Introduction

Space physics started over 50 years ago with the launches on October 4 and November 3, 1957, of Sputnik I and II by the Soviet Union, and Explorer 1 and 3 by the United States on January 31 and March 26, 1958. Explorers 1 and 3 carried James Van Allen's Geiger counters. He had hoped to measure the lowenergy portion of the differential intensity of cosmic rays (particles with energies of hundreds of MeV of non-terrestrial origin), which could not be observed from the ground or with balloons because of atmospheric absorption. Yet the few minutes of data, received whenever the satellite was within range of the tracking station, were puzzling. At low geocentric distances of the 2500 km apogee orbit of Explorer 1 particle intensities were as expected. However, at higher altitudes the intensity dropped to zero. Explorer 3 carried a tape recorder and solved the puzzle. Again, the particle intensity or counting rate was normal at low altitudes, but then it increased rapidly until the maximum transmittable level of 128 counts/s was reached. A constant rate of 128 counts/s was observed for some time but then suddenly dropped to zero, recovering to 128 counts/s later and finally returning to normal at low altitudes. The actual counting rate was increasing rapidly far beyond the 128 counts/s limit, reaching such high rates that the Geiger counter “froze”, that is discharged so frequently that it could not properly recover between counts, yielding pulses too small to be detected by the circuitry used.