On December 6, 1957, a Vanguard rocket carrying a satellite exploded on the launching pad, less than one month after the Soviets' successful launch of two Sputnik satellites. But on January 31, 1958, Explorer I was launched. On March 15, 1958, Vanguard I became the country's second satellite in space.

This satellite was the backup for Vanguard I. Its payload is identical to those of the first two successful American satellites.