The Villain: Radiation
This left only radiation damage. We had other good reasons to suspect this, too. As far back as October 1961, scientists at Bell Labs and Brookhaven National Laboratories had made an important discovery about the effect of radiation on a transistor. They found that, when radiation penetrates the outer shell of a transistor and ionizes the gases inside, electrically charged particles (ions) tend to collect on the surface and change the transistor’s electric properties. This effect is particularly noticeable when a transistor is operating under reverse bias voltage. We knew that some of the 37 transistors used in each Telstar I decoder circuit were operating under continuous reverse bias and that they also had less metal shielding than did those semiconductors in the telemetry and receiver circuits.
Telstar’s radiation detectors had been telling us that the concentration of high-energy electrons near the inner edge of the Van Allen belt was greater than we had expected. We now know this may have come as a result of man-made high-altitude nuclear explosions, one of which took place the day before Telstar I was launched. But we had had no reason to anticipate this extra radiation—it was more than one hundred times the predicted level—when we tested the transistors to be used in Telstar I. So we were not too surprised that they were more susceptible to radiation damage than we had thought they would be.
All this seemed to give us a theoretical explanation for the trouble. So, after November 23rd, we began looking for laboratory evidence to confirm our radiation theory. First, two engineers traveled down to Johannesburg, South Africa. At this time the highest point of Telstar’s orbit—when it passes through the least Van Allen belt radiation—was over the southern hemisphere, and it seemed a good idea to command the satellite when it was under the condition of lowest radiation and see if anything would happen. However, everything the engineers tried proved fruitless.
A duplicate of one of Telstar I’s command decoders, containing 37 transistors and 191 diodes, being placed in a small elevator that will lower it into a gamma radiation chamber.
At our Murray Hill, New Jersey, laboratories we worked on a different approach. We exposed transistors like those used in the decoders to large doses of radiation. We also exposed entire spare decoder units to accelerated radiation to find out where their weakest points were (see [illustration]). And then we built and tested decoders using radiation-resistant transistors to see if they worked better. After a week of intensive laboratory work, we had some pretty good evidence. The tests of individual transistors definitely showed that heavy radiation would cause them to deteriorate. Testing of the complete decoders also led to some failures, and, when we analyzed them, they turned out to be the kind that would be caused by faulty transistors. We also discovered that the most sensitive part of a decoder circuit was the zero gate, which recognizes the zeros in the one-and-zero code that commands the satellite.
A typical command signal sent to Telstar I. It consists of seven pulses: a three-unit “start” pulse and a binary code made up of three two-unit “one” pulses and three one-unit “zero” pulses.