Abstract Protecting Jupiter’s Icy Moons from the Earth’s biological contamination is a key consideration for the Europa Clipper mission. The mission’s goal is to explore Jupiter’s moon, Europa, and investigate its habitability. The mission must satisfy NASA’s Planetary Protection (PP) requirements specified in the NASA Procedural Requirement (NPR) 8020.12D. The NPR states, “The probability of inadvertent contamination of an ocean or other liquid water body must be less than 1 × 10−4 per mission.” The NPR defines contamination as “the introduction of a single viable terrestrial microorganism into a liquid-water environment”. The probabilistic nature of the requirement demands a probabilistic response. The Europa Clipper team at NASA’s Jet Propulsion Laboratory has developed an end-to-end probabilistic risk assessment (PRA) in order to demonstrate requirement compliance and inform design decisions. This Planetary Protection PRA conservatively assesses the unanticipated events that would need to coincide for the Europa Clipper mission to contaminate an ocean on one of Jupiter’s icy moons. This quantification requires a model of: spacecraft failure scenarios and the potential that these scenarios result in an icy body impact; expected geological resurfacing timescales that may introduce transported Earth biology to interstitial liquid water; and an assessment of biological mortality throughout the journey from Earth to the subsurface ocean. The motivation behind the development of this PRA is to determine the proper amount of bioburden reduction required prelaunch in order to achieve the NPR risk threshold, while minimizing programmatic and mission risk. Previous mathematical approaches have failed to offer such guidance, resulting in over or under-specified microbial reduction protocols and greater risk either to the exploration target or to the mission. If sterilizing spacecraft hardware is the sole solution employed to meet NASA’s planetary protection requirement, PRA results show a 13-log bioburden reduction prelaunch is necessary. Most spaceflight electronics and optics cannot endure such microbial reduction protocols, or doing so would decrease part reliability to the point of being counter-productive. The Project, therefore, seeks to show compliance with the NPR by demonstrating the probability is sufficiently small that Europa Clipper inadvertently impacts an icy moon and subsequently delivers a fragment of hardware onto a piece of Europa that resurfaces by the year 3000 (within the 1000 year period of biological exploration). The Project does not seek to demonstrate that the hardware would be sterile if such an unlikely event sequence occurs. This model introduces key improvements over previous planetary protection models. It affords an exact, interdependent, mathematically rigorous, end-to-end methodology for quantifying the probability of transmitting biologically viable contaminants from one planetary body to another. The model and specific findings, as they relate to the Europa Clipper mission, are explored at a summary level in this paper, along with implications of the model’s improvements. This work provides a cornerstone for future missions that are required to perform a planetary protection probabilistic risk assessment, such as Europa Lander and Mars Sample Return.

中文翻译：

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Europa Clipper 行星保护概率风险评估总结

摘要保护木星的冰卫星免受地球生物污染是欧罗巴快船任务的一个关键考虑因素。该任务的目标是探索木星的卫星木卫二，并调查其宜居性。该任务必须满足 NASA 程序要求 (NPR) 8020.12D 中规定的 NASA 行星保护 (PP) 要求。NPR 指出，“每次任务意外污染海洋或其他液态水体的概率必须小于 1 × 10−4。” NPR 将污染定义为“将单一活的陆地微生物引入液态水环境”。需求的概率性质要求概率响应。NASA 喷气推进实验室的 Europa Clipper 团队开发了端到端的概率风险评估 (PRA)，以证明要求合规性并为设计决策提供信息。这个行星保护 PRA 保守地评估了意料之外的事件，这些事件需要同时发生，以便欧罗巴快船任务污染木星冰冷卫星上的海洋。这种量化需要一个模型：航天器故障情景以及这些情景导致冰体撞击的可能性；可能将运输的地球生物学引入间隙液态水的预期地质重铺时间尺度；以及对从地球到地下海洋的整个旅程中的生物死亡率的评估。制定此 PRA 背后的动机是确定发射前所需的适当生物负载减少量，以达到 NPR 风险阈值，同时最大限度地减少计划和任务风险。以前的数学方法未能提供这样的指导，导致微生物减少协议过度或不足，以及对探索目标或任务的更大风险。如果对航天器硬件进行消毒是满足 NASA 行星保护要求的唯一解决方案，那么 PRA 结果表明，发射前生物负载减少 13 对数是必要的。大多数航天电子和光学设备都无法忍受这种微生物减少协议，否则会降低部件的可靠性，以至适得其反。因此，该项目 试图通过证明木卫二快船无意中撞击冰冷卫星的可能性足够小，随后将硬件碎片传送到一块木卫二上，该碎片在 3000 年（在生物探索的 1000 年期间）重新浮出水面，以此来证明符合 NPR . 该项目并不试图证明如果发生这种不太可能发生的事件序列，硬件将是无菌的。该模型对以前的行星保护模型进行了重大改进。它提供了一种精确的、相互依赖的、数学上严格的、端到端的方法，用于量化从一个行星体到另一个行星体的生物活性污染物的概率。模型和具体发现，因为它们与欧罗巴快船任务相关，在本文的摘要级别进行了探讨，以及模型改进的影响。这项工作为未来执行行星保护概率风险评估所需的任务提供了基石，例如欧罗巴着陆器和火星样本返回。