Newest Nuclear Tests: What They Hope to Prove

Atomic Energy Commission scientists last week were "keyed up and ready" to take the U.S. into a new high level of nuclear testing. First shot on the agenda: a giant skyrocket exploding not more than 60 miles above Johnston Island, its sub-megaton flash visible in Hawaii 700 miles away, its power sending waves of electrical disturbance around the earth to be picked up by sensitive instruments.

Later shots in this latest series of U.S. nuclear tests will carry a bigger bang and go off far higher above the surface of the earth. If not obscured by clouds, these explosions will be seen in Hawaii as brilliant flashes of light in the night sky, probably followed by enormous and rapidly swelling fireballs. They will dazzle the naked eye like glimpses of the sun, doing no permanent damage; but the AEC warns that it will be extremely dangerous to look at the lofty explosions with binoculars or telescopes. Concentrated on the eye's retina, the light will be strong enough to cook a fair-sized blind spot.

Smoke Screen. Purpose of the high-altitude shots is both military and scientific. Nuclear explosions in the vacuum of space or in the thin fringe of the atmosphere do not behave as they do in the dense air near sea level. Little or none of their energy goes into shock waves; most of it escapes as X rays, neutrons, and other varieties of radiation that are relatively unimportant in ground-level bursts. But military men are most anxious to learn what this radiation will do to missiles and satellites, and even to aircraft.

The tests will also demonstrate the electrical effects of high-altitude blasts. When a nuclear weapon explodes in the thin air more than ten miles above the earth, it creates vast numbers of long-lasting free electrons. If they are numerous enough, the electrons can absorb and reflect many kinds of radio waves. The AEC estimates that a one-megaton weapon bursting at a 50-mile altitude will disrupt high-frequency radio waves (the most useful kind for long-distance communication) for 600 miles around.

The most serious military effect probably concerns radar--particularly the powerful radars that are being developed to spot ballistic missiles plunging down from space. A high-altitude nuclear explosion, the AEC explains, acts like an enormous, radar-blinding smoke screen. Radar beams that search the sky for invading warheads may be either absorbed or totally reflected by bomb-ionized air. An enemy hoping to hit a target defended by radar-guided anti-missile missiles might well explode a warhead several hundred miles up to create an electronic smoke screen that would blind defensive radars to other warheads racing toward their targets.

Small Worry. Beyond such military implications, the high-altitude tests have an additional purpose that is purely scientific: to find out how nuclear explosions above the atmosphere affect the Van Allen radiation belt. Some scientists have denounced this part of the experiment as a crime against man's own environment; they fear it will damage the belt for many years. But after earnest consultation with U.S. experts, the AEC is convinced that there is little to worry about. The lower and stronger of the two space explosions will release electrons that will be promptly trapped by the earth's magnetic field. Most of them will get entangled in the atmosphere, creating artificial auroras. A few that travel higher may drift around the earth until they are over the Atlantic Ocean, where the lopsidedness of the magnetic field will make them swoop lower; then the atmosphere will absorb them too.

Most interesting test scientifically, and the most hotly criticized, is a sub-megaton shot to be exploded at about 500 miles altitude. It will be high enough to have a direct effect on the lower levels of the Van Allen belt. AEC scientists think it will create a "magnetic bubble" hundreds of miles in diameter that will distort the lines of force of the earth's magnetic field. The bubble will probably disappear after 30 to 40 minutes, but while it lasts, it will be an obstacle to the free movement of particles in the radiation belt. Some of them will detour around it and continue on their way. Others, diverted downward, will be caught by the earth's atmosphere. But AEC scientists are certain that the bubble will be too small to affect more than a few percent of the belt's particles. Any major effect on the belt is expected to disappear within a few weeks.

To observe the effects of its space explosions, the AEC has deployed a net of supersensitive instruments, some of them halfway around the world. These mechanical observers will watch the sky for auroras, feel the soil for electric currents, measure changes in the earth's magnetic field. At least two U.S. satellites now in orbit also carry equipment to observe the experiments, and foreign scientists are preparing for their own test watch. Before each explosion, the AEC has promised, all the world's laboratories will get advance word of when to watch for electrical disturbances, spreading like gigantic ripples from far-off Johnston Island.

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