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The equations of motion for a lightly damped spherical pendulum are considered. The suspension point is harmonically excited in both vertical and horizontal directions. The equations are approximated in the neighborhood of resonance by including the third order terms in the amplitude. The stability of equilibrium points of the modulation equations in a four-dimensional space is studied. The periodic orbits of the spherical pendulum without base excitations are revisited via the Jacobian elliptic integral to highlight the role played by homoclinic orbits. The homoclinic intersections of the stable and unstable manifolds of the perturbed spherical pendulum are investigated. The physical parameters leading to chaotic solutions in terms of the spherical angles are derived from the vanishing Melnikov-Holmes-Marsden (MHM) integral. The existence of real zeros of the MHM integral implies the possible chaotic motion of the harmonically forced spherical pendulum as a result from the transverse intersection between the stable and unstable manifolds of the weakly disturbed spherical pendulum within the regions of investigated parameters. The chaotic motion of the modulation equations is simulated via the 4th-order Runge-Kutta algorithms for certain cases to verify the analysis.

In this paper Deprit's variables are used to describe the Hamiltonian equations for attitude motions of a gyrostat satellite spinning about arbitrarily body-fixed axes. The Hamiltonian equations for the attitude motions of the gyrostat satellite in terms of the Deprit's variables and under small viscous damping and nonautonomous perturbations are suitable for the employment of the Melnikov's integral. The torque-free homoclinic orbits to the symmetric Kelvin gyrostat are derived by means of the elliptic function integral theory. With the help of residue theory of complex functions, the Melnikov's integral is utilized to analytically study the criterion for chaotic oscillations of the attitude motions of the symmetric Kelvin gyrostat under small, damping and periodic external disturbing torques. The Melnikov's integral yields an analytical criterion for the chaotic oscillations of the attitude motions in the form of an inequality that gives a necessary condition for chaotic dynamics in terms of the physical parameters. The dependence of the onset of homoclinic orbits on quantities such as body shapes, the initial conditions of the angular velocities or the two constants of motions of the torque-free gyrostat satellite is investigated in details. The dependence of the onset of chaos on quantities such as the amplitudes of the external excitation and the damping coefficients' matrix is discussed. The bifurcation curves based upon the Melnikov's integral are computed by using the combined parameters versus the frequency of the external excitation. The theoretical criterion agrees with the result of the numerical simulation of the gyrostat satellite by using the fourth-order Runge-Kutta integration algorithm. The numerical solutions show that the motions of the perturbed symmetric gyrostat satellite possess a lot of "random" characteristic associated with a nonperiodic solution.

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