On July 9, 1962, the United States detonated a 1.4-megaton thermonuclear device 400 km above Johnston Island in the Pacific. The test, codenamed Starfish Prime, was meant to study high-altitude nuclear effects on radar and missile-defense systems. [1] What it actually demonstrated was an unexpectedly powerful electromagnetic pulse. Street lights failed on Oahu, about 1,300 km away, hundreds of burglar alarms rang in Honolulu and circuit breakers on power lines opened. [2] The discovery led to a decades-long study of the high-altitude electromagnetic pulse — HEMP — and to the realization that a single nuclear weapon, well-placed, could disable entire continents.
How a nuclear bomb generates EMP
A nuclear detonation releases about 0.3% of its energy as prompt gamma rays — high-energy photons that can penetrate matter. At ground level, gamma rays are absorbed within meters of the detonation, contributing to the prompt radiation pulse. But at high altitude (above 30 km, where the air is thin), the gammas can travel hundreds of kilometers before they interact with anything. [2]
When a gamma ray strikes an air molecule, it knocks an electron loose at relativistic speed (the Compton effect). The accelerated electron is then deflected by the Earth's magnetic field, producing a brief but intense electromagnetic pulse. Because the gammas spread over a large area before they interact, the pulse covers an enormous footprint — the line-of-sight area from the burst altitude.
A burst at 400 km altitude has a line-of-sight footprint of roughly 2,200 km radius, enough to cover almost all of the continental United States. [2]
E1, E2, and E3 components
The HEMP signal has three temporally distinct components, each with different damage characteristics. E1 is the prompt pulse: a sub-microsecond spike of intense electromagnetic energy. It is fast enough to defeat most surge protectors and induces voltages high enough to destroy semiconductors directly. Modern electronics — phones, computers, vehicle ignitions — are particularly vulnerable.
E2 is an intermediate-time pulse, similar to lightning, lasting from about one microsecond to one second after the burst. [3] It is generated by neutrons interacting with the atmosphere. E2 is well within the range that lightning protection can handle, so it is generally not the threat component.
E3 is a much slower pulse, lasting tens to hundreds of seconds, caused by the bomb's expanding fireball distorting the geomagnetic field. [3] E3 is dangerous to long-distance power lines and transformers, where it can induce ground-induced currents (GIC) similar to those produced by severe geomagnetic storms. E3 is the component most likely to trigger continent-wide power-grid failures.
What gets damaged
Modern unhardened electronics are extremely vulnerable to E1. Computer chips, phones, electronic ignitions, controllers in industrial systems, and the supervisory hardware of power-grid substations can all be damaged or destroyed. Long cables and power lines collect far more E1 energy than short ones, though even short cables can carry some of the pulse into equipment. [2]
The power grid is the most concerning vulnerability. Long transmission lines pick up substantial E3 currents. Large transformers that step voltage up and down at substations can be damaged when ground-induced currents drive them into magnetic saturation. EMP damage to grid protection systems could extend to large power transformers. [4]
Critical infrastructure — water treatment, hospital systems, emergency services, financial transactions — all depend on electric power. A continent-wide grid outage caused by HEMP could last weeks to months and cause cascading failures across the economy.
How seriously experts take the threat
Assessments of HEMP threat have ranged widely. Presenting the EMP Commission's 2008 findings to Congress, its chairman called EMP "one of a small number of threats that can hold our society at risk of catastrophic consequences." [4] Critics have argued that the report overstates vulnerabilities and understates the difficulty of executing an attack.
The technical consensus is somewhere in between: HEMP is a real and serious threat, particularly to the power grid, but the damage is highly dependent on the specifics of the burst (yield, altitude, latitude relative to magnetic field, atmospheric conditions). And executing a HEMP attack requires either a delivery vehicle capable of reaching high altitude over the target — which only nuclear-armed states with ICBMs currently have — or a ship-launched short-range missile lofted from offshore.
Hardening
EMP hardening — designing electronics to survive HEMP — is well understood but expensive. Military systems (strategic bombers, missile silos, command-and-control nodes) are routinely hardened. Civilian infrastructure is mostly not.
FERC Order No. 830 (2016), which approved a standard requiring grid planners to assess their vulnerability to geomagnetic disturbances every five years, was a small step in this direction. [5] But comprehensive hardening would require sustained policy attention and funding that has so far not materialized.
EMP in modern threat assessments
HEMP appears in modern threat assessments primarily in the context of North Korean and Russian capabilities. North Korea's Hwasong-14 ICBM, first flight-tested in July 2017, showed a potential range of more than 10,000 km. [6] Russia and China both possess clear HEMP-capable delivery systems.
The simulator does not model HEMP effects directly — its scenarios show ground-level blast and thermal effects, which are the dominant kill mechanisms for surface and low-altitude air bursts. HEMP is a different attack profile entirely, optimized to disable infrastructure rather than to destroy targets. See the glossary entry on EMP for a definition, and the sources page for primary references.