Stationkeeping of the First Earth-Moon Libration Orbiters: The ARTEMIS Mission

spacecraft, named P1 and P2, entered Earth-Moon Lissajous orbits on August 25 th and October 22 nd 2010, respectively. Once the Earth-Moon libration point orbits were achieved they were maintained there for 11 months, with the P1 spacecraft orbiting EM L2 and P2 orbiting EM L1. During this stationkeeping phase, P1 was transferred from L2 to L1. From these EM libration orbits, both spacecraft were inserted into elliptical lunar orbits on June 27 th and July 17 th 2011 respectively. The challenge of ARTEMIS stationkeeping was that libration point orbits near collinear locations, including quasi-periodic Lissajous trajectories, are inherently unstable and must be controlled. For EarthMoon applications stationkeeping is more challenging than in the Sun-Earth system, in part because of the shorter time scales, the larger orbital eccentricity of the secondary, and the fact that the Sun acts as a significant perturbing body both in terms of the gravitational force and solar radiation pressure. To accurately assess the impact of these significant differences, the orbit must be modeled as a true four-body problem. Besides these inherent issues associated with the Earth-Moon system, ARTEMIS had mission requirements to be met and spacecraft constraints on the direction of delta-velocity (∆v). Although a general trajectory was defined for the mission, there was no required reference motion. Since the ARTEMIS spacecraft were originally designed for a passive mission in Earth elliptical orbits and were already flying, fuel was extremely limited. Thus, with the unique operational constraints, accomplishment of the maintenance goals with the minimum cost in terms of fuel was the highest priority. Background