Navigating THEMIS to the ARTEMIS Low-Energy Lunar Transfer Trajectory

THEMIS – a NASA Medium Explorer (MIDEX) mission – is a five-spacecraft constellation launched in February 2007 to study magnetospheric phenomena leading to the aurora borealis. During the primary mission phase, completed in the summer of 2009, all five spacecraft collected science data in synchronized, highly elliptical Earth orbits. Both mission design and efficient navigation and flight operations during the primary mission resulted in appreciable fuel reserves. Therefore, an ambitious mission extension, ARTEMIS, became feasible. ARTEMIS involves transferring the outer two spacecraft from Earth to lunar orbits where they will conduct measurements of the Moon’s interaction with the solar wind and its crustal magnetic fields. Earth departure of these two spacecraft is accomplished by successively raising the apogees of their orbits until lunar perturbations become the dominant forces significantly altering their trajectories. This orbit raise sequence requires over forty maneuvering events, with multiple lunar approaches and fly-bys, before setting the two spacecraft on low-energy transfer trajectories to lunar orbit in February and March 2010. This paper addresses overcoming the navigation and operational challenges presented by the ARTEMIS mission, consisting of two spacecraft that were not designed to leave Earth orbits.