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Current Mars Planetary Protection Categories and Bioburden Requirements
Planetary protection Category IV applies to missions to bodies of significant interest to the process of chemical evolution and/or origin of life and where scientific opinion provides a significant chance that contamination could compromise future investigations. The following Category IV requirements are from NASA Procedural Requirements Document NPR8020-12D, Detailed Planetary Protection Requirements, Chapter 5, Section 3.
Trajectory Biasing Requirements
- A probability of impact assessment shall be provided for all launch vehicle elements leaving Earth’s orbit, covering the first fifty years after launch.
- Cruise stages shall avoid Mars impact at a probability no less than 0.99 for 20 years after launch and a probability no less than 0.95 for the period 20-50 years after launch.
- Spacecraft that do not meet impact avoidance constraints shall limit their total (surface, mated, and encapsulated) bioburden level to 5 × 105 spores.
Sub-categories
Category IV missions to Mars are subdivided into IVa, IVb, and IVc.
Category IVa missions are those not carrying instruments designed to investigate extant Martian life. Cat IVa missions include Pathfinder, Mars Exploration Rovers, Mars Science Laboratory/Curiosity, and Insight. Cat IVa planetary protection requirements are:
- The total exposed bioburden of the landed system shall not exceed 3 × 105 spores at launch.
and - The bioburden density shall not exceed 300 spores per square meter of exposed external and internal spacecraft surfaces.
and - Missions must provide an assessment of Entry, Descent, and Landing (EDL) expected performance against environmental and other design cases, identifying included and excluded factors, and, to the extent available, quantitative assessments of confidence levels.
Category IVb missions are those carrying instruments designed to investigate extant martian life. Cat IVb missions include Phoenix and Mars 2020.1 Category IVb missions shall comply with all of the requirements of PP Category IVa and also with one of the following requirements:
- The entire landed system is restricted to a surface biological burden level of 30 spores or to levels of biological burden reduction driven by the nature and sensitivity of the particular life-detection experiments, whichever are more stringent, and protected from recontamination.
or - The subsystems which are involved in the acquisition, delivery, and analysis of samples used for life detection are sterilized to these levels.
Category IVc missions are those accessing special regions on Mars, even if not carrying life-detection instrumentation. The Viking landers were Cat IVc.
Cat IVc missions shall comply with all of the requirements of planetary protection Category IVa and also the following:
- For missions landing within a special region, the entire landed system shall be restricted to a surface biological burden level of 30 spores.
- For missions accessing a special region through horizontal or vertical mobility, one of the following requirements shall be imposed:
either- The entire landed system is restricted to a surface biological burden level of 30 spores;
or - The subsystems which directly contact the special region are sterilized to these levels, and a method of preventing their recontamination prior to accessing the special region is provided.
- The entire landed system is restricted to a surface biological burden level of 30 spores;
- If the probability of a non-nominal landing in a special region (including EDL and spacecraft-induced special regions) is greater than 0.01, then the entire landed system shall be sterilized to the following levels: a surface biological burden level of 30 spores and a total (surface, mated, and encapsulated) bioburden level of 1.5 × 104 spores.
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1 Even though Mars 2020 is designed to be the first step of Mars Sample Return, it was not categorized as Category V, as discussed in National Academies of Sciences, Engineering, and Medicine, 2018, Review and Assessment of Planetary Protection Policy Development Processes, Washington, DC: The National Academies Press.
