• lunisolar effect
• lunisolar resonance
• Molniya orbits
• resonances overlapping

In this thesis, the lunisolar perturbation affecting the dynamics of a Molniya satellite on a long-term timescale is investigated. A doubly-averaged model is built, including the expansion of the lunisolar disturbing function up to the third order. Some numerical experiments on the intermediate-period lunisolar effects are given, in order to validate the doubly-averaged model.
By exploiting the Hamiltonian dynamical structure, the long-term evolution of the Delaunay variables of the satellite is carried on the basis of numerical experiments focusing on the following indicators: the amplitudes of the harmonic coefficients, the periods of the arguments involved and the ratio between them.
The resonant region, associated to the dominant terms identified, are studied by both the standard resonance model and an alternative approach, assuming the isolated resonance hypothesis. The results obtained show when the standard approach does not give a reliable description of the resonant region. In particular, in case of a deep asymmetry between the stable and the unstable equilibria of the given dynamics, the pendulum-like Hamiltonian does not describe appropriately the features of the dynamical structure. Finally, the main overlapping resonances in the the Molniya region are identified.