The Danger of EMP

Scientists and military planners assigned to devise defenses against nuclear blasts have had their hands full.

They must shield prospective targets against thunderous shock waves, searing heat, deadly X rays, gamma rays and neutrons. They must also guard against a lesser-known product of atomic explosions called electromagnetic pulse, or EMP. In a recent Washington speech, Senator Henry Jackson, atomic-weapons specialist of the Armed Services Committee, insisted that despite five years of research, EMP still poses a "serious problem" to the nation's communications, radar and missile systems.

EMP is created when gamma rays from an exploding weapon strike electrons in the surrounding air, causing them to move rapidly away from the center of the burst. Because of the shape of the warhead, the irregularity of the atmosphere or the proximity of the explosion to the earth, the pattern of the outward-speeding electrons is seldom symmetrical; the overall effect is similar to that caused by a flow of accelerating electrons in only one direction.

Like electrical current pulsing through a wire, the stream of electrons produces a rapidly expanding electromagnetic field—the EMP. Just as a moving magnetic field induces currents in the wires of a generator, the expanding EMP produces powerful currents in any electrical conductor it crosses. At considerable distances from the blast, these induced currents are strong enough to blow fuses or melt wiring and other metallic components in ground installations and aircraft. They would probably have the same effect on a missile's guidance and firing systems.

Before approval of the limited atomic test ban treaty five years ago, Jackson told the Senate, his committee was assured by scientists that enough had been learned from atmospheric tests to design electronic components that could withstand EMP's current surges. But Jackson is not convinced. Now that researchers are limited exclusively to confined underground tests for guidance, he said, they are prevented from solving the EMP problem completely—especially for missiles in flight.

Although the military has placed some circuitry underground and installed surge arresters (which safely dissipate sudden pulses of current) on other equipment, Jackson says that much of the nation's electronic defense—and its offensive missiles, too—may well be susceptible to EMP. Military men and scientists who do not share his concern are as frustrated as those who would like to continue testing. Secrecy prevents them from airing their arguments.