GOES I-M

The Geostationary Operational Environmental Satellite (GOES) I-M series use solar torque to help their attitude control systems. Momentum wheels point the spacecraft, but can build up speed over time until they reach the limit of their bearings. One solar array sticks out one side of the GOES I-M satellites, which is balanced by a small conical "solar sail" on the end of a long boom on the other side. A trim tab on the end of the solar array controls the solar torque, which can slow down the wheels by torquing in opposite directions. This saves propellant that would otherwise be required to slow down the wheels.

Reference

 * GOES I/M Databook
 * Chapter 2: The GOES Spacecraft Configuration