From perplexity to understanding with Explorer III

As mentioned in Chapter 10, the full interrogation log shows that many of the early Explorer III interrogation attempts of the onboard tape recorder either did not elicit observable responses from the satellite or usable ground station recordings were not obtained. During the first hours, both Van Allen and I eagerly sought information on the performance of the newly orbiting instrument, especially its onboard data recorder. As Van Allen was returning to Iowa City the evening of the launch, he called John Mengel at NRL during a plane change in Chicago.21 He learned of the interrogation success history up to that point, that is, of the passes over Antigua (immediate post­launch check—good response), San Diego (no response), Quito (not heard—response possibly heard at Lima but not recorded), and Antofagasta (no response). Thus, by that evening, there were only two indications of possibly successful interrogations. Only one of them was from the instrument after it had spent some time in orbit, and that response was uncertain.

I was en route from the Cape to Huntsville, Alabama, during that first evening and not able to find out about any successful postinjection interrogations until nearly midday the next day, when I was finally able to talk to Mengel in Washington. By then, additional at least partly successful interrogations (commands followed by an audible response) had been achieved from Quito, Santiago (two passes), Antofagasta, Quito (observed at Lima), and Havana. During that same period, five other passes elicited no response.22

The first interrogation that resulted in the receipt of a strong signal carrying a complete data dump (that is, of an onboard recording covering a complete orbit of the satellite), and that was successfully recorded on the ground, was finally achieved at about 21:00 UT (6:00 PM EST) on 27 March at Quito, over 27 hours after launch! A few more interrogations of variable quality were obtained during the next nine hours by South American stations. Before any of those recordings could be displayed in human-readable form, however, they had to make their way from South America, through customs, to the NRL Processing Center in Washington, D. C.

Within the next few days, the situation began to show substantial improvement. On Friday, 28 March, successful playbacks were recorded at Santiago (2), Antofagasta, Lima, Fort Stewart (2), San Diego, and Quito. During that same time, there were failures on only three attempts.

Enigmatically, the San Diego station operator reported, based on their listening to the audible playback signal while recording their 28 March pass, that there was a possible data dropout of three seconds about halfway through the roughly six second data readout. The data at the time of the gap had been recorded on the satellite when it was near its highest position in orbit. That suggestion of an apparent zero counting rate became tremendously important as events unfolded.

CHAPTER 12 • DISCOVERY OF THE TRAPPED RADIATION 331

On Saturday, additional successful recordings were made at Santiago (2), Lima, Antigua, Havana, Fort Stewart, and San Diego, with three other failures. By that time, the first ground station recordings of the data playbacks were just arriving at the processing center in Washington, D. C.

I reached Iowa City from Huntsville in mid-morning that Saturday. Driving imme­diately to the physics building, I checked with Van Allen, Carl McIlwain, and Ernie Ray on their progress with the Explorer I data. Eight weeks had elapsed since that satellite had been launched. Significant quantities of its data had arrived and were being examined. Although there was a growing feeling that Explorer I was detecting some unexpected real physical phenomenon, we had not yet concluded that we were seeing a high radiation intensity.

During that weekend meeting, in addition to the discussion of the Explorer I data, I briefed the group on the results of the Friday meeting at Huntsville. I had planned on putting two GM counters with different amounts of shielding in our IGY Heavy Payload instrument. But Van was hesitant about that, expressing his opinion that two GM counters would increase the chances for instrument failure.23 Although he insisted during that discussion on making one of the two counters a dummy, after we realized later that the Explorer I and III instruments were detecting high-intensity radiation of a largely unknown character, a live second counter was reinstated with added absorber material to help us in further understanding the new phenomenon.24

Also during that Saturday meeting, Van Allen and I agreed that I needed to move back to Iowa City as quickly as possible. We settled on a date of 6 April (only eight days thence) for my switch back to the SUI payroll. My family would return as quickly as we could settle our affairs in Pasadena.

I returned to my office in Pasadena on Monday for a week of whirlwind activities to wrap up my work there and prepare for our move.

Iowa’s role in the burgeoning space program was gaining wide publicity. On Sunday, 30 March, a CBS television camera crew arrived on the Iowa campus and began setting up their equipment. Walter Cronkite interviewed Van Allen on Monday (Figure 12.1), and his crew continued taping additional footage the next day.

