NUCLEAR EXPLOSIONS IN SPACE:
THE THREAT OF EMP (ELECTROMAGNETIC PULSE)
MINI BRIEF NUMBER MB82221
George N. Chatham
Science Policy Research Division
THE LIBRARY OF CONGRESS
CONGRESSIONAL RESEARCH SERVICE
MAJOR ISSUES SYSTEM
DATE ORIGINATED 03/02/82
DATE UPDATED 12/15/83
FOR ADDITIONAL INFORMATION CALL 287-5700
I S S U E DEFINITION
A high-altitude nuclear explosion produces electrons which may
be c a u g h t
by t h e Earth's magnetic field a n d convert their energy i n t o radio waves.
n e t effect i s a microsecond burst of i n t e n s e , broad
frequency r a d i o wave
e n e r g y , Which i s o n e form of a n Electromagnetic P u l s e or EMP.
d o e s n o t threaten living things, i t can disrupt or destroy unprotected
s t a t e e l e c t r o n i c s , including those used i
r a d i o and telephone
computers and computer magnetic memory b a n k s , aircraft instrumentation and
f l i g h t controls, ignition and control modu1e.s in a u t o m o b i l e s , protective
devices Which sustain electric power distribution in t h e United
S t a t e s , and
satellites and their ground stations. At i s s u e i s the vulnerability o f the
Nation and the measures that can be implemented to minimize the d a m a g e t h a t
c a n b e caused by EMP.
T h e high-altitude EMP was f i r s t encountered during nuclear testing over
Johnson Island i n t h e Pacific i n 1958. D u r i n g this test p e r i o d , 1 9 5 8 - 1 9 6 3 ,
EMP was not a threat to the relatively primitive vacuum tube electronics nor
to t h e electromechanical relays o f telephone systems.
It w a s regarded a s a
mystery and a n "interference."
In 1 9 6 2 the Rand Corporation i n California
published a n explanation of how the pulse w a s generated along with
Electronic vulnerability to t h e EMP emerged i n t h e next decade with
electronic revolution which began in the early 1960s.
T h e vacuum
gave way to solid s t a t e electronics.
Only milliwatts of power could perform
electronic tasks which required watts using t h e older technology.
As efficiency r o s e , sensitivity to transient surges of current a l s o rose.
T h e solid state circuitry, a l m o s t universal by 1 9 7 0 , proved to be a thousand
to a million times a s easily damaged by an E M P a s the older vacuum
T h e growth of understanding within
the military establishment of EMP
effects a s well a s the problems of EMP countermeasures i s revealed i n the
history of the Safeguard antiballistic missile (ABM), a system of r a d a r s ,
c o m p u t e r s , and nuclear-armed interceptor missiles to have been d e p l o y e d a t
twelve sites a c r o s s t h e United S t a t e s starting i n the early 1970s.
understanding of EMP developed i n the military,
cost i n the
("hardening") and EMP
simulation equipment became a major
Although the missiles and ground
c o n t r o l s y s t e m s were
hardened, the possibility w a s recognized t h a t t h e e ~ p 1 0 ~ i O nf r o m the
Safeguard or S o v i e t nuclear warhead
isolate t h e sPte from all
Essential links to NORAD (the North American D e f e n s e Command)
and N O R A D v s l i n k s to the P r e s i d e n t could b e t e r m i n a t e d by
extent to which Safeguard might
impair the overall defensive capability
remained a d e b a t e throughout t h e l i f e of the project (1969-1976).
Ironically, the threat posed by Safeguard w a s significantly reduced by the
May 1 9 7 2 anti-ballistic missile treaty with t h e ' Soviet Union.
T h e treaty
permitted t w o ABM s i t e s ; a 1 9 7 4 protocol to t h e treaty reduced that number to
A s i n g l e Safeguard base became operational in April 1 9 7 5 but closed i n
February 1 9 7 6 , primarily d u e to concern that the utility of a single ABM site
could be nullified by t h e growing S o v i e t missile force.
Concern over the
l o s s of communications to EMP remained.
Solid s t a t e electronics initiated several trends i n the early 1 9 6 0 s which
began to accelerate a n d still continue to do s o today.
Circuitry i s n o w more
six or seven orders of magnitude
compared to vacuum
Power consumption has decreased by a similar. amount.
T h e cost
of circuitry has a l s o decreased by f i v e t o six orders of magnitude.
combination of f a c t o r s has led to a geometric growth of tasks
Small electronic c h i p s n o w control a n d monitor vehicle
engines, provide instrument displays, and provide $20.00
t h e mathematical power t h a t would h a v e c o s t over a million
dollars i n the
l a t e 1950s.
They c o n t r o l and program
factory m a c h i n e s , h o m e kitchen
equipment, toys and games.
Along with this revolution their sensitivity t o transient charges of
electricity h a s risen i n about the s a m e proportion.
circuitry i s
a b o u t a million times a s easily destroyed by a n EMP a s the older vacuum
T h i s sensitivity may be expected to rise steadily along with growth
of the r o l e played by electronics.
