U.S. Centennial of Flight Commission home page
Explorer I launch  Jan. 1958

The first upper stage to fly successfully was a cluster of small solid-propellant Baby Sergeant rockets mounted inside a cylindrical tub that was spun by an electric motor atop a Jupiter C/Juno I rocket. The Baby Sergeant pushed the first U.S. satellite, Explorer 1, into orbit on January 31, 1958.

Credits - NASA