VOICE FROM SPACE

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 brief electrical pulses when radiation particles hit them. Some of them are shielded with lead so that they will register only high-energy particles. Others are sensitive only to particles from one direction. The pulses from each instrument are fed to individual audio-frequency (audible sound) oscillators, changing the frequency of the pulses slightly. These modified audio tones are imposed on the high-frequency carrier wave sent out by the satellite's transmitter. Fed into a loudspeaker, they sound like strange, unbeautiful music.

When this mixed signal reaches the ground, it is recorded on magnetic tapes. In Van Allen's laboratory the tapes pass through a machine that separates each imposed audio tone from the carrier frequency and records it visually as a jiggly red line on a wide band of graph paper (see chart).

The four lower lines on the chart were made by the instruments of Explorer IV while it was passing through the inner Van Allen belt of heavy radiation. Channel I is from a Geiger tube set up to give a pulse when 64 radiation particles have passed through it, is thus intended to record areas of relatively low radiation. Here the radiation is so heavy that the counter is swamped and no meaningful count is recorded. Its small oscillations are mere radio noise.

Channel II came from a scintillation counter sensitive to radiation from one direction. Each wave records 2,048 particles above a certain energy level. Because the satellite is tumbling, the waves are spaced at varying intervals--closer together when the instrument points into the radiation, further apart when it turns away. Thus this channel tells both the radiation's strength and directional character, also indicates the rate of tumble, and the satellite's attitude at any given moment.

Channel III, a Geiger tube, records higher rates of radiation, is designed to take over when Channel I is swamped. Here its square-topped waves are indicating about 4,000 counts per second, enough to kill a man exposed to them in a matter of days.

Channel IV, from a directional scintillator, measures the amount of radiation energy passing through the instrument's window each second. The significant parts of the line are its depressions below the flat parts: the deeper they are, the greater the energy. The depressions come in cycles reflecting the tumbling of the satellite, but some of the energy is recorded when the scintillator is "looking away" from the direction of the radiation. This reveals that the lower part of the Van Allen radiation belt contains particles powerful enough to pass through the shielding around the scintillator.

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