Low-thrust trajectory optimization for the solar system pony express

We present a method for designing and optimizing Earth–Mars cycler orbits for the Solar System Pony Express (SSPE) mission concept. The SSPE is aimed at augmenting the capabilities of the Deep Space Network (DSN) by enabling high latency and high bandwidth communications through the use of interplanetary data mules. The SSPE will leverage a network of smallsats, outfitted with optical laser communications, and by exploiting cycler orbits it will perform flybys of Mars at least once a year, retrieving 1–3 petabits of data per flyby from missions already operating at the planet. This data will then be downlinked to Earth during the next Earth flyby, also occurring about once a year. The SSPE satellites are launched as a rideshare payload with another Mars-bound mission. Each data mule flying in the SSPE network is equipped with a low-thrust propulsion system that is used for cycler orbit injection and for targeting flybys of Earth and Mars. Cycler orbits are first computed using a patched conic approximation and impulsive thrust. These candidate solutions are then transitioned into a higher-fidelity ephemeris model, and using an indirect optimization method, fuel-optimal low-thrust transfers are computed between each pair of flybys. We present three high-fidelity candidate solutions where a 500 kg data mule is inserted into an Earth–Mars cycler orbit using the NASA Evolutionary Xenon Thruster (NEXT). In all three cases, the results show that the fuel consumption is feasible from a mission design perspective and that solutions obtained in the patched conics model can be successfully transitioned to the ephemeris model using low-thrust propulsion.