(Files in red–history)

Index

9. Magnetic trapping

9H. Poincaré, 1896

10. Trapped Motion

10H. Einstein, 1910

10a. Particle Drift

11. Explorers 1/3

11a. Geiger Counter

12. Rad. Belts

12H. Argus 1958

12a. Inner Belt

12b. Outer Belt

13. Fast Particles

14. Synch. Orbit

15. Energy

Launch of Explorer 1
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In 1957 an "International Geophysical Year" (IGY) was organized, later extended to 1958, and both the Soviet Union and the USA announced their intention to launch that year artificial Earth satellites. The USSR was first, sending off its first "Sputnik" ("satellite") on October 4, followed by Sputnik II on November 3. However the official US entry, the Vanguard satellite, went up in flames in a launch failure in December. The US then authorized a back-up spacecraft mission, initiated unofficially a few years earlier by Wernher Von Braun. Von Braun had built large missiles for the US Army and had all the hardware ready, but until then was given no permission to launch a satellite.

The spacecraft, named Explorer 1, was launched 31 January 1958 and was designed and built by a group of scientists from the University of Iowa, led by James Van Allen. That group had been previously credited with the first observation of auroral electrons from a rocket; incidentally, the idea of the IGY itself started in 1950 at a dinner party at Van Allen's home (at the time, near Washington).

Van Allen equipped the spacecraft with a Geiger counter, a device for detecting high-energy ions and electrons. The goal was to measure the intensity of cosmic rays, fast ions that come from space, and in particular its variation with distance from the magnetic equator. Van Allen hoped to learn from this about the low end of the cosmic ray energy range, particles too slow to penetrate the full thickness of the atmosphere and reach the ground.

Discovery of the Radiation Belt

Unlike the orbits of the Sputniks, that of Explorer 1 was quite elliptical and it rose to an altitude of about 2500 kilometers. Furthermore, since it had been decided to omit the spacecraft's tape recorder on the first flight, data could only be collected when Explorer 1 was within range of a tracking station, for at most a few minutes each time. The data were puzzling. At low points of the orbit the number of energetic particles was near the expected value, but at the high portions of the orbit none were counted at all.

Explorer 2 failed to orbit, but Explorer 3, launched March 26, was successful, and it did carry a tape recorder. Its trace of the number of counts was normal at low altitudes, then it rose rapidly to fill the transmittable limit of 128, but at the highest level it fell to zero. Laboratory experiments with similar counters confirmed that this was characteristic of extremely high counting rates, when the counter discharged so frequently that it could not properly recover between counts, yielding pulses too small to trigger the counting circuit.

Sputnik III, carrying more elaborate scientific instruments, was launched May 12 and confirmed the discovery. It was later realized that Sputnik II had also detected the belt at the highest part of its orbit, but that occured above Australia, where the USSR did not track it. The Australians did get the signal, but the USSR would not reveal to them the broadcast code. Further studies were conducted by Explorer 4 later that year (trapped radiation, history) and of course, by many spacecraft ever since.

Exploring Further

A web site with a picture of the Explorer-1 rocket and with links.
For more on the story of Explorer 1 and Sputnik, click here.
Click here for a web page devoted to James Van Allen, and here for his autobiography.

A commemorative article "Discovering Earth's Radiation Belts: Remembering Explorer 1 and 3" by Frank McDonald and John Naugle appeared in "Eos" 89, 23 September 2008, p. 361-3.