In Part I of this series, we discussed how a single, relatively simple device exploded over the U.S. Midwest would generate an electromagnetic pulse, or EMP, that could destroy the whole U.S. power grid and virtually everything connected to it. An EMP attack poses perhaps the ultimate threat to today’s contingency planner. Unlike other disasters, which are regional in scope, an EMP could affect the entire nation and flash us back almost instantly into a 19th-century economy. There would be no outside help and no one to turn to. An EMP attack would wipe out computers and the Internet. There would be no electricity, perhaps for years. Emergency services would stop, and hospital operating rooms would close. Food distribution would stop. In the ensuing chaos, most Americans would die from starvation in months.
How Lethal Is an EMP Weapon?
Understanding more about what EMP is and how it manifests itself is the first step in planning for such a disaster. Several factors govern the lethality of a nuclear EMP weapon:
- Device yield
- Other factors
A nuclear detonation high in the atmospherein fact, at the edge of spaceproduces an immediate flux of gamma rays. These gamma rays in turn produce high-energy free electrons. This occurs at altitudes between roughly 10-25 miles. The electrons are then trapped in the Earth's magnetic field, giving rise to an oscillating electric current. This current in turn gives rise to the rapidly rising radiated electromagnetic field known as an electromagnetic pulse (EMP).
The “line of sight” (LOS) from dozens or hundreds of miles above the earth means the EMP effect can easily span entire continents-sized areas. For example, large device detonated 250 to 312 miles over Kansas would affect the entire continental U.S. The energy from such an event extends LOS to the visual horizon as seen from the burst point. Note that this altitude is above the orbit of many LEO (Low Earth Orbit) satellites. Therefore, such an EMP could be expected to wreak havoc over those systems as well. Geosynchronous satellites, on the other hand, orbit 22,300 miles up. These systems could in fact survive. For other electronic equipment to be affected, the weapon would need to be within LOS above the visible horizon. In most cases it would be, even if it was partially blocked by mountains or other topography.
Typical nuclear weapon yields used during Cold War planning for EMP attacks were in the range of 1 to 10 megatons, or roughly 50 to 500 times the size of the bombs used at Hiroshima and Nagasaki. Physicists have testified during U.S. Congressional hearings that weapons with yields as low as 10 kilotons or less, roughly half the size of the Hiroshima bomb, can produce a very large EMP. In fact, small "pure fission” weapons like these combined with thin cases are much more efficient at causing EMP than most megaton thermonuclear bombs. Ironically, these are somewhat cruder designs, which can be presumed to be easier to build if fissionable material was available.
An important aspect of nuclear EMP is that all of the components of the pulse are actually generated outside of the device. For high-altitude nuclear explosions, this means that much of the EMP is actually generated at a large distance from the detonation point, when gamma radiation hits the upper atmosphere. According to the standard reference text on nuclear weapons effects published by the U.S. Department of Defense:
The peak electric field (and its amplitude) at the Earth's surface from a high-altitude burst will depend upon the explosion yield, the height of the burst, the location of the observer, and the orientation with respect to the geomagnetic field. As a general rule, however, the field strength may be expected to be tens of kilovolts per meter over most of the area receiving the EMP radiation.
It is the peak electric field of the EMP that determines the peak voltage induced in equipment and other electrical conductors on the ground, and most of the damage is determined by induced voltages
Other governing factors such as local topography and the local strength of the earth’s magnetic field (as discussed previously based on latitude) also come into play when predicting the lethality of an EMP device.