Vanguard SLV-3 and Vanguard 2 were earth-orbiting satellites designed to measure cloud-cover distribution over the daylight portion of its orbit. The spacecraft was a magnesium sphere 50.8 cm in diameter. It contained two optical telescopes with two photocells. The sphere was internally gold-plated and externally covered with an aluminum deposit coated with silicon oxide of sufficient thickness to provide thermal control for the instrumentation. Radio communication was provided by a 1 W, 108.03 MHz telemetry transmitter and a 10 mW, 108 MHz beacon transmitter that sent a continuous signal for tracking purposes.

A command receiver was used to activate a tape recorder that relayed telescope experiment data to the telemetry transmitter. Both transmitters functioned normally for 19 days. The satellite was spin stabilized at 50 rpm, but telemetry data were poor because of an unsatisfactory orientation of the spin axis. The power supply for the instrumentation was provided by mercury batteries.