Science: Detecting the Tests

As soon as the Russians started their new series of nuclear tests in the atmosphere, it was clear that they could not have cared less how quickly the rest of the world learned about their latest tactic. Atmospheric tests are impossible to conceal; they shout their presence in varied voices, some of which carry for thousands of miles and can be detected in many ways.

>Radio receivers near the borders of the U.S.S.R. were the first U.S. instruments to report the news. Nuclear explosions in air generate sharp pulses of radio energy that can be picked up at great distances and clearly distinguished from everyday static. When two or more stations note the direction from which a pulse comes and the instant that it arrives, its point of origin can be calculated accurately.

> Radar was also quick to discover the Russian tests. The radiation from a nuclear explosion causes changes in the ionosphere, the electrically charged layers of the upper atmosphere. The searching beam of a long-distance radar is reflected by this disturbance, making the aftermath of the explosion visible to receiving apparatus at the radar station.

> Microbarographs (sensitive recording barometers) got the word slightly later. An explosion in the lower atmosphere puts almost half of its energy into shock waves that travel through the air, turning first into audible sound waves with a thunderlike bang, then into fluctuations of pressure. Microbarographs can detect this pressure wave more than 1,000 miles away. The U.S. has a ring of microbarographs waiting for interesting waves to wash down from the Soviet border. A clear reading from a microbarograph gives a good estimate of an explosion’s punch.

> Radioactive residue, carried high in the air by the rising Russian mushroom, brought the news last. As in every nuclear explosion, some of the dangerous residue fell near the test site; the rest climbed into the stratosphere and was carried around the earth by high-altitude winds. Collected by high-flying airplanes (the U.S. has many patrol planes equipped for this job), the residue was rushed to laboratories and carefully analyzed. It identified the materials used and permitted a close estimate of the efficiency of the nuclear reaction.

Low & High Tests. If the Russians were trying to hide their tests, they would have held them underground. Underground explosions send no ordinary radio signals or barometric waves. They are invisible to radar, and they scatter no telltale fallout. But they do create powerful earth waves that travel in the earth’s crust and deep through its interior. A powerful underground explosion registers on seismographs all over the world, and smaller explosions are detected at shorter distances. The fault of this system is that weak bomb waves are hard to distinguish from the waves of natural earthquakes. Some experts claim that underground explosions send very low-frequency (less than ten kilocycles) electromagnetic waves through the solid rock. Since earthquakes do not do this, a special underground receiver might be able to distinguish them from nuclear explosions.

U.S. experts have speculated that the most important purpose of the Russian tests may be to try out warheads for anti-missile missiles designed to destroy incoming intercontinental missiles while they are still high above the earth. If this is so, the present tests are preliminary. The anti-missile missile must attack its target well above the atmosphere, and nuclear explosions in dense, low-level air cannot test the effectiveness of such a weapon in the vacuum of space.

When a nuclear bomb explodes in the atmosphere, it forms a hot fireball that expands and cools until its pressure has fallen to that of the air around it. Then it rises like a balloon, while a destructive shock wave leaves its surface and races through the air. An explosion in a vacuum behaves differently. There is no conventional fireball. The main nuclear reaction is finished in less than a millionth of a second, and many spherical shells of radiation move outward from the explosion, unhampered in the emptiness of space. First comes a deadly burst of X rays and gamma rays, traveling with the speed of light and carrying about seven-tenths of the explosive energy. Such a radiation would be stopped by a few miles of air; in space it has unlimited range and can do terrible damage at a great distance. Along with the X rays and gamma rays travel heat, light and ultraviolet waves; just behind come beta rays (high-speed electrons) and neutrons, both of them highly destructive.

Eyes on Space. Bombs have already been exploded above the atmosphere, but so far as the public has been told, their destructive effects have not been tested. There is a good chance that their X rays, neutrons, and other far-ranging emanations can destroy or explode an incoming missile or hamper its flight from much farther away than would be possible in the atmosphere, but no one will know for sure until many space tests have been made.

The U.S. is surely alert for Soviet space tests, and it has the means with which to watch. An explosion just above the atmosphere cannot be detected effectively by microbarographs, and probably not by radio waves. But when X rays from the detonation hit the top of the atmosphere, they will cause the emission of visible light, which can be detected by sensitive, upward-looking instruments. The bomb’s emanations will also cause changes in the ionosphere that can be detected by long-range radar.

Another giveaway is the Argus effect. When U.S. Project Argus exploded its nuclear armed rockets above the atmosphere in 1958, electrons from the blasts were caught by the earth’s magnetic field and whirled around the world, causing conspicuous electrical effects. Russian instruments deep in Siberia detected the effects easily, but the Russians did not know what they had recorded until Project Argus was declassified. Both the U.S. and Russia certainly have Argus detectors watching for space bursts now.