Abstract:In recent years, electric propulsion is widely used in deep-space missions. The specific impulse is varied in electric engines. Therefore, the variable-specific-impulse mathematical model is considered more practical in mission design. The research on optimization of variable-specific-impulse continuous low-thrust trajectories is of broad prospect in engineering practice. There are three commonly-used variable-specific-impulse simplified model: the specific impulse varies within a certain range, the specific impulse varies with the Sun-spacecraft distance and the thruster model with a finite number of operation points, which are characterized by different pairs of thruster input power. They are all corresponding to the working principle and manner of the real engine system. Considering the trajectory optimization methods for the three models, direct and indirect methods are commonly-used. However, due to the deficiency of the methods and the particularity of the models, the strategies for improvement are added based on direct and indirect methods. This paper surveys the state of the art in variable-specific-impulse simplified models and variable-specific-impulse continuous low-thrust trajectory optimization methods.