Mission design options for human Mars missions

Background: Interplanetary trajectory selection will be a significant driver for the design of human Mars missions, impacting propulsive, habitation, and atmospheric entry system requirements. Conjunction-class interplanetary trajectories are the leading candidate for human Mars missions, due to their short in-space durations, long surface stays, and lower propulsion requirements, in contrast to the long in-space durations, short surface stays, and high propulsion requirements characteristic of opposition-class missions. Within conjunction-class mission trajectories, considerations for providing an abort option to return the crew to Earth without stopping at Mars are also worthwhile. Approach: This paper presents Earth-Mars and Mars-Earth conjunction trajectories across a series of mission opportunities and transfer times in order to provide human Mars mission designers with an option space of possible crew and cargo transfer trajectories. For the specific case of crew transfer from Earth to Mars, the potential for aborting the mission without capture into Mars orbit is also examined. Two additional sub-classes of trajectories are thus presented: free return trajectories, where the outbound trajectory would return the crew to Earth after a fixed period of time without major propulsive maneuvers; and propulsive-abort trajectories, where the propulsive capability of the transfer vehicle is used to modify the trajectory during a Mars swing-by. Beyond propulsive requirements, trajectory selection also has a significant impact on the entry velocity and therefore the aeroassist system requirements, which are also examined in this paper. Results: Conjunction-class interplanetary trajectory data across a range of mission and architecture options is provided for use by designers of human Mars missions. Our investigation suggests potential constraints for entry velocities at Earth and Mars due to aeroassist considerations and describes feasible trajectories within these constraints. Based upon Mars entry velocity, the 2-year period free return abort trajectory is found to be less desirable for many mission opportunities than previously considered.