With the recent implementation of the Artemis Accords, interest in the cis-lunar space is rapidly increasing, necessitating the development of more precise and accurate modeling tools. While general-purpose mission design tools are available, this study proposes an open-source mission design tool, HALO, for High-precision Analyser for Lunar Orbits. This work presents a comprehensive review of the modeling approaches, structural design, and algorithms employed, aiming at facilitating ease of use and adaptation for other research in the cis-lunar domain. Furthermore, accuracy studies of the propagator are provided for various orbits of interest within this domain, including low lunar orbits, elliptical frozen orbits, and 3-body problem orbits, such as Near Rectilinear Halo Orbits and Distant Retrograde Orbits.
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