The main body of the spacecraft consists of a wheel of aluminum alloy separated into nine wedge-shaped compartments. Five of these compartments contain apparatus for five experiments not requiring solar orientation. The remaining four compartments house the electronic controls,
batteries, telemetry equipment, and radio-command equipment. Each wheel compartment provides approximately 1000 cubic inches of volume, and each
wheel experiment can weigh up to approximately 45 pounds. The wheel also contains three extendable arms which are folded down around the third stage of the launch vehicle during the launch phase.
When the spacecraft is injected into orbit, the arms are extended which increases the effective diameter of the wheel from 1.1 meters to 2.4 meters.