GETTING information from a satellite is tricky business. "If you want to measure the temperature up there," says Van Allen, "you can't put a mercury thermometer in your bird. You have to read temperature as an electrical signal." This is done with a tiny "thermistor," whose resistance to current put out by the satellite's batteries varies with temperature. The change affects the frequency of the electronic signal sent out by the satellite's transmitter, thus reporting the temperature to the ground.

Van Allen's instrument packages to measure radiation are much more complicated. Their sensing elements are Geiger tubes and scintillation counters that give...

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