Explorer-I, officially known as Satellite 1958 Alpha, was the first United States earth satellite and was sent aloft as part of the United States program for the International Geophysical Year 1957-1958. It was designed and built by the Jet Propulsion Laboratory (JPL) of the California Institute of Technology under the direction of Dr. William H. Pickering. The satellite instrumentation of Explorer-I was designed and built by Dr. James Van Allen of the State University of Iowa.
The satellite was launched from Cape Canaveral (now Cape Kennedy) in Florida at 10:48 P.M. EST on 31 January 1958 by the Jupiter-C vehicle--a special modification of the Redstone ballistic missile--that was designed, built, and launched by the Army Ballistic Missile Agency (ABMA) under the direction of Dr. Wernher Von Braun.
Jupiter-C, a direct descendant of the German A-4 (V-2) rocket, was originally developed in 1955-1956 as a high-performance rocket for testing purposes. The Jupiter-C rocket was originally developed to test the ablative re-entry nose cone of the Jupiter IRBM, although its satellite-launching capabilities were recognized at the time it was designed. The Jupiter-C has its origins in the United States Army's Project Orbiter in 1954. The project was canceled in 1955, however when the decision was made to proceed with Project Vanguard.
The vehicle consists of a modified Redstone ballistic missile topped by three solid-propellant upper stages. The tankage of the Redstone was lengthened by eight feet to provide additional propellant. The instrument compartment is also smaller and lighter than the Redstone's. The second and third stages are clustered in a "tub" atop the vehicle, while the fourth stage is atop the tub itself. The second stage is an outer ring of eleven scaled-down Sergeant rocket engines; the third stage is a cluster of three scaled down Sergeant rockets grouped within. These are held in position by bulkheads and rings and are surrounded by a cylindrical outer shell. The webbed base plate of the shell rests on a ball-bearing shaft mounted on the first-stage instrument section. Two electric motors spin in the tub at a rate varying from 450 to 750 rpm to compensate for thrust imbalance when the clustered motors fire. The rate of spin is varied by a programmer so that it does not couple with the changing resonant frequency of the first stage during flight.
The upper-stage tub was spun-up before launch. During first-stage flight, the vehicle was guided by a gyro-controleld autopilot controlling both air-vanes and jet vanes on the first stage by means of servos. Following a vertical launch from a simple steel table, the vehicle was programmed so that it was travelling at an angle of 40 degrees from the horizontal at burnout of the first stage, which occurred 157 seconds after launch. At first-stage burnout, explosive bolts fired and springs separated the instrument section from the first-stage tankage. The instrument section and the spinning tub were slowly tipped to a horizontal position by means of four air jets located at the base of the instrument section. When the apex of the vertical flight occurred after a coasting flight of about 247 seconds, a radio signal from the ground ignited the eleven-rocket cluster of the second stage, separating the tub from the instrument section. The third and fourth stages were fired in turn to boost the satellite and fourth stage to an orbital velocity of 18,000 miles per hour.
When used as a satellite launching vehicle, the Jupiter-C is sometimes referred to as the Juno-I.
Following the launch of the Soviet Sputnik I on 4 October 1957, ABMA was directed to proceed with the launching of a satellite using the Jupiter-C, which had already been flight-tested in nose-cone re-entry tests for the Jupiter intermediate-range ballistic missile (IRBM). Working closely together, ABMA and JPL completed the job of modifying the Jupiter-C and building the Explorer-I in 84 days.
Once in orbit, the cosmic ray equipment of Explorer-I indicated a much lower cosmic ray count than had been anticipated. Dr. Van Allen theorized that the equipment may have been saturated by very strong caused by the existence of a belt of charged particles trapped in space by the earth's magnetic field. The existence of these Van Allen Belts, discovered by Explorer-I, was confirmed by Explorer-III, which was launched by a Jupiter-C on 26 March 1958.
The discovery of the Van Allen Belts by the Explorer satellites was considered to be one of the outstanding discoveries of the International Geophysical Year.
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