7

Conclusions and Recommendations

As a result of its deliberations, the task group was able to reach the following conclusions on appropriate planetary protection measures for future missions to Europa. All statements express consensus views unless noted otherwise.

  1. To meet planetary protection requirements and obligations, each Europa-bound spacecraft must be cleaned, sterilized, and/or subjected to sufficient radiation prior to contact with Europa’s surface so that the probability of contaminating a possible europan ocean with a viable terrestrial organism at any time in the future is less than 10-4.*

  2. Current cleaning and sterilization techniques are satisfactory to meet the needs of future space missions to Europa. These techniques include Viking-derived procedures such as washing surfaces with isopropyl alcohol and/or sporicides and sterilization by dry heating, as well as more modern processes such as sterilization by hydrogen peroxide, assuming that final sterilization is accomplished via exposure of the spacecraft to Europa’s radiation environment. The technological drawbacks of current prelaunch sterilization techniques are such that the use of these techniques is likely to increase the complexity and, hence, the cost of a mission.

  3. The current culture-based method used to determine the bioload on a spacecraft should be supplemented by screening tests for specific types of extremophiles, such as radiation-resistant organisms.

  4. Modern molecular methods, such as those based on the polymerase chain reaction (PCR), may prove to be quicker and more sensitive for detecting and identifying biological contamination than NASA ’s existing culturing protocols for planetary protection.

Knowledge of the planetary protection requirements for Europa will be enhanced by the data returned from future missions and the continuing analysis of Galileo observations. The task group recommends that, in addition, a series of scientific and technical investigations be conducted to reduce uncertainty in calculating the probability of contaminating Europa as a result of spacecraft missions. These investigations include targeted research in the following areas:

  • Ecology of clean room and spacecraft-assembly areas, with emphasis on extremophiles such as radiation-resistant microbes—Research on the variety and abundance of such organisms in these areas will allow targeting bioload-reduction techniques to the specific organisms present in these artificial environments.

  • Detailed comparisons of bioload assay methods—What are the strengths and weaknesses of the various types of molecular-based techniques? Quantitative results should be compared in order to determine which methods can best extend current culturing techniques. Can quick PCR methods replace culture-based assays? Can improved detection techniques be developed to readily distinguish between living and dead organisms?

*  

Two minority views were expressed by two subsets of the task group. Both would allow future missions to meet (possibly updated versions of) the planetary protection standards currently applied to Mars missions. These minority viewpoints are based on two different arguments. One minority subset argued that the planetary protection provisions for Europa should be broadly consistent with the current practice of categorization based on the type of mission (e.g., flyby, orbiter, or lander) and the degree to which the spacecraft’s target is of interest to studies of processes related to chemical evolution. The other minority subgroup argued that studies of the organisms found in extreme terrestrial environments suggest that no known terrestrial organism has a significant probability of surviving and multiplying in a europan ocean. The practical consequence of both of these views is that Europa missions should be subject to essentially the same planetary protection requirements that are currently applied to Mars missions. That is, spacecraft (including orbiters) without biological experiments should be subject to at least Viking-level cleaning, but sterilization is not necessary.



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Preventing the Forward Contamination of Europa 7 Conclusions and Recommendations As a result of its deliberations, the task group was able to reach the following conclusions on appropriate planetary protection measures for future missions to Europa. All statements express consensus views unless noted otherwise. To meet planetary protection requirements and obligations, each Europa-bound spacecraft must be cleaned, sterilized, and/or subjected to sufficient radiation prior to contact with Europa’s surface so that the probability of contaminating a possible europan ocean with a viable terrestrial organism at any time in the future is less than 10-4.* Current cleaning and sterilization techniques are satisfactory to meet the needs of future space missions to Europa. These techniques include Viking-derived procedures such as washing surfaces with isopropyl alcohol and/or sporicides and sterilization by dry heating, as well as more modern processes such as sterilization by hydrogen peroxide, assuming that final sterilization is accomplished via exposure of the spacecraft to Europa’s radiation environment. The technological drawbacks of current prelaunch sterilization techniques are such that the use of these techniques is likely to increase the complexity and, hence, the cost of a mission. The current culture-based method used to determine the bioload on a spacecraft should be supplemented by screening tests for specific types of extremophiles, such as radiation-resistant organisms. Modern molecular methods, such as those based on the polymerase chain reaction (PCR), may prove to be quicker and more sensitive for detecting and identifying biological contamination than NASA ’s existing culturing protocols for planetary protection. Knowledge of the planetary protection requirements for Europa will be enhanced by the data returned from future missions and the continuing analysis of Galileo observations. The task group recommends that, in addition, a series of scientific and technical investigations be conducted to reduce uncertainty in calculating the probability of contaminating Europa as a result of spacecraft missions. These investigations include targeted research in the following areas: Ecology of clean room and spacecraft-assembly areas, with emphasis on extremophiles such as radiation-resistant microbes—Research on the variety and abundance of such organisms in these areas will allow targeting bioload-reduction techniques to the specific organisms present in these artificial environments. Detailed comparisons of bioload assay methods—What are the strengths and weaknesses of the various types of molecular-based techniques? Quantitative results should be compared in order to determine which methods can best extend current culturing techniques. Can quick PCR methods replace culture-based assays? Can improved detection techniques be developed to readily distinguish between living and dead organisms? *   Two minority views were expressed by two subsets of the task group. Both would allow future missions to meet (possibly updated versions of) the planetary protection standards currently applied to Mars missions. These minority viewpoints are based on two different arguments. One minority subset argued that the planetary protection provisions for Europa should be broadly consistent with the current practice of categorization based on the type of mission (e.g., flyby, orbiter, or lander) and the degree to which the spacecraft’s target is of interest to studies of processes related to chemical evolution. The other minority subgroup argued that studies of the organisms found in extreme terrestrial environments suggest that no known terrestrial organism has a significant probability of surviving and multiplying in a europan ocean. The practical consequence of both of these views is that Europa missions should be subject to essentially the same planetary protection requirements that are currently applied to Mars missions. That is, spacecraft (including orbiters) without biological experiments should be subject to at least Viking-level cleaning, but sterilization is not necessary.