Explorer 1

• 1. Project Orbiter Origins

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  The U.S. Earth satellite program began in 1954 as a joint Army and Navy proposal called Project Orbiter. This plan aimed to place a scientific satellite into orbit during the International Geophysical Year using a military Redstone missile. The Eisenhower administration rejected this proposal in 1955. They chose the Navy's Project Vanguard instead because its booster was advertised as more civilian in nature. Following the launch of Sputnik 1 on the 4th of October 1957, the initial Project Orbiter program revived. It became known as the Explorer program to catch up with the Soviet Union.

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• 2. Rapid Development And Launch

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  Explorer 1 was designed and built by the Jet Propulsion Laboratory under Dr. William Hayward Pickering. A Jupiter-C rocket modified by the Army Ballistic Missile Agency became the Juno I booster for the February 1958 launch. Working closely together, ABMA and JPL completed modifying the rocket and building the satellite in just 84 days. Before work finished, the Soviet Union launched Sputnik 2 on the 3rd of November 1957. The U.S. Navy attempted to put their first satellite into orbit but failed with the Vanguard TV-3 launch on the 6th of December 1957. After a jet stream delay on the 28th of January 1958, the Juno I rocket finally lifted off at 03:47:56 GMT on the 1st of February 1958.

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• 3. Satellite Design And Payload

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  The total mass of the satellite weighed approximately 14 kilograms with about 3.5 kilograms dedicated to instrumentation. The instrument section orbited as a single unit spinning around its long axis at 750 revolutions per minute. Because limited space required low weight, engineers used germanium and silicon transistors in the electronics. A total of 20 transistors were used in Explorer 1 plus additional ones in the Army's micrometeorite amplifier. Electrical power came from mercury chemical batteries making up roughly 40% of the payload weight. The external skin was sandblasted stainless steel with white stripes determined by studies of shadow-sunlight intervals.

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• 4. The Van Allen Radiation Discovery

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  The elongated body of the spacecraft was designed to spin about its long least-inertia axis but refused to do so. It instead started precessing due to energy dissipation from flexible structural elements. Engineers later understood that the body ends up in the spin state minimizing kinetic rotational energy for fixed angular momentum. This being the maximal-inertia axis motivated further development of Eulerian theory of rigid body dynamics. Nearly 200 years passed before this kind of momentum-preserving energy dissipation required new theoretical work.

  Mercury batteries powered the high-power transmitter for 31 days and the low-power transmitter for 105 days. Explorer 1 stopped transmission

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• 5. Orbital Mechanics Anomaly

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  of data on the 23rd of May 1958 when its batteries died. The satellite remained in orbit for more than 12 years after that failure. It reentered the atmosphere over the Pacific Ocean on the 31st of March 1970 after completing more than 58,400 orbits. The acoustic micrometeorite detector recorded 145 impacts of cosmic dust during a twelve-day period. This calculated to an average impact rate of roughly 8 times 10 to the minus 3 per second per square meter.

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