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.