What was the satellite that was launched by americans in the Cold War?

Explorer I (the first US satellite to orbit the Earth) was launched on January 31, 1958, and discovered the radiation belts known as the Van Allen Belt. Additional satellites were launched to study the radiation. Unfortunately Explorer 2 (March 5, 1958) failed to reach orbit. Explorer 3, also known as Gamma 1, was successfuly orbited on March 26, 1958, becoming the second US satellite in space.