 Juno-I launched the Explorer 1 satellite on January 31, 1958, becoming the first U.S. satellite, as well as discovering the Van Allen radiation belt.The Juno I consisted of a Jupiter-C first stage, based on the Redstone missile; with three additional solid fuel stages based on the Sergeant missile to provide the added impulse to achieve orbit.The fourth stage was mounted on top of the "tub" of the third stage, and fired after third-stage burnout to boost the payload and fourth stage to an orbital velocity of .