ConclusionsAn approximate method for computing near-optimal, minimumfuel, planar lunar trajectories for low-thrust spacecraft has been developed. Our method approximates the long-duration powered Earth-escape and moon-capture spirals with curve fits from universal low-thrust trajectory solutions and numerically computes the translunar coasting trajectory between the curve-fit boundaries using the restricted three-body problem dynamics. The approximate method requires only four optimization variables, and solutions are readily obtained by using sequential quadratic programming. Nearoptimal solutions for a wide range of initial thrust-to-weight ratios and initial and terminal circular orbit altitudes are obtained both quickly and with little computational load. The performance and orbit transfer characteristics of the near-optimal solutions exhibit a very close match with published optimal lunar trajectory solutions. This method would be a useful preliminary design tool for spacecraft and mission designers.