The Effect of the Io Plasma Torus on Precise Orbit Determination and Gravity Recovery: Application to Europa Clipper

Abstract

The neutral gases released by the intense volcanic activity of the Jupiter moon Io, once ionized by the Jupiter magnetic environment, give rise to a toroidal plasma distribution known as the Io plasma torus (IPT). Radio signals passing through charged-particle environments such as the IPT are heavily perturbed proportionally to the rate of change of the charged-particle distribution along the path of the radio wave. If not properly calibrated, the IPT may induce significant perturbation on the radiometric tracking link to Earth. The radio tracking signal is the main observable for the Gravity and Radio Science (G/RS) investigation on NASA's Europa Clipper, whose aim is to measure the gravity field, tidal response, and moment of inertia of Europa, and to precisely reconstruct the trajectory of the probe in support of other scientific investigations. In this work, we quantify the detrimental effects of the IPT on the radiometric observables. We show how these affect the products of the G/RS, and prove the necessity of accurate calibrations. We simulate different calibration strategies to mitigate its net perturbative effect, based on currently available models of the IPT. Considering that these models have been developed with Juno in mind, they are tailored to its orbital geometry and we show that they cannot be easily applied to other geometries. We conclude that, although the model-based calibration strategies can be very effective, further work will be needed to make them applicable to probes with a significantly different orbital geometry such as Europa Clipper or ESA's JUICE.