Mars Free Return Trajectories

Abstract

Trajectories to Mars have been separated into various classes by previous a ~ t h o r s . l ~ These classes include Opposition, Sprint, Conjunction, Free Return, and Cycler. Opposition class missions are characterized by a high-energy trajectory and a relatively short Mars stay time (<3 months). The name stems from the fact that the Earth leaves opposition with Mars a t the Mars arrival. The total mission duration for this class ranges from 1 to 2.5 years. Also, the Opposition class trajectories have high arrival velocities a t both Mars and Earth. Since the periapsis of the transfers can be lower than the orbit of Venus, the addition of a Venus swingby can improve mission performance. A subset of the Opposition class is the Sprint class. This class has a mission duration of approximately 1 to 1.4 years with a 30 day stay time. Sprint missions are intended for piloted missions because of their short flight times, but they have higher AV requirements. The most traditional mission class is the Conjunction class. In this class the Earth is moving into conjunction with Mars at the time of Mars arrival. These missions are characterized by low-energy trajectories and have a relatively long stay time (0.8 to 1.5 years). They can be used during the early exploratory phase where many tasks need to be done on the planetary surface. For initial piloted missions, the Free Return class will most likely be the mission of choice since these trajectories do not require a deterministic maneuver to return the spacecraft to the Earth in the event of an emergency (e.g., Apollo 13). A subset of the Free Return class is the Cycler class,3 which includes