UNDERSTANDING SPACECRAFT AGILITY FOR ORBIT TRANSFERS ON THE DAWN LOW-THRUST MISSION

摘要

Dawn is a low-thrust interplanetary spacecraft currently orbiting the asteroid Vesta. The spacecraft launched in September 2007 on a mission to better understand the early creation of the solar system, and recently arrived at Vesta in May of 2011. Three solar electric ion-propulsion engines provide the primary thrust for the Dawn spacecraft. Ion engines very efficiently produce a small thrust magnitude, and therefore must thrust almost continuously for long periods to realize the necessary change in velocity to reach Vesta, and eventually Ceres. The amount of time that must be spent thrusting presents unique challenges to executing orbit transfers on low-thrust missions. The necessary delta-V cannot be easily decomposed into a subset of fixed inertial attitudes, leading to the need to use time-varying thrust attitudes. Changing spacecraft orientation while thrusting imposes dynamic constraints on orbit transfer thrust designs, in addition to geometric pointing constraints. The dynamic constraints increase the coupling between the navigation design and the attitude control system, further challenging the design of flyable orbit transfers. Additionally, software commanding limitations and the need to maintain positive solar array power made developing simplified constraints for the navigation team a very non-linear problem. Post-launch development of new tools and processes was necessary to aid in the design of flyable orbit transfer trajectories. This paper briefly discusses the general attitude control issues with orbit transfers on a low thrust mission, and details their specific resolution on the Dawn mission. The discussion presents the tools and strategies developed to ensure successful design of orbit transfer thrust trajectories that remain safely within the capabilities of the Dawn attitude control system. With Dawn having recently completed a significant percentage of the orbit transfers at Vesta, this paper compares predicted performance and capabilities with actual spacecraft performance during orbit transfers. Being NASA's first ion-propulsion mission to orbit an asteroid provided numerous lessons that will be beneficial to all future low thrust missions.