Low-Thrust Trajectory Optimizatoin for the Solar System Pony Express

We present a method for designing and optimizing Earth-Mars cycler orbits forthe Solar System Pony Express (SSPE) mission concept. The SSPE is aimedat augmenting the capabilities of the Deep Space Network (DSN) by enablinghigh latency and high bandwidth communications through the use of interplanetarydata mules. The SSPE will leverage a network of smallsats, outfitted withoptical laser communications, and by exploiting cycler orbits it will perform flybysof Mars at least once a year, retrieving 1-3 petabits of data per flyby frommissions already operating at the planet. This data will then be downlinkedto Earth during the next Earth flyby, also occurring about once a year. TheSSPE satellites are launched as a rideshare payload with another Mars-boundmission. Each data mule flying in the SSPE network is equipped with a lowthrustpropulsion system that is used for cycler orbit injection and for targetingflybys of Earth and Mars. Cycler orbits are first computed using a patchedconic approximation and impulsive thrust. These candidate solutions are thentransitioned into a higher-fidelity ephemeris model, and using an indirect optimizationmethod, fuel-optimal low-thrust transfers are computed between eachpair of flybys. We present three high-fidelity candidate solutions where a 500kg 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 thefuel consumption is feasible from a mission design perspective and that solutions obtained in the patched conics model can be successfully transitioned tothe ephemeris model using low-thrust propulsion.