On March 26, 1958, the United States launched its third satellite into space. [‘On This Day in Space’ Video Series on Space.com](https://www.space.com/39251-on-this-day-in-space.html)
Explorer 3 was almost identical to Explorer 1, which was the first satellite the U.S. ever launched. Explorer 3 launched on a Juno I rocket from Cape Canaveral and entered an eccentric orbit. This means it was following a long elliptical path around the Earth. Its payload included a cosmic ray counter and a micrometeorite detector. The data from Explorer 3 and Explorer 1 led to the discovery of the Van Allen radiation belt. This is a region around the Earth where charged particles from the sun get trapped by Earth's magnetic field. Explorer 3 spent 93 days orbiting the Earth before its orbit decayed.
