Speakers

• Dr George Voyatzis — Aristotle University of Thessaloniki
• Dr. Alexis Coyette — University of Namur

Abstract

Research in celestial mechanics addresses challenges in spacecraft mission design and the dynamics of planetary bodies. One study investigated terminator periodic orbits (TPOs) for the ESA Hera mission in the Didymos–Dimorphos binary asteroid system. Using analytic continuation, eight distinct TPO families were identified in a non-rotating frame, classified into unstable (Group I, with resonant angle ϕ_R = 0) or quasi-stable/stable (Group II, with ϕ_R = π) configurations. In a rotation model, the Coriolis force caused slow orbital drift, but characteristic terminator properties (inclination ≈90°, ascending node ≈±90°) were preserved for approximately 200 days. Optimized initial conditions limited orbital deviations to below 25 m for 30 days and 40 m for two months, demonstrating suitability for Hera's parking.

A separate study developed a second-order dynamical model for Cassini states in large satellites, specifically Cassini state 1 for the Galilean moons. This model incorporated the full gravitational torque and coupled obliquity with polar motion, quantifying nutations in obliquity and longitude. Inertial obliquity exhibited primarily semidiurnal nutation, while body-frame obliquity showed predominantly diurnal nutation. For Callisto, diurnal nutations reached 400 milliarcseconds (40 m on surface), and second-order polar motion was resonantly amplified by factors up to 60. The presence of a subsurface ocean was shown to significantly influence Cassini state 1, causing resonant amplification of obliquity and polar motion for certain ocean thicknesses (e.g., 100-500 km for Ganymede), offering constraints for internal structures.

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