Explorer 1

• 1. Introduction

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  At 03:47:56 GMT on the 1st of February 1958, Explorer 1 lifted from Launch Complex 26A at Cape Canaveral, Florida. America had been watching the Soviet Union orbit the Earth for nearly four months. Two Soviet satellites were already circling above, and the United States Navy had just failed publicly in its own attempt. The satellite that finally broke through was built and launched under intense pressure. But speed alone does not explain why this mission matters. Explorer 1's instruments recorded something so unexpected that its discoverers initially doubted their own data. What the satellite found would be counted among the outstanding discoveries of the International Geophysical Year. The discovery would carry the name of a physicist in Iowa. He had been thinking about cosmic rays long before anyone built a rocket to carry his detector.

• 2. Humiliation and Urgency

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  In 1955, the Eisenhower administration rejected a joint Army and Navy proposal called Project Orbiter. The proposal aimed to put a scientific satellite into orbit during the International Geophysical Year, using a military Redstone missile. The administration chose instead the Navy's Project Vanguard, whose booster was advertised as more civilian in nature. On the 4th of October 1957, the Soviet Union launched Sputnik 1, and those optics stopped mattering. Sputnik 2 followed on the 3rd of November 1957. America's response came on the 6th of December 1957, when the Navy's Vanguard TV-3 failed to reach orbit. Project Orbiter's concept was revived as the Explorer program, this time pairing the Jet Propulsion Laboratory with the Army Ballistic Missile Agency. The resulting rocket was designated the Juno I. It built on a Jupiter-C design already proven in nose cone reentry tests for the Jupiter intermediate-range ballistic missile.

• 3. Eighty-Four Days

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  Dr. William Hayward Pickering directed the satellite work at JPL, which operated under the California Institute of Technology. Together with the Army Ballistic Missile Agency, his team completed both the rocket modification and the satellite itself in 84 days. The satellite that emerged was small by the standards of what the Soviets had put in orbit. Its total mass was 13.97 kilograms, of which 8.3 kilograms were instrumentation. Sputnik 1 had weighed 83.6 kilograms. Explorer 1's instrument section and the empty fourth-stage rocket casing orbited as a single unit. That unit spun around the long axis at 750 revolutions per minute.

  The outer casing of the instrument section was sandblasted stainless steel with white stripes. Engineers had tested other configurations, including alternating white and green, and blue alternating with copper. The final scheme emerged from studies of shadow and sunlight intervals, based on the planned firing time, trajectory, orbit, and inclination. Inside, the electronics used germanium and silicon transistors. Twenty transistors were used in total. Mercury chemical batteries provided the electrical power, making up approximately 40 percent of the payload weight.

  Two antennas handled communications with the ground. A 60-milliwatt transmitter fed a dipole antenna built into the satellite's body, operating on 108.03 MHz. Four flexible whips forming a turnstile antenna were driven by a separate 10-milliwatt transmitter on 108.00 MHz. The science payload those transmitters would relay to Earth had been designed by Dr. James Van Allen of the University of Iowa.

• 4. Van Allen's Instruments

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  Dr. James Van Allen of the University of Iowa designed and built Explorer 1's scientific payload. At the center was an Anton 314 omnidirectional Geiger-Müller tube, developed by Dr. George H. Ludwig of Iowa's Cosmic Ray Laboratory. It could detect protons above 30 MeV and electrons above 3 MeV. Five temperature sensors rounded out the basic measurements: one inside the satellite, three on the exterior, and one on the nose cone.

  Two separate detectors hunted for micrometeorite impacts. One was an acoustic device, a crystal transducer with an effective area of 0.075 square meters. Its average threshold sensitivity was 2.5 grams per centimeter per second. It registered impacts as a function of a particle's mass and velocity. The second detector was a wire grid of 12 cards, each wound with two layers of nickel alloy wire just 17 micrometers in diameter. A micrometeorite of about 10 micrometers could fracture the wire, break the circuit, and record the event.

  The Iowa Cosmic Ray Instrument flew without a key component. A tape data recorder that would have stored readings for later playback was not modified in time to make it onto the spacecraft. That absence would make the real-time data coming down from orbit much harder to interpret.

