What was the first successfully launched United States satellite?

Explorer 1 was the first successfully launched United States satellite.

The satellite was launched on January 31, 1958, by a Juno I rocket from Cape Canaveral, Florida. The mission followed the Soviet Union’s launch of Sputnik 1 in October 1957 and Sputnik 2 in November, events that intensified the early Space Race.

Explorer 1 carried instruments designed by a team led by James Van Allen at the University of Iowa. Its measurements detected the radiation belts surrounding Earth, later known as the Van Allen radiation belts. This made the mission important not only politically but also scientifically.

The United States had attempted to launch Vanguard 1 earlier, but that rocket failed shortly after liftoff in December 1957. Vanguard 1 eventually reached orbit in March 1958 and became the oldest human-made satellite still in space, but Explorer 1 was the first successful American launch.