Vanguard 1 was the first satellite to use solar cells for electrical power.
The United States launched Vanguard 1 on 17 March 1958. Small solar cells supplied electricity to its radio transmitter, allowing the satellite to operate after its chemical batteries were depleted.
Vanguard 1 was designed to study the effects of the space environment on satellites and to test tracking systems. Although its radio transmissions stopped in 1964, the spacecraft remains in orbit and is the oldest human-made object still in space.
The satellite is sometimes confused with Sputnik 1 or Explorer 1 because all three were early space milestones. Sputnik 1 was the first artificial satellite overall, while Explorer 1 was the first U.S. satellite to reach orbit. Vanguard 1 introduced solar-cell power to spaceflight.