Explorer-1 was the first U.S. satellite that went into space, and the third in the world, behind the Soviet Union’s historic Sputnik 1 and, later, Sputnik 2. It was the second satellite to carry a payload—Sputnik 2 was the first one.
Explorer-1 was launched from a Juno I rocket. It was designed by the Jet Propulsion Laboratory (JPL) at the California Institute of Technology.
In charge of the Explorer-1 program was Dr. William H. Pickering. In charge of the design and construction of the scientific instrumentation aboard the satellite was Dr. James Van Allen of the University of Iowa.
The Juno I was a rocket from the U.S. Army Ballistic Missile Agency (ABMA) that was modified to carry a payload. The payload was a cosmic ray instrumentation satellite designed by Dr. George H. Ludwig, of the Cosmic Ray Laboratory at the University of Iowa.
Explorer-1 was inserted into a 224 by 1,575 mile (360 by 2,520 kilometer) orbit. It made one orbit every 114.8 minutes, for a total of 12.54 orbits per day. The satellite was 80 inches (203 centimeters) long and 6.25 inches (15.9 centimeters) in diameter. Its payload was basically a Geiger counter that detected cosmic rays, along with temperature sensors, micrometeorite impact microphone, and various gauges.
At certain altitudes, researchers noted that the Geiger counter would not sense any cosmic rays. After further satellites were launched to study cosmic rays, it was decided that these minimal readings had overloaded the instrument due to an overabundance of cosmic rays over about 1,250 miles (2,000 kilometers) above the surface of Earth. It was later discovered that a belt of highly charged particles were trapped by the Earth’s magnetic field at that altitude and above. This belt of radiation is now called the Van Allen Radiation Belt.
The satellite was powered by nickel-cadmium chemical batteries. They provided a high source of power to transmit data back to Earth for about thirty-one days and a low source of power for 105 days. Explorer 1 stopped transmitting data on May 23, 1958 when its batteries were expended.
The orbit of Explorer continued to decay over a period of about twelve years. It eventually re-entered the Earth’s atmosphere on March 31, 1970 over the Pacific Ocean—after approximately 58,000 orbits about the Earth.
Further information on Explorer-1 and the Explorer series of satellites is found at:
NASA, Solar System Exploration: https://solarsystem.nasa.gov/missions/profile.cfm?MCode=Explorer_01
NASA: 50th Anniversary of the Space Age (1957-2007): https://www.nasa.gov/externalflash/SpaceAge/index.html
Semiconductor Museum: A Transistor Museum Interview with Dr. George Ludwig (The First Transistors in Space—Personal Reflections by the Designer of the Cosmic Ray Instrumentation Package for the Explorer I Satellite): https://semiconductormuseum.com/Transistors/LectureHall/Ludwig/Ludwig_Index.htm.