• 5. Into Orbit

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  A jet stream forced a launch postponement on the 28th of January 1958. After the delay cleared, the Juno I placed Explorer 1 into orbit. Its perigee was 358 kilometers; its apogee reached 2,550 kilometers. The orbit's period was 114.80 minutes and its inclination 33.24 degrees. Goldstone Tracking Station was supposed to report within 90 minutes whether the launch had succeeded. That confirmation did not arrive on schedule. The orbit turned out to be larger than mission planners had expected, delaying the satellite's passage within tracking range. At about 06:30 GMT, the confirmation finally came. A news conference was convened in the Great Hall at the National Academy of Sciences in Washington, D.C.

  The original expected orbital lifetime before decay was three years. Mercury batteries powered the high-power transmitter for 31 days before they ran out. The low-power transmitter continued for 105 days. Explorer 1 stopped transmitting data on the 23rd of May 1958. The satellite itself kept orbiting for more than 12 years after that. On the 31st of March 1970, it reentered the atmosphere over the Pacific Ocean, having completed more than 58,400 orbits.

  The data it had returned in those 105 active days had already posed a question that physicists could not yet answer from the ground.

• 6. The Baffling Zeros

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  Explorer 1's Geiger counter sometimes reported about 30 cosmic ray counts per second, which was the expected rate. Other times it returned zero. The real-time data reaching the ground was sparse and puzzling. Dr. Van Allen's team at the University of Iowa observed a pattern: every zero-count reading came from passes above 2,000 kilometers over South America. Passes at 500 kilometers returned normal cosmic ray levels. Without more data, the cause remained unclear.

  Explorer 3, which flew later that year and carried a tape data recorder, supplied the confirmation the team needed. The original Geiger counter had not malfunctioned. It had been saturated. Radiation from a belt of charged particles, trapped by Earth's magnetic field, was so intense that it overwhelmed the instrument entirely. The counter registered silence where there should have been a count. That belt of charged particles is now known as the Van Allen radiation belt.

  Meanwhile, the satellite had been quietly demonstrating something else odd about its own behavior in space.

• 7. Unexpected Physics

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  Explorer 1 had been designed to spin around its long axis, the one with the least inertia. After launch, it began precessing instead. Energy dissipated through the satellite's flexible structural elements, and the spin migrated toward a different configuration. Eventually the satellite settled into its maximal-inertia axis, the spin state that minimizes kinetic rotational energy for a fixed angular momentum. Analyzing this behavior after the flight prompted the first significant development of the Eulerian theory of rigid body dynamics in nearly 200 years.

  The acoustic micrometeorite detector kept gathering data throughout the satellite's active life. In 78,750 seconds of observation, it logged 145 impacts of cosmic dust. That works out to an average of 29 impacts per hour per square meter. Explorer 3 was just one of four follow-up satellites that the Juno I launch vehicle would carry into orbit in 1958.

• 8. A Lasting Program

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  Four satellites of the Explorer series were launched by the Juno I booster in 1958. Explorer 3 and 4 reached orbit successfully; Explorer 2 and 5 failed to do so. A final Juno I flight, carrying a satellite called Beacon-1, also failed. In 1959, the Juno I was replaced by the Juno II launch vehicle. The Explorers program itself continued long after those early missions.

  In late October 2011, a successor project called Explorer-1 Prime sent Unit 2 into orbit aboard a Delta II launch vehicle. The satellite was built using modern construction techniques. It served as a backup: Unit 1, launched on the 4th of March 2011, had failed to reach orbit due to a launch vehicle failure.

  The Smithsonian Institution's National Air and Space Museum in Washington, D.C. holds an identically constructed flight backup of Explorer 1. It is displayed in the Milestones of Flight Gallery. At Cape Canaveral, LC-26A was deactivated in 1963 and designated for museum use in 1964. It became the Air Force Space and Missile Museum, which displays a full-scale mock-up of the satellite. Explorer 1 was assigned Satellite Catalog Number 00004. Its Harvard designation, 1958 Alpha 1, was a forerunner to the international system still used today to catalog every object in Earth's orbit.