Choice of the Optimal Launch Date for Interplanetary Missions

Interplanetary missions are strictly dependent on the launch date. Mission planning requires a knowledge not only of the technological and cost constraints but also the study of the influence of the launch opportunity on the spacecraft performance and on the feasibility of the mission. Pork-chop plots are effective tools to design interplanetary missions, providing a graphical overview of the relationship between the fundamental parameters of the mission design, namely, the launch date, the duration and the energy requirements. In this way it is possible to evaluate the best timing to accomplish the mission under current constraints. Plots of a similar type can be drawn also for optimizing missions based on low thrust or on propellantless propulsion—like solar sails. The cost function described in these plots may be the square of the hyperbolic excess speed, the ΔV, the ratio between the initial mass and the payload mass or between the mass of the propulsion system and the payload or the cost function J used in the optimization of the trajectory. In case of two-way missions, it is possible to plot a cost function of the same type, by adding the values related to the forward and the backward journeys, as a function of the duration of the two legs of the travel, once the stay on the planet has been fixed. In this way it is possible to optimize also missions which are intrinsically two ways, like human exploration missions or sample return missions.