable organisms at levels above those specified below, through final deposition of sample tubes on Mars:
- The probability that a single viable organism is introduced into each sample shall be less than the limit obtained by multiplying the internal surface area of a sealed sample tube, in m2, by the Viking post-sterilization surface bioburden limit of 0.03 viable organisms per m2,
- Terrestrial organic contamination shall be limited to levels below <1 ppb of any Tier 1 organic compound per sample; <10 ppb Total Organic Carbon per sample,
- And sample tubes shall be designed for opening after return to Earth in a manner that prevents additional contamination of samples during extraction.