Atomic Tests: The Blast at Lop Nor

The bleak land around Lop Nor, a salt lake in the Takla Makan Desert of Red China's Sinkiang province, is one of the most remote and unpleasant places on earth. But last week Lop Nor was suddenly familiar to all the world when President Johnson pinpointed it as the place where the Chinese had conducted their first atomic test.

U.S. authorities have not yet told all they know about the Chinese test, presumably because disclosure would divulge too much about their detection methods, which are extremely efficient. They predicted the blast weeks in advance, reported it almost as soon as it happened, named its fissionable material, estimated its energy and followed the spreading cloud of radioactivity as it circled the Northern Hemisphere on the fast westerly winds that prevail at high altitudes.

Slim Tower. The Chinese test was in the atmosphere; the nuclear device was probably perched on a slim tower several hundred feet high to keep the fireball out of contact with the ground. This type of test, outlawed by the U.S.. Russia and Britain by the 1963 test ban treaty, has much to recommend it to the novice nuclear power. The explosion's position is known precisely, and it can be watched by hundreds of instruments, some of them so close that they are vaporized a few microseconds after they send their data.

Underground tests, such as the one that the U.S. conducted last week in a salt dome near Baxterville, Miss., are much more expensive and not as convenient to observe. They are also harder to detect and might well be carried out in secret.

Near-surface explosions can never be secret. They proclaim themselves loudly in many different ways. The shock wave smacks the ground hard, starting characteristic earth waves that may be detected by seismographs thousands of miles away. In the air the shock wave turns into a sound wave that weakens as it travels until it dwindles into a brief rise of barometric pressure. In its last weak form, the wave can cover thousands of miles before it becomes too faint for microbarographs to distinguish it from natural variations of atmospheric pressure. The U.S. undoubtedly had many seismographs and microbarographs stationed around China to be on the alert for its maiden test.

Prattling Particles. Radios and radars were also alert. Any nuclear explosion sets off a great variety of electromagnetic waves, some of which are in the radio segment of the spectrum. They travel great distances, guided around the curve of the earth by ionized layers in the upper atmosphere, and they are not difficult to detect. The explosion-born pulse of radio waves disappears quickly, but another radio effect lingers on. As the mushroom cloud climbs into the stratosphere, its radioactivity releases a vast number of electrons that ionize a mass of air and turn it into a radio wave reflector. This air mass shows up on long-distance radars, and it may distort radio waves coming from beyond it. A combination of all these long-distance methods of measurement can pinpoint the explosion accurately and give a good idea of its strength.