The Coupled Orbital and Rotational Dynamics of Europa from Radiometric Data of NASA Europa Clipper Mission

Abstract

Europa Clipper is a NASA mission that will be launched in October 2024 to study Europa, one of the Galilean moons of Jupiter, and it will arrive in the Jovian system in 2030. During its operational time, it will perform multiple flybys of Europa with different altitudes and geometries. The gravity field and the rotational state of Europa can be precisely determined by reconstructing the Europa Clipper trajectory during the moon flybys. The observable quantity used in the estimation is the spacecraft range-rate, obtained from the frequency shift due to the Doppler effect of a highly stable microwave carrier transmitted from an Earth ground station to the spacecraft, that coherently retransmits the signal to Earth by means of the on-board transponder. As a result, the Europa Clipper mission will enable the estimation, with unprecedented accuracy, of the gravity field, orbit, and rotational state of Europa, thus dramatically improving our knowledge in its internal structure, origin, evolution, and habitability. The objective of this research activity is the estimation of the static gravity field and tidal parameters of Europa, along with other related physical parameters of interest, such as the rotational state and orbit around Jupiter. The typical approach in radio science gravity data analysis is to estimate the rotational state as a parametric function of time, effectively considering the orbit and rotation of a celestial body as decoupled. However, this is just an approximation. In fact, the orbital dynamics of the moon is not only influenced by the gravity field of the central body and the other moons, but also by the non-sphericity of the moon itself, which creates an acceleration that depends on the rotational state. Besides, the triaxiality of the moon determines the torques acting on it and its inertia matrix, and therefore the rotation of the moon through the Euler equations. The goal of this project is to improve the radio science data analysis, performing the orbit determination of Europa Clipper modeling the coupling dynamics between orbit and rotation, to improve the reliability and accuracy of the estimate of Europa static gravity, tidal parameters, orbit and rotational state, and ultimately of its internal structure.