John Wohlstetter - Sleepwalking with the Bomb

Knowledge of EMP dates back to the first nuclear explosion. Enrico Fermi shielded all nearby electronics before the Trinity test in 1945 in expectation of an electromagnetic pulse. When it came, it still knocked out data, despite the shielding. But the phenomenon was at first highly localized and dwarfed by blast, heat, and lethal radiation effects, and nuclear explosions in the 17 years afterwards were either surface bursts, like Trinity, or low-altitude air bursts, like Little Boy and Fat Man.

High-altitude EMP first revealed its surprising destructive potential in 1962, when the U.S. detonated its Starfish Prime 1.4 megaton H-bomb 250 miles over Johnston Island in the Pacific Ocean. The explosion occurred in the intense bands of radiation in the ionosphere known as the Van Allen radiation belts. Discovered by James Van Allen (from data collected by America’s first successful satellite, Explorer I, launched in 1958), the belts interact with EMP and amplify its intensity. The pulse of electromagnetic energy from the test reached Honolulu, 900 miles to the east, and knocked out 300 streetlights, a telephone microwave radio link, and lots of burglar alarms.

Russia conducted atmospheric tests over land, its leaders secure in the knowledge that its citizens dared not protest. Russian Test 184, part of a series of detonations over Kazakhstan later in 1962 called the “K Project,” knocked out a 625-mile underground electric power line, interfered with diesel generators, and started some electrical fires in damaged equipment. At 300 kilotons yield, Test 184 was one-fifth the power of Starfish Prime; it also was detonated at just over 70 miles up, less than one-third the American test’s altitude. EMP effects are not overly sensitive to explosive yield, according to EMP expert Dr. William Graham, who chaired the congressional EMP panel. But effects are far more intense over land, because EMP perturbs the Earth’s magnetic field, which is far stronger over land than water. The lower altitude of the Russian test limited EMP radius. But having been detonated over land, the Russian test caught more ground installations within reach of its footprint than did Starfish Prime.

A chain of events causes EMP. First come the intense gamma emissions from the fissioning atoms of the bomb. These ionize the air, knocking electrons out of atoms. When the electrons hit the earth’s magnetic field, it knocks them sideways, and it is this interaction that generates the electromagnetic pulse.

Three successive, different pulses generate the EMP effect: E1 is the superfast high-voltage pulse that bypasses surge protectors (which are designed to stop pulses that build up less rapidly) and “fries” all affected electronics. E2 is akin to the electromagnetic charge emitted as lightning strikes. This pulse is low frequency and ordinarily would not get past surge protectors, except that if the first pulse has fried the protective equipment, then the second pulse, like a burglar who enters an open door, gets in. The third pulse, E3, is akin to the geomagnetic storms generated by solar flares. This pulse is the Earth’s magnetic field being disturbed by the bomb, then settling back to normal—an extremely low frequency pulse that lasts tens, even hundreds of seconds. Though weak, because of its duration this third pulse builds up enough strength to knock out large components like electric grid power transformers, and reaches deep into the ground.

EMP confounds some common expectations of the relative dangers of different nuclear weapons. Aside from the displacement of the Earth’s magnetic field (E3), the intensity of the phenomenon does not, as indicated above, scale with warhead yield. A Hiroshima-size blast (14 kilotons, less than 1/20th the K Project yields and 1/100th that of Starfish Prime) will do very nicely. Counter-intuitively, a hydrogen bomb with yields in the megaton range short-circuits EMP by the phenomenon of pre-ionization. The hydrogen bomb’s trigger (an A-bomb) prematurely strips electrons from the atoms of the air, so that the later, larger quantity of electrons shorts out in the atmosphere rather than interacting with the Earth’s magnetic field. H-bombs thus cause less EMP damage than A-bombs.

EMP means that a foe does not need the massive explosive yield of an H-bomb to inflict grave damage. An A-bomb will perform the EMP task with greater effect. North Korea already has an A-bomb, and Iran is getting tantalizingly closer to one.

Altitude is a critical factor in EMP effects—the bomb must go off at the right height to hit the Earth’s magnetic field. Deep space detonations do not cause EMP effects on earth—the gamma ray emissions from the bomb dissipate long before reaching Earth’s magnetic field. And low-altitude nuclear detonations, which generate vast blast, heat, and radiation damage, explode below most of Earth’s magnetic field and hence generate less EMP.

An attacker could detonate an EMP weapon 300 miles over Kansas, and cover a 1,470-mile radius that would encompass the entire lower 48 states. On a smaller scale, even a ground burst weapon’s EMP effects could devastate local electric power in a radius of roughly 6 to 12 miles, potentially hugely effective over a major metropolis. EMP can create imbalances within the grid that cause the system to shut down and inflict severe damage. Because newer infrastructures rely so much on digital technologies, far fewer people are required to run them; thus, in a crisis, there are far fewer workers available to rapidly reconstitute damaged parts of the system. Infrastructures driven by modern silicon computer chips are much more vulnerable to disruption via EMP than are infrastructures built with older-generation electric power technologies.

In 1998, Iran, which lacks strategic bombers and missile subs, test-fired a missile from a floating barge, validating its ability to launch a ballistic missile from a platform less stable than a ground launch (and thus more susceptible to inaccurate guidance). Barges could easily cruise offshore in international waters, i.e., outside the 12-mile limit. [54] Some countries claim a 200-mile limit to territorial waters, but such claims are not currently recognized under international law. Iran would have to launch its missiles a few hundred miles at sea, to limit the chance of detection. Reaching 1,500 miles inside the U.S. homeland would require a missile range of about 2,000 miles. New Iranian models are approaching this range. Such missiles could be launched from either the Atlantic or the Pacific Ocean, or from the Gulf of Mexico.

In 1999, Iran tested an armed ballistic missile in an “EMP mode”: this means that the missile was fired in a steep trajectory whose angle of ascent matched that required for an EMP attack. The missile’s conventional warhead detonated at high altitude. This test validated Iran’s ability to carry out a coastal EMP strike. When newer, longer-range missiles enter service, Iran could target the lower 48 states by detonating an EMP weapon centered over the interior.

Using shorter-range missiles Iran could target selected cities easily from offshore, with lower-level EMP bursts. Ship-launch scenarios were one threat specifically identified by the 1998 Rumsfeld Commission report on growing ballistic missile threats. Not that this threat was new: in his 1949 memoirs Dr. Vannevar Bush, wartime science adviser to Presidents Roosevelt and Truman, warned of the threat posed by “atomic bombs smuggled in by innocent-appearing ships, to be detonated at the chosen moment.” The U.S. successfully launched a Polaris ballistic missile off a commercial ship in 1962.

AMONG THE warnings issued by the congressional EMP panel in 2008, perhaps the strongest concerned the risks of infrastructure interdependency, specifically its tendency to increase the time needed to recover after an EMP attack: