The Europa Clipper Gravity and Radio Science Investigation

Abstract

Europa Clipper is the next NASA Flagship mission that will explore Jupiter's moon Europa. It has a rich payload with ten instruments and investigations, including the Gravity and Radio Science investigation (G/RS). The synergistic science made possible will provide a synoptic view of the Europa system. The overarching goal of Europa Clipper is to Explore Europa to Investigate its Habitability with a number of science objectives and themes related to its ice shell and ocean, its composition, its geology, and its potential recent activity. The Europa Clipper spacecraft is currently undergoing assembly and testing at NASA JPL (follow live on Youtube! https://bit.ly/clippercam) and it will be shipped to KSC in May 2024. The launch window opens on October 10, 2024.With 49 planned flybys, the tour trajectory samples Europa globally, but not evenly, with gaps at 90° and 270° longitude due to the constraints of the multiple-flyby mission design strategy to limit radiation impacts. Flyby altitudes typically vary between 25 km and 100 km, providing for higher sensitivity to shorter-wavelength gravity signal. The primary raw data for the G/RS investigation are collected from DSN 70-m antennas through Open-Loop Receivers (OLRs) in the ±2h periods around each flyby, leveraging the telecom subsystem's three fan beam (FBA) and two low-gain (LGA) antennas because the high-gain antenna (HGA) is not steerable.The highest priority for the G/RS investigation is to obtain an accurate measurement of the tidal Love number k2, which describes the amplitude of the gravitational response of Europa to the forcing tidal potential imposed by Jupiter. The measurement requirement is set at an uncertainty of 0.06 to provide an unambiguous independent assessment of the presence of an ocean. Expectations from orbit determination simulations show a robust margin of 3-4 times. Simulations of the gravity field recovery show that the low-degree gravity field can be resolved to degrees 5-10, depending on the assumptions for the level of gravity anomalies in the truth field.The interior structure of Europa will be informed by its hydrostatic equilibrium state (a current assumption as the Galileo data is not sufficient to independently estimate J2 and C22) and its moment of inertia. Given the uneven low-altitude spatial sampling, "Line-of-Sight" (LOS) analysis techniques will be important to extract the most from the signatures in the radio Doppler data. Other constraints on the ice shell, ocean, and seafloor will be possible especially in combination with the data collected by the other Europa Clipper instruments. Moreover, Europa Clipper will probe Europa's ionosphere with radio occultations, with geographic coverage complementary to in situ instruments.