Significantly, during that interview, Van Allen still made no mention of the pos­sibility of a region of unexpectedly high radiation intensity. The Explorer III data had not yet been examined, and he was, as was his usual practice, being careful to avoid premature speculation. Even later, on 20 April, when that interview was aired, no public mention was made of the discovery, even though, by that time, we were gaining an increased understanding of what our instrument was seeing, and a meeting had already been arranged in Washington for us to disclose our new findings.

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FIGURE 12.1 Van Allen (right) briefing television newsman Walter Cronkite during a break in videotaping at the University of Iowa on 1 April 1958. Van Allen was describing the satellite program and the results from the cosmic ray experiment that we had seen up to that date. A very poor mockup of the Explorer III satellite is on the bench behind them. The top half is in the center, and the cosmic ray instrument package is behind Van Allen’s arm. A spare model of the onboard tape recorder is located behind Van Allen’s hand. The interview was aired on 20 April. (Courtesy of the Department of Physics and Astronomy Van Allen Collection, The University of Iowa, Iowa City, Iowa.)

Even while the interview was being taped in Iowa City, the first Explorer III tapes arrived in Washington from several of the Minitrack ground stations. H. J. (Jack) Peake, the NRL engineer in charge of processing our data there, reported upon the arrival of those tapes in a short letter, as reproduced here.25

Van Allen’s First Lookat the Explorer III Data The day after his Cronkite interview, Van Allen went again to Washington to discuss a number of Explorer I and III matters. On Wednesday, 2 April, he took a taxi from his downtown hotel to the NRL facility in the southern corner of the District, where he conferred with Joseph Siry (relative to orbital data), John Mengel (data acquisition and data processing), and others of the Vanguard team.26

This is a good place to reemphasize the fact that NRL’s Vanguard staff provided outstanding support for the early Explorer program. Their Vanguard I had been launched just two weeks before the Explorer III launch, and after that launch, they were working around the clock to deal with tracking, orbit determination, data acquisition, and data processing simultaneously for three satellites.

As he concluded his business at the NRL, Van Allen took a taxi to the Vanguard Computing Center at 615 Pennsylvania Avenue. There he received a copy of Jack Peake’s letter and the film that had been prepared the previous day27:

CHAPTER 12 • DISCOVERY OF THE TRAPPED RADIATION

1 April 1958

To: Van Allen and Ludwig

Received from Minitrack stations at San Diego and Ft. Stewart, Ga. 4 1958 gamma recordings (2 from each station) yesterday. One record, San Diego at 1748 Z [UT] on 28 March, was of such high S/N [signal-to-noise] ratio that we transcribed it to the enclosed film record. On the film is pulse time code (1748 Z is marked on the film), detected signal, and 10 kc (needs some magnification to see peaks clearly).

Please accept the film record, compliments of the U. S. Navy! Will be sending along 1/4-inch tape transcriptions shortly.

Sincerely,

H. J. Peake Code 6413 Applications Branch Solid State Division

Putting that precious first Explorer III data readout into his briefcase, he returned to his DuPont Plaza Hotel room, stopping en route at a Peoples Drug Store to pick up chart paper and a few drawing supplies. Late into the night, Van read the data from substantial portions of the record, roughly computed the pulse rates with his slide rule, and plotted the data on a graph. He then charted the counting rate as a function of time. He completed it at about 3:00 in the morning, packed his work sheets and graph into his briefcase, and retired for the rest of the night.

He later recounted that, at that point, he was convinced “that our instruments on both Explorers I and III were working properly, but that we were encountering a mysterious physical effect of a real nature.”28

Although the original film from which he worked that night was apparently not preserved, his original notes and the resulting graph do exist in the University of Iowa Libraries, Department of Special Collections.29 Figure 12.2 is a reproduction of a portion of that graph.

Back in Iowa City Jack Peake also shipped copies of his 1 April letter and the strip- film recording of the San Diego data readout to Iowa City. Carl McIlwain recalled that he met a delivery person early in the afternoon, almost certainly on 2 April, at the entrance of the old Physics and Mathematics Building. Excitedly, he took the film to the basement, where he, Ernie Ray, Joe Kasper, and Herbert (Herb) Sauer quickly mounted it on a reel-to-reel microfilm reader. Carl had been saying for over a month that the transition from normal cosmic ray rates to zero counts was the key to whether it was from an instrument failure or high radiation fluxes. Quoting Carl, “and there it was!”30 Smooth transitions in the counting rate were instantly apparent.

