On January 31, 1958, Explorer I (officially Satellite 1958 Alpha) became the first Earth satellite of the United States. It was launched at 10:48 pm EST from Cape Canaveral Air Force Station Launch Complex 26 in Florida, onboard a Juno I rocket.

Following the successful launch of the Soviet satellite Sputnik I on October 4, 1957 and the embarrassing failure in December 1957 of the first American attempt to launch a satellite, the US Army launched a scientific satellite using a rocket that had been developed to test guided missile components.

Explorer I carried an instrument package developed by a team at the State University of Iowa under the direction of Professor James A. Van Allen. Data returned by Explorer I and Explorer III (launched in March 1958) provided evidence that the Earth is surrounded by intense bands of radiation, now called the Van Allen radiation belts. This was the first major scientific discovery of the space age.