Keeping the Satellites Cool

Looked at casually, the three shiny, silvery balls, 20 in. in diameter, seem to be made of polished aluminum. A closer look shows that they have an elusive iridescence, like the rainbow colors of soap bubbles. This is the only visible hint that there is something tricky about their shiny surfaces. The three balls in the Naval Research Laboratory at Washington are satellites for Project Vanguard to place on orbits circling the earth.

Heat Balance. To make observations beyond the earth's atmosphere, the instruments in a satellite must be kept fairly cool. The job is harder than it sounds. Space is neither hot nor cold, but an object exposed to sunlight in space may get pretty hot. The temperature that it reaches will depend on how its surface absorbs and emits radiation. If the energy that it absorbs from sunlight is greater than the energy that it emits as heat rays, the body's temperature will rise. The amount of heat rays that it emits will rise too. Eventually, the outgoing heat will balance the incoming solar energy, and the body's temperature will stop rising.

The task of the Naval Research Laboratory was to find a surface for the satellite that 1) does not absorb much solar energy, and 2) gets rid of the energy that it does absorb without becoming too hot. When the satellites came from their manufacturer, Brooks & Perkins, Inc. of Detroit, they were thin-skinned magnesium spheres plated with gold. Aluminum is better for reflecting sunlight, but since aluminum will not stick to gold, the gold had to be covered with a thin film of chromium. Aluminum will stick to chromium, but it also mixes with it and loses part of its reflecting power. So the chromium film in turn had to be coated with glassy silicon monoxide, and then with aluminum.

The delicate work of depositing the coatings was done by the Army Engineers at Fort Belvoir, Va. Each satellite was put in a vacuum chamber and turned, like a chicken on a spit while the materials in the coatings were evaporated electrically and deposited on its surface. The final coat was a second layer of silicon monoxide.

No One to Freeze. The two outer layers determine the satellite's reaction to solar radiation. Silicon monoxide is transparent to visible light, so the powerful visible part of the sun's radiation penetrates to the shiny aluminum and is mostly reflected back into space. The temperature of the aluminum rises slowly, both because it is a poor absorber and because the silicon monoxide layer in contact with it is a comparatively good radiator of infra-red (heat) rays. By experimenting with different thicknesses of silicon monoxide, the Navy's scientists think they can keep the temperature of the skin below 140DEG F. The sensitive instruments inside will be comfortable at about 120DEG F. When the satellite passes into the shadow of the earth, its temperature will drop suddenly to below zero Fahrenheit, but since it will be inhabited only by frostproof instruments, the Navymen do not care how cold it gets.

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