At that point, Carl stopped looking at the film to pursue the then-paramount question. If high radiation intensities were actually being encountered, what would account for the apparent zero counting rates? He quickly lashed together a set of

equipment, using a GM counter and coupling circuit similar to those in the satellite instrument. Setting the counter in front of the X-ray machine that he had been using for his rocket instrument calibrations, he made a quick set of runs. Lo and behold, initially the pulse rate at the output of the coupling circuit rose as expected with increasing X-ray intensity, but it then fell off at higher exposure levels! At rates that should have been about 30,000 per second, the rate actually observed at the output of the circuitry was zero.

That behavior resulted from the characteristics of the GM counter and its coupling and scaling circuits. When an ionizing particle entered the counter and interacted with its internal gas, it produced a pulse on the counter’s central anode. That pulse took only a few microseconds to build. It decayed, however, at a much slower rate, depending on the resistance and capacitance in the circuit. At normal primary cosmic ray rates, the relatively leisurely decay was essentially complete by the time another particle arrived. Thus, all of the arriving particles produced pulses of full amplitude that were counted by the scaler circuits.

When two events occurred much closer together, however, the pulse for the second event started, not from a zero value, but from a higher value that still lingered from the previous event. The transition was from that higher value to, still, the fixed power supply voltage. Thus, the amplitude of the second pulse at the input

FIGURE 12.3 The note that Ernie Ray left on Van Allen’s door after Ernie and Carl Mcll – wain saw the results of Carl’s exposure of the GM counter to an X-ray beam. The note was scrawled on a sheet of paper that was used either earlier or later for some extrane­ous doodling. The exact date of the note is uncertain—the date at the top in Van Allen’s handwriting (5 April) could be the date that he filed it, not necessarily the date that he first saw it. The annotation at the bottom (3 April) is probably the date of the doodling, not necessarily the date of the note. This au­thor’s best judgment is that the note was written on either 2 or 3 April. (Courtesy of the University Archives, Papers of James A. Van Allen, Department of Special Collections, University of Iowa Libraries.)

to the scaler circuit was less than its full normal value. As the GM counter pulses became closer and closer together, the pulse amplitudes at the scaler input became smaller and smaller. Eventually, the pulse amplitudes became too small to trigger the scaler. When that happened, the scaler registered none of them, seeming to signify a zero rate.

Bolstering the tentative conclusion With that information, Ernie excitedly wrote his famous “Space is Radioactive” note (reproduced here as Figure 12.3) and posted it on Van Allen’s door.31 Although, of course, Ernie and Carl knew that space was not radioactive in the true sense of the word, the note clearly expressed their excitement and firm belief that the instrument was, in fact, encountering a real very high radiation level.

Van Allen returned from Washington to the Iowa campus sometime on 3 April. He met, either late that day or the next day, with Ernie and Carl to show his data plot. Carl then offered his data showing the GM counting rate during its exposure to the X-ray beam. The three, Van Allen, Ernie, and Carl, were immediately convinced that the only possible interpretation of the data was that Explorers I and III were encountering very high fluxes of radiation—radiation at least a thousand times as intense as the normal cosmic ray rate.

They quickly set about to make a more complete and detailed plot of the 28 March San Diego data. On Friday, 4 April, Ernie, representing his coworkers, penciled a

OPENING SPACE RESEARCH

second note to Van Allen that accompanied their data plot. Its first page read, “An analysis of data from 1958/. San Diego interrogates: 28 March 1948 Z.” Its second page read32:

From the preliminary information from Vanguard computing center, perigee occurs five minutes after the maximum latitude. Evidently interrogation also occurs near perigee. Thus the data show low (reasonable) counting rates near perigee. The interesting portion of the graph has equatorial crossings and apogee in it.

Evidently there is no hole in space. Rather, space is radioactive.

Very rough preliminary work with an Anton Geiger tube indicates that it stops putting out usable pulses when, if its dead time were zero, it would be counting 104-106/sec.

Data read and analyzed and supporting work carried out courtesy of Kasper, Mcllwain, Sauer, and Ray.

[Signed] Ernie Ray 4 April 1958