Geodetic Investigations of the Europa Clipper Mission

One of the objectives of the Gravity and Radio Science (G/RS) instrument on NASA’s EuropaClipper mission is to improve our knowledge of Europa’s ephemerides, to trace the moon’sthermal-orbital evolution. Here, we present a complementary methodology to use the Radarfor Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument to enhancethese measurements by tracking echoes from Europa at greater distances, beyond its primaryrole of subsurface sounding during close flybys. We investigate the possibility of occasionalREASON measurements during the non-nadir phase at altitudes beyond 40,000 km to extendthe ephemerides dataset. Through radar simulations of REASON’s echoes, we evaluate anddemonstrate this approach by determining the maximum altitude for surface return detection,achieved by coherently processing hundreds to thousands of pulses within the signal’s round-triptime. The received signal’s power can be further enhanced by maximizing pulse length, targetingEuropa’s most reflective (leading sub-Jovian) hemisphere, and summing many compressedpulses. We also explore a phase shift approach for velocity measurements, which uses existingranging pulses. Using Europa Clipper’s most recent trajectory design, we identify flybys thatalign with the best working scenarios for this approach. Our performance estimates for rangeand velocity resolution demonstrate that this approach could obtain measurements relevant toEuropa’s ephemerides. This method offers key insights into Europa’s orbital evolution and,ultimately, its long-term habitability.

美国国家航空航天局（National Aeronautics and Space Administration, NASA）欧罗巴快帆（Europa Clipper）任务搭载的重力与射电科学（Gravity and Radio Science, G/RS）仪器，其核心目标之一便是提升我们对木卫二星历表的认知，以追溯该卫星的热轨道演化历程。在此，我们提出一种互补方法，借助欧罗巴评估与探测雷达：从海洋到近地表（Radar for Europa Assessment and Sounding: Ocean to Near-surface, REASON）仪器，通过在更远距离上追踪木卫二的雷达回波来优化上述测量——该仪器的核心本职是在近距离飞掠期间开展地下探测。我们探究了在非天底相位、4万千米以上高度开展偶发性REASON测量的可行性，以扩充星历表数据集。通过对REASON回波开展雷达仿真，我们通过在信号往返时间内对数百乃至数千个脉冲进行相干处理，确定了地表回波探测的最大高度，以此评估并验证了这一方法。接收信号的功率可通过以下方式进一步提升：最大化脉冲长度、瞄准木卫二反射率最高的前导亚木星半球，以及对多个经过压缩的脉冲进行叠加求和。我们还探索了一种基于相移的速度测量方法，该方法可复用现有的测距脉冲。借助欧罗巴快帆最新的轨道设计方案，我们筛选出了适配该方法最优工作场景的飞掠任务。我们对距离与速度分辨率的性能评估表明，该方法可获取与木卫二星历表相关的有效测量数据。此方法可为木卫二的轨道演化乃至其长期可居住性研究提供关键洞见。