The First Explorer
Inaugurating the era of space exploration for the US, the First Explorer, a thirty pound satellite, was launched into Earth orbit on February 1, 1958 by the Army Ballistic Missle Agency.
Explorer I carried instruments to measure temperatures, micrometeorite impacts, and an experiment designed by James A. Van Allen to measure the density of electrons and ions in space. The measurements made by Van Allen's experiment led to an unexpected and startling discovery -- an earth-encircling belt of high energy electrons and ions trapped in the magnetosphere now known as the Van Allen Radiation Belt.
Explorer I ceased transmitting on February 28 of that year but remained in orbit until March of 1970. Pioneering astrophysicist James Van Allen died on August 9th at the age of 91.