After the Second World War, the Defense Department launched serious research push into the fields of rocketry and upper atmosphere sciences to ensure American leadership in technology. As part of this push, President Dwight D. Eisenhower approved a plan to orbit a scientific satellite as part of the International Geophysical Year (IGY) for the period from July 1 1957 to December 31 1958, a cooperative effort to collect scientific data about the Earth. Quickly, the Soviet Union jumped in, announcing plans to orbit its own satellites.
The Naval Research Laboratory's Vanguard project was selected on September 9 1955 to support the IGY effort, but while it enjoyed exceptional publicity throughout the second half of 1955, and all of 1956, the technological requirements in the program were too big and funding levels too small to ensure success.
The launch of Sputnik 1 on October 4, 1957 pushed the U.S. satellite program in crisis mode. Playing technological catch-up, the United States launched its first Earth satellite on January 31, 1958, when Explorer 1 documented the existence of radiation zones encircling the Earth.