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C
Event Sequence Diagram for the
Determination of Planetary Protection
Measures for Missions to Icy Bodies
The binary decision-making framework outlined in Chapter 2 provides an alternative to probabilistic estimates
of contamination constrained by the uncertain and/or unknowable factors included in the Coleman-Sagan equa -
tion. The decision-making framework can be visualized in a number of different ways. The committee’s preferred
depiction (see Figure 2.2) may not be the one most familiar to all relevant scientific and technical communities.
Indeed, engineers tend to visualize decision networks as event sequence diagrams.
The event sequence diagram presented in Figure C.1 is included to provide mission planners with the functional
equivalent of the decision-making framework in Chapter 2, but in a more familiar format.
Figure C.1 indicates the process to be applied for the two determinations necessary, the first of which is
related to potential habitability of the icy body target (that is, its “fragility” against bio-propagation), and the
second related to the type of mission proposed so as to address the potential for “initiating” a bio-contamination
of a potentially habitable icy body. This bimodal determination process (that is, the determination of the fragility
of the process, design, target) and the determination of the potential for damage initiation are consistent with the
general process of risk determination used across a variety of applications. 1,2
The left-hand portion of Figure C.1 represents the decision of whether the planetary body of interest should
be considered to be potentially habitable. Four criteria are used to judge the habitability of the planetary body and
specifically question whether the planetary body is known to possess liquid water, the key elements considered
essential for terrestrial life, environments known to be compatible with known extreme conditions of terrestrial life,
and accessible sources of chemical energy. If the planetary body does not possess one or more of these attributes,
then it is judged as uninhabitable by terrestrial life and, although assembly of spacecraft intended for these bodies
should be performed in a clean room, no bioload reduction is required for planetary protection. If the planetary
body does possess these four essential attributes for habitability by terrestrial life, or if this information remains
undetermined at the time of the mission, then the planetary body is deemed to be potentially habitable.
The right-hand portion of Figure C.1 considers the nature of the mission itself (e.g., flyby, orbiter, lander) as
relevant to determining planetary protection requirements for missions to potentially habitable planetary bodies.
Consideration must be given to whether the mission employs a lander and/or an orbiter and whether a flyby attempt
will be made of the given planetary body. If a lander is employed, the likelihood of the spacecraft interacting
with a habitable region must be evaluated, and for all missions the probability of the lander crashing or otherwise
interacting with a region where surface-subsurface transport is possible must be assessed. If this likelihood is less
than 10–4 over a period of 103 years, then no bio-load reduction measures are required for planetary protection
70
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71
APPENDIX C
beyond clean-room assembly. If the probability for interacting with habitable regions exceeds 10 –4 over a period
of 103 years, then specific consideration must be given to whether the lack of complex and heterogeneous organic
nutrients in aqueous environments of icy moons would preclude the propagation of any microbes that may have
survived extreme irradiation and desiccation environments in transport. If the lack of nutrients indeed precludes
propagation, then clean-room assembly is deemed sufficient; however, if the potential for propagation remains,
then at least minimal planetary protection methods are required, and the final-decision question then considers
whether heat treatment at 60°C for 5 hours would fail to eliminate all physiological groups that could potentially
propagate on the target body. If so, then stringent planetary protection methods are required for the mission to
proceed, or else the mission must either be reformulated or cancelled.
REFERENCES
1 . J. Fragola, B. Putney, and J. Minarck III, An Evaluation of Containment Assurance Risk for Earth Entry Vehicle and
Space Shuttle Sample Return, Earth Entry Vehicle Office, NASA Langley Research Center Hampton, Va., September 30, 2002.
2 . J. Fragola, B. Putney, and J. Minarck III, Mars Sample Return Probabilistic Risk Assessment Final Report: An Evalua -
tion of Containment Assurance Risk for Earth Entry Vehicle and Space Shuttle Sample Return, Contract No. 123-4119, NASA
Langley Research Center, Hampton, Va.
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72 PLANETARY PROTECTION REQUIREMENTS FOR SPACECRAFT MISSIONS TO ICY SOLAR SYSTEM BODIES
Habitability
Potentially
2. Key 3.Physical 4. Chemical Transfer
of Planetary 1. Water? Habitable
Elements? Conditions? Energy? Out
Body
Clean Room Clean Room
Clean Room
Clean Room
Assembly Assembly
Assembly
Assembly
Key to Decision Questions
1. Do current data indicate that the destination lacks liquid water essential for
terrestrial life? (Decision Point 1)
2. Do current data indicate that the destination lacks any of the key elements C, H,
N, P, S, K, Mg, Ca, O, and Fe, required for terrestrial life? (Decision Point 2)
3. Do current data indicate that the physical properties of the target body are
incompatible with known extreme conditions for terrestrial life? (Decision Point
3)
4. Do current data indicate that the environment lacks an accessible source of
chemical energy? (Decision Point 4)
5.1. Is a lander available? (Decision Point 5)
5.2. Is an orbiter available? (Decision Point 5)
5.3. Is a close flyby possible? (Decision Point 5)
5a. Do current data indicate that the probability of the spacecraft contacting a
habitable environment within 1,000 years is less than 10-4 ? (Decision Point 5)
5b. Do current data indicate that the probability of the spacecraft crashing or
otherwise contacting an active fissure or other region where surface-subsurface
transport is possible within 1,000 years is less than 10-4 ? (Decision Point 5)
6. Do current data indicate that the lack of complex and heterogeneous organic
nutrients in aqueous environments of icy moons will prevent the survival of
irradiated and desiccated microbes? (Decision Point 6)
7. Do current data indicate that heat treatment of the spacecraft at 60˚C for 5
hours will eliminate all physiological groups that can propagate on the target
body? (Decision Point 7)
FIGURE C.1 Event sequence diagram for the determination of planetary protection measures for missions to icy bodies
(continues next page).
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73
APPENDIX C
Habitability Legend
of Planetary
Body
Topic
Decision
Transfer No
5.1 Lander? 5.2 Orbiter? 5.3 Flyby?
In
Yes
Transfer
5a. Habitable
End State
Region?
Stringent
7. 60C for Planetary
6. Organic
5b. Subsurface 5 Hours Protection
Nutrients?
Transport? Effective? Required
Minimal
Clean Room Planetary
Clean Room
Assembly Protection
Assembly
Required
Key to Planetary Protection Endpoints
Clean room assembly but no bioload reduction required for
Clean Room planetary protection
Assembly
Minimal planetary protection required, including NASA
Minimal
standard cleaning and bioload monitoring, heating sealed
Planetary
components to 60C for 5 hours, and molecular bioload
Protection
analysis
Required
Stringent planetary protection required, including NASA
Stringent
standard cleaning and bioload monitoring, molecular bioload
Planetary
Protection analysis, and Viking-level, terminal bioload reduction; or
Required
cancel mission
FIGURE C.1 Continued.
