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Explorer-I Satellite 1958 Alpha
Organization:	Army Ballistic Missile Agency
Major contractors:	Jet Propulsion Laboratory
Mission type:	Earth Science
Satellite of:	Earth
Launch date:	February 1, 1958 at 03:48 UTC
Launch vehicle:	Juno I
Decay:	March 31, 1970
Mission duration:	111 days
NSSDC ID:	1958-001A
Webpage:	NASA NSSDC Master Catalog
Mass:	13.9703 kg (30.799 lb)
Semimajor axis:	7,832.2 km (4,866.6 miles)
Eccentricity:	.139849
Inclination:	33.24°
Orbital period:	114.8 minutes
Apoapsis:	2,550 km (1,585 miles)
Periapsis:	358 km (222 miles)
Orbits:	~56,000
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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 10:48 pm 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.

Contents 1 Mission 2 Launch vehicle 3 Spacecraft design 4 Mission results 5 References 6 External links

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.

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).

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 scientific instrumentation of Explorer-I satellite was designed and built under the direction of Dr. James Van Allen of the University of Iowa. The cosmic ray instrumentation package was designed by Dr. George Ludwig, of Iowa's Cosmic Ray Laboratory.

Explorer-I was the second satellite to carry a mission payload (Sputnik 2 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 (approximately thirty counts per second) but sometimes it would show a peculiar zero counts per second. The Iowa group (under Van Allen) noted that all of the zero counts per second reports were from an altitude of 2000+ km (1,250 + miles) over South America, while passes at 500 km (310 miles) 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 the Van Allen radiation belt.

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 mi) and an apogee of 2,520 kilometers (1,575 mi) 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. (In comparison the first Soviet satellite, Sputnik I, weighed 83.6 kg. [184 lb]) 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.

The Explorer I instrumentation payload used transistor electronics, consisting of both germanium and silicon devices. This was a very early timeframe in the development of transistor technology, and represents the first documented use of transistors in the U.S. earth satellite program. A total of 29 transistors were used in Explorer I, plus additional ones in the Army's micrometeorite amplifier.

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.

• Data Sheet, Department of Astronautics, National Air and Space Museum, Smithsonian Institution.

• Explorer 1 Mission Profile by NASA's Solar System Exploration
• NASA's 50th Anniversary of the Space Age including Explorer 1 - Interactive Media
• First Transistors in Space - Personal Reflections by the Designer of the Cosmic Ray Instrumentation Package for the Explorer I Satellite

Preceded by none

Explorer program January 31, 1958

Succeeded by Explorer 2