Explorer 1 was the United States' first satellite in space. The 1958 launch of the satellite — twice the size of a basketball — was an important moment for the country, as the Space Race with the Soviet Union was just beginning.
The satellite, which was designed, built and operated by the NASA Jet Propulsion Laboratory, carried a cosmic ray detector, which led to Explorer Principal Investigator Dr. James Van Allen's discovery of radiation belts around Earth held in place by the planet's magnetic field. The findings were later named Van Allen belts in honor of their discoverer.