GRAIL Trajectory Design: Lunar Orbit Insertion through Science

Abstract

The Gravity Recovery and Interior Laboratory (GRAIL) mission will be travelling to the Moon in September 2011 as the eleventh selection from NASA’s Discovery program. The three month Science Phase is preceded by a 10 week period used to maneuver the twin GRAIL spacecraft from the low-energy trans-lunar cruise trajectory to the final science orbits. The two orbiters are individually operated and the trajectory design strategy described in this paper assures that the orbiters attain the required formation in time to begin science while remaining adequately separated throughout this period. I. Introduction HE primary objectives of the Gravity Recovery and Interior Laboratory (GRAIL) mission are to determine the structure of the lunar interior and gain a better understanding of the thermal evolution of the Moon. To accomplish this goal, the twin GRAIL spacecraft will work together to make high-accuracy measurements of the lunar gravity field on a global, regional, and local scale. During the 82-day Science Phase, the GRAIL spacecraft will fly in formation about the Moon in low-altitude, near-polar, coplanar orbits. The close formation allows for constant evaluation of the Ka-band signal between them, which provides a direct and precise measurement of the range-rate between the two orbiters. Post-processing of the range-rate data produces the final gravity field. The six months leading up to the Science Phase take the orbiters through five other mission phases: Launch, Trans-Lunar Cruise (TLC), Lunar Orbit Insertion (LOI), Orbit Period Reduction (OPR), and Transition to Science Formation (TSF). The series of maneuvers to go from the trans-lunar trajectory to the final Science Phase orbits is the focus of this paper.