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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cape Canaveral—March 26, 1958. The Jupiter C stood iridescent in the dawn sky, shrouded by plumes of liquid oxygen a few hours before launch time. George Ludwig, Jack Froelich of JPL, and other JPL satellite engineers watched it with subdued anxiety when it finally soared skyward in a bolt of scarlet flame. Explorer II, launched three weeks earlier, had failed to fire into orbit. But eight minutes into Explorer IIIs launch, the ground stations began to catch a high-pitched hum. The JPL team burst into jubilant cheers but Ludwig focused most of his attention on the Iowa Geiger counter. For him and Van Allen, Explorer III held the promise of vindication from a baffling mystery and suspicions of a possible instrument malfunction on Explorer I. • • • The Geiger tube on Explorer I counted cosmic rays like a faulty adding machine. Ground stations received data in a chorus of rising and falling tones that correlated to levels of cosmic ray counts. Then, the instrument seemingly blanked out and the counts dropped to zero before returning to reasonable levels. Sparse data sets of a minute’s duration transmitted from uncertain altitudes only intensified the gaps in the puzzle. “I’ve got bad news for you. My people tell me that your counter stopped working,” Pickering called to say as the pattern emerged. “I thought he was wrong because we had all these pieces where it was and wasn’t working,” Van Allen recalled, though he couldn’t explain the anomaly either. “When I got back from my Fort Churchill expedition, they were busy looking at the data and scratching their heads,” McIlwain noted. “I pointed out that another possibility was Discovery of the Radiation Belts 12 that the flux might sometimes be very high, driving the Geiger tube into such hard saturation that it didn’t count at all.” Getting a satellite in space within four months of the first Sputnik required nothing short of an engineering miracle but Van Allen candidly described ExplorerIasa“shakedown”operationthatsucceededon“fool’sluck.”Yeteven its fragmentary data produced miles of reel-to-reel audiotapes that delivered cosmic ray counts in the form of a staccato symphony of rising and falling tones. The tapes arrived via regular mail days after they were recorded and the backlog piled ever higher in the basement of the physics building where tapesplayednonstoponCrownRoyalconsoletapeplayers.Pensonlongarms pulsed and slithered across paper tapes to graph the audio tones. A squadron of students measured the peaks and valleys of the graphs and converted them to numerical counts recorded in notebooks. Van Allen, Ray, Kasper, and McIlwain scanned the paper tapes for a transition from the rising counts to the sudden plunges. If a transition existed, it was lost in one of the gaping holes in the data. But Explorer III had Ludwig’s data recorder to pin down the roller-coaster ride of the cosmic ray counts. The recorder collected continuous data. A groundstationtransmittedasingleradiocommandandtriggeredtherecorder to dump all the data from a complete orbit of Earth in a matter of six seconds. Van Allen pinned his hopes on the recorder as Explorer I delivered messages that marked a major technical failure or a mystifying new property of space. The tape recorder had already captured the public’s imagination. Walter SullivanoftheNewYorkTimesvisitedIowainOctober1957asLudwigcompleted the prototype of the Deal 1 Geiger tube, a “masterpiece of miniature electronics ,”Sullivanwrote.“Mostimpressivetothelaymanwasthetaperecorderwhich was to give the satellite its memory. It was the size of a small alarm clock, designed so that the (40.5-inch) magnetic tape would jump forward once a second for as long as two hours, winding a spring as its wheels revolved.” A satellite with a memory was just the thing Van Allen needed to figure out how a journey through space could paralyze a Geiger counter and then restore it to normal activity. Ludwig started working on the recorder in mid-1956. In additiontoliningupminiaturizedwatchparts,heswitchedfromMylarrecording tape that could stretch in high temperatures to a rugged “0.001-inch thick phosphor bronze recording tape with an electroplated nickel-cobalt recording surface.” The tape operated on a time-release take-up spool, moving in short stops like the hands on a mechanical clock. It recorded 200 seconds of data per inch of tape until the playback command from the ground released a spring that pulled the tape across a playback head. Then the recorder deliv176 Discovery of the Radiation Belts ered a global epic of data before the recorder erased it and wound the tape back in position to record again...

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