Managing momentum on the dawn low thrust mission

Dawn is low-thrust interplanetary spacecraft enroute to the asteroids Vesta and Ceres in an effort to better understand the early creation of the solar system. After launch in September 2007, the spacecraft will flyby Mars in February 2009 before arriving at Vesta in summer of 2011 and Ceres in early 2015. Three solar electric ion-propulsion engines are used to provide the primary thrust for the Dawn spacecraft. Ion engines produce a very small but very efficient force, and therefore must be thrusting almost continuously to realize the necessary change in velocity to reach Vesta and Ceres. Momentum must be carefully managed to ensure the spacecraft has enough control authority to perform necessary turns and hold a fixed inertial attitude against external torques. Along with torques from solar pressure and gravity-gradients, ion-propulsion engines produce a torque about the thrust axis that must also be countered by the four reaction wheel assemblies (RWA). New constraints were placed on the 8-year mission shortly prior to launch that required Dawn to minimize time spent in the sub-Elasto-Hydro-Dynamic (sub-EHD) region and minimize the total revolutions of all four RWAs. Accurate prediction of wheel speeds is the first step in developing a strategy to minimize both wheel speeds and total revolutions. Due to schedule and staffing constraints, the ground tools needed to accomplish this momentum management process were developed post launch, in parallel with the missions initial checkout phase. This paper discusses the momentum management issues of ion-propulsion missions and specifically the Dawn spacecraft. The discussion includes the tools and strategies developed to manage the spacecraft momentum with the goal of preserving RWA health.