  The optimization of low-thrust, multi-revolution orbit transfer trajectories
is often regarded as a difficult problem in modern astrodynamics. In this
paper, a flexible and computationally efficient approach is presented for the
optimization of low-thrust orbit transfers under eclipse constraints. The
proposed approach leverages a new dynamic model of the orbital motion and a
Lyapunov-based initial guess generation scheme that is very easy to tune. A
multi-objective, single-phase formulation of the optimal control problem is
devised, which provides a convenient way to trade off fuel consumption and time
of flight. A distinctive feature of such a formulation is that it requires no
prior information about the structure of the optimal solution. Simulation
results for two benchmark orbit transfer scenarios indicate that minimum-time,
minimum-fuel and mixed time/fuel-optimal instances of the control problem can
be readily solved via direct collocation, while incurring a significantly lower
computational demand with respect to existing techniques.
