Explorer I

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Explorer I
Explorer 1
Organization: Army Ballistic Missile Agency
Major contractors: Jet Propulsion Laboratory
Mission type: Earth Science
Satellite of: Earth
Launch: February 1, 1958 at 03:48 UTC
Launch vehicle: Juno I
Decay: March 31, 1970
Mission duration: 111 days
Mass: 13.9703 kg
NSSDC ID: 1958-001A
Webpage: NASA
Orbital elements
Semimajor axis: 7,832.2 km
Eccentricity: .139849
Inclination: 33.24°
Orbital period: 114.8 minutes
Apogee: 2,550 km
Perigee: 358 km
Orbits: ~56,000
Geiger-Mueller detector : Detection of cosmic rays
Wire grid array : Micrometeorite detection
Acoustic detector : Micrometeorite detection

Explorer-I, officially Satellite 1958 Alpha (and sometimes referred to as Explorer 1), was the first Earth satellite of the United States, having been launched at 10:48pm EST on January 31 (03:48 on 1 February in GMT), 1958, as part of the United States program for the International Geophysical Year. The satellite was launched from LC-26 at Cape Canaveral Air Force Station in Florida onboard a Juno I rocket.


Following the launch of the Soviet satellite Sputnik I on October 4, 1957, there was a frenzied effort by the United States to launch a satellite of its own, beginning the Space Race. Explorer-I was designed and built by the Jet Propulsion Laboratory (JPL), while the Jupiter-C rocket was modified by the Army Ballistic Missile Agency (ABMA) to accommodate a satellite payload, the resulting rocket becoming known as the Juno I. Working closely together, ABMA and JPL completed the job of modifying the Jupiter-C and building the Explorer-I in 84 days. Before work was completed, however, the Soviet Union launched a second satellite, Sputnik 2, on November 3, 1957.

Launch vehicle

Explorer-I's launch vehicle, the Juno I, 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 Jupiter-C used for the launch had already been flight-tested in nose cone reentry tests for the Jupiter IRBM).

Spacecraft design

Explorer-I was designed and built by the Jet Propulsion Laboratory 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 University of Iowa.

Explorer-I was the second satellite to carry a mission payload (Sputnik II was the first). Most notably, it was equipped with a geiger counter for the purposes of detecting cosmic rays. Sometimes the instrumentation would report the expected cosmic-ray count (~30 counts per second) but sometimes it would show a peculiar 0 counts per second. The Iowa group (under Van Allen) noted that all of the 0 counts per second reports were from an altitude of 2000+ km over South America, while passes at 500 km would show the expected level of cosmic rays. After Explorer III, it was decided that the original geiger counter had been overwhelmed by strong radiation coming from a belt of charged particles trapped in space by the Earth's magnetic field, now known as a Van Allen radiation belt.

Mission results

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.

Explorer-I was placed in an orbit with a perigee of 360 kilometers (224 miles) and an apogee of 2520 kilometers (1575 miles) having a period of 114.9 minutes. The total weight was 13.97 kilograms (30.8 lb), of which 8.3 kilograms (18.3 lb) were instrumentation. The instrument section at the front end of the satellite and the empty scaled-down fourth-stage rocket casing orbited as a single unit, spinning around its long axis at 750 revolutions per minute.

Instrumentation consisted of a cosmic-ray detection package, an internal temperature sensor, three external temperature sensors, a nose-cone temperature sensor, a micrometeorite impact microphone, and a ring of micrometeorite erosion gauges. Data from these instruments were transmitted to the ground by a 60-milliwatt transmitter operating on 108.03 megahertz and a 10 milliwatt transmitter operating on 108.00 MHz.

Transmitting antennas consisted of two fibreglass slot antennas in the body of the satellite itself and four flexible whips forming a turnstile antenna. The rotation of the satellite about its long axis kept the flexible whips extended.

The external skin of the instrument section was painted in alternate strips of white and dark green to provide passive temperature control of the satellite. The proportions of the light and dark strips were determined by studies of shadow-sunlight intervals based on firing time, trajectory, orbit, and inclination.

Electrical power was provided by nickel-cadmium chemical batteries that made up approximately 40 percent of the payload weight. These provided power that operated the high power transmitter for 31 days and the low-power transmitter for 105 days.

Because of the limited space available and the requirements for low weight, the Explorer-I instrumentation was designed and built with simplicity and high reliability in mind. It was completely successful.

Explorer I stopped transmission of data on May 23, 1958, when its batteries died, but remained in orbit for more than 12 years. It made a fiery reentry over the Pacific Ocean on March 31, 1970. Explorer I was the first of the long-running Explorer program, which as of November 2004 has launched 83 Explorer probes.

The identically-constructed flight backup of Explorer I is currently located in the Smithsonian Institution's National Air and Space Museum, Milestones of Flight Gallery.

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