3.4.2 Limiting contamination on icy worlds
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What planetary protection category would a mission to an icy world fit into?
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Missions to icy worlds are classified based on their mission type, target body, and scientific objectives. Using Table 5 we can see that icy world missions might be classified as Category III, IV or even V (Restricted Earth return).
One of the underlying concepts in the planetary protection policy is that of the ‘period of biological exploration (PBE)’. This is the time needed for a robotic mission to operate at a potentially habitable environment and detect biological life. However, in Activity 4 you considered what might happen to the spacecraft at the end of its mission, since that might also result in a potential contamination event. Hence, the PBE is longer than the mission lifetime.
For icy worlds, the PBE has been defined as 1000 years, and the probability that a robotic mission could potentially contaminate an icy world in that period is less than 1 × 10−4. This probability is based on the following equation:
Where
- Pc = the probability of contamination of a single terrestrial organism
- N0= the number of microorganisms on the spacecraft initially
- R = reduction due to sterilisation pre-launch
- PS = Probability that the microorganisms on the spacecraft will reach the surface of the planet
- PI= Probability that the spacecraft will impact the planet (1 for landers)
- PR = Probability of microorganisms being released into the environment after landing (usually set at 1 for landers and crash-landers)
- Pg = Probability of growth (usually set at 1 where the target is liquid water)
You can see that calculating the probability of contamination takes into consideration several factors, including whether the spacecraft might crash into the surface, deliberately or otherwise. For the JUICE mission, which is a Category II mission, the mission team addressed the planetary protection requirements by limiting the probability of impact on Europa to a level of 1 × 10−4, which by default limits the probability of contamination of the sub-surface ocean to levels below 1 × 10−4.