Explorer 1 was the first U.S. satellite to reach orbit, launching on January 31, 1958.
The satellite was built by the Jet Propulsion Laboratory and the Army Ballistic Missile Agency, while NASA had been created only months earlier. Its launch followed the Soviet Union’s Sputnik missions and marked America’s entry into the Space Age.
Explorer 1 carried instruments designed to study cosmic rays. Its most important discovery was the Van Allen radiation belts, zones of charged particles trapped by Earth’s magnetic field. The mission’s scientific success made it more than a symbolic response to Sputnik.
Explorer 1 remained in orbit for more than a decade before reentering Earth’s atmosphere in 1970. It is sometimes confused with Vanguard 1, which was the second U.S. satellite and remains one of the oldest human-made objects still in orbit.