The attitude control system is responsible for determining the spacecraft's orientation in space, or "attitude," and providing control for maintaining and changing that attitude. Its hardware consists of two star trackers, three two-axis inertial reference units, 16 sun sensors and four reaction-wheel assemblies.
The system controls gimbals to keep the solar arrays pointed towards the sun. In addition, it controls gimbaling of the ion thrusters, which can be moved in two axes. The system usually determines the spacecraft's attitude using its star trackers to sight known stars.
The spacecraft's attitude is usually controlled by the reaction wheels, devices somewhat similar to traditional gyroscopes that use the momentum of spinning mass to maintain or change the spacecraft's orientation. However, the attitude can also be maintained or modified by a set of twelve 0.9-newton hydrazine thrusters that are collectively called the reaction control system. Dawn's current plans use two reaction wheels for attitude control. One of the four reaction wheels developed excessive friction in June 2010, and another in August of 2012, and the spacecraft automatically powered each off. Engineers have developed plans to enable the mission to meet its objectives at Ceres with or without reaction wheels.
The current configuration of three reaction wheels allows Dawn to fulfill its science goals. But, to provide flexibility in the case of another reaction wheel anomaly, engineers have also uploaded software to the spacecraft so Dawn can use two wheels in combination with thrusters to help with attitude control.
How does the flight team communicate with the Dawn spacecraft? It's all through code written on Earth and sent to the spacecraft's computer through the Deep Space Network. The telecommunication subsystem provides communication with Earth through any of three low-gain antennas and one 1.52-meter-diameter (5-foot) parabolic high-gain antenna. The high-gain antenna is the primary one used for most communication. The low-gain antennas are used when the spacecraft is not pointing the high-gain antenna toward Earth. Only one antenna can be used at a time.
The Dawn spacecraft's command and data handling system provides overall control of the spacecraft and manages the flow of engineering and science data. The system consists of redundant RAD6000 processors, each with 8 gigabits of memory.