General Mission Analysis Tool
General Mission Analysis Tool is open-source space mission analysis software developed by NASA and private industry.
It has been used for several missions, including LCROSS, the Lunar Reconnaissance Orbiter, OSIRIS-REx, the Magnetospheric Multiscale Mission, and the Transiting Exoplanet Survey Satellite mission.
GMAT is an open-source alternative to software like Systems Tool Kit and FreeFlyer.