Explorers Program

The Explorer program was the United States' first successful attempt to launch unmanned space missions. It began as an attempt to place an artificial satellite into orbit during the International Geophysical Year, and became an attempt to catch up with the Soviet Union after that nation's launch of Sputnik I on October 4, 1957.

Explorer I was launched on January 31, 1958 and attained Earth orbit. Its instruments detected the Van Allen radiation belts.

(other craft in the Explorer program here)