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How Do We Get There?

The design of Dawn's trajectory is difficult, unusual, and interesting because of the use of solar electric propulsion, implemented on Dawn as an ion propulsion system (IPS). While providing performance far in excess of what conventional chemical propulsion would deliver, the IPS necessitates the use of design tools and methods quite different from what has been used for the development of trajectories since the dawn of the solar system (or, at least, since the dawn of space exploration). Rather than finding a few points at which impulsive maneuvers are required, this problem involves the determination of IPS thrust vectors over years of continuous thrusting. Unlike trajectories for ballistic missions, Dawn's depends sensitively on the spacecraft's power system (because power translates directly into IPS thrust). The tools that generate the trajectories require much more coaxing and cajoling (and sometimes pleading) than the tools that have been used for conventional missions.

In addition to the different underlying mathematical problem, the use of the IPS necessitates unfamiliar constraints on the mission. For example, because IPS thrusting is needed for years at a time, the mission could be vulnerable to an unexpected loss of thrust. Therefore, a substantial effort is devoted to designing a trajectory with enough "mission margin" that most spacecraft problems that interfere with IPS thrusting do not jeopardize reaching both Vesta and Ceres. (Missions relying on chemical propulsion tend to have greater vulnerability for shorter times.)

The initial work is focused on obtaining an understanding of the sensitivity of the trajectory to parameters that we can control. Ultimately we will develop a baseline trajectory that accounts for constraints such as the finite launch period, launch window, Vesta arrival window (to ensure good lighting for framing camera and mapping spectrometer observations of the south pole), Ceres arrival window (for lighting at one of the poles), mission margin, periods in which spacecraft activities preclude thrusting in the optimal direction, spacecraft power characteristics, flybys of other asteroids during the interplanetary cruise, and others. We separately analyze the orbit insertion, departure, and orbit transfers at each primary science target, where the complexity of spiraling around the bodies requires different analytical techniques.

Steve Williams and Dr. Greg Whiffen of JPL are the principal trajectory analysts on Dawn. Steve designed the trajectory for Deep Space 1 (DS1), the mission that tested the IPS design Dawn uses. Many issues that an operational IPS flight would face were revealed during that work; prior analyses had rarely, if ever, exceeded the depth necessary for conceptual studies. Greg has written a powerful new trajectory design tool that complements the one used for DS1. With his new software, Greg has generated our first looks at the Vesta orbit transfers. The first baseline trajectory will be completed by early April. Although preliminary, it will be significantly more accurate than previous calculations.