T h e EMP Phenomenon
T h e gamma ray flash from a high-altitude nuclear
explosion produces a
spherical w a v e front. T h e portion of this sphere expanding toward t h e Earth
impacts t h e upper a t m o s p h e r e tearing l o o s e a heavy c h a r g e of electrons.
f r e e electrons a r e immediately captured by the Earth's magnetic field.
dissipate their velocity by spinning around t h e magnetic
field l i n e s ,
converting their energy i n t o radio waves.
The--net e f f e c t i s a microsecond
burst of i n t e n s e , broad frequency radio wave energy, which i s o n e form of a n
Electromagnetic P u l s e o r EMP.
Since t h e EMP contains a broad
spectrum of f r e q u e n c i e s , a n electrical
conductor o f a n y length c a n serve a s a n antenna.
T h e heaviest
jolt i s
received from the electrical service lines.
H o w e v e r , wiring internal to
electronic equipment a l s o receives a n d conducts t h e s u r g e directly i n t o t h e
T h e disruptive capability of t h e EMP i s due more to i t s s h a p e than to t h e
total energy i t contains.
T h e duration of the pulse i s a b o u t a millionth
a second (a microsecond) but the peak power density i s nearly i n s t a n t a n e o u s
-- a f e w billionths of a second (about 10 nanoseconds).
T h e t o t a l energy
received from a nominal one-megaton burst a b o v e t h e a t m o s p h e r e i s small
a b o u t half a joule per
T h e peak
power - - d e n s i t y , however,
T h e onset of t h e peak power density i s
reaches 6 megawatts per square meter.
a hundred t i m e s more
rapid than t h a t of lightning.
AS a c o n s e q u e n c e ,
conventional lightning s u r g e protective devices a r e n o t effective against t h e
Any electrical conductor may serve a s a n antenna f o r t h e pulse, even short
lengths of internal wire.
All electronic circuitry more advanced
using vacuum tubes may be affected by
T h i s may
communication networks, transmitters a n d receivers, and a l l telephone systems
except those i n which g l a s s fiber has been substituted for wire.
may be destroyed a n d magnetic tapes and discs erased. Machines a n d vehicles
which employ electronic modules
in their operation would be disabled.
Electronic instruments such a s a i r c r a f t NAVCOM a n d f l i g n t control systems may
Satellites exposed to the expanding spherical wave f r o n t of gamma and
x-rays could a l s o r e c e i v e a crippling or destructive EMP.
T h e radiation
could f r e e electrons from the metal
the s a t e l l i t e , creating a n
electric charge. T h e intensity of the charge could reach a million volts per
T h e electronic payload would then r e c e i v e large induced c u r r e n t s , not
EMP o r
unlike a lightning strike. T h i s effect i s called a system-generated
Protection or hardening against SGEMP presents a problem
entirely solved. E v e n a double walled satellite would not be i m m u n e to a
pulse strong enough to induce a charge i n the i n n e r shell.
A single nuclear
warhead could Conceivably disable a l l satellites n o t shielded by
T h i s would include s y n c h r o n o u s , a s well a s l o w a l t i t u d e civil and military
Defensive systems such a s laser battle
stations and o f f e n s i v e
s y s t e m s such a s k i l l e r satellites o r enroute missiles could a l s o be disabled
or damaged by the SGEMP.
T h e E f f e c t of EMP
A nuclear explosion 1 5 0 miles a b o v e the geographic center of the country
i s t o o distant to i n f l i c t direct damage or threaten life.
H o w e v e r , a t this
a l t i t u d e , the expanding sphere of gamma radiation would excite a l e n s shaped
segment of the upper atmosphere several thousand square miles in area.
resulting EMP would cover most of the United States.
Large-scale power blackouts could occur. P o w e r plants could automatically
shut down a s the pulse triggered fault sensors. T h e power distribution grid
would then be imbalanced causing many more generating facilities to s h u t d o w n
i n response to s u r g e s and overloads.
However, with n o a d d i t i o n a l EMP's,
power distribution could be restored i n a matter of hours o r a day.
More serious would be the destruction of electronic circuitry.
On l i n e
equipment would r e c e i v e t h e EMP through the electrical service connection.
New equipment i n s t o r e s or warehouses would r e c e i v e the pulse through w i r e
l e a d s which would s e r v e a s antennae.
In brief the main effects of t h e EMP would be t e m p o r a r y , large-scale power
l o s s e s followed by
dependent banking f u n c t i o n s , a i r f l e e t o p e r a t i o n s , and s u r f a c e transportation
s y s t e m s could a l l b e affected.
T h e r e a r e concerns that d e f e n s i v e and
retaliatory capability a s well a s the operation . o f military communication
systems could a l s o be disrupted.
Protection from t h e EMP
T h e word "harden," borrowed from the military, has'come to mean "protect."
For example, a computer which has been "hardened" i s shielded o r o t h e r w i s e
protected against t h e EMP.
Protection from the pulse i s s i m p l e in principle.
C o n d u c t o r s such a s power cables or a n t e n n a l e a d s may
be fitted with
a r r e s t o r s or filters designed specifically for the fast r i s e t i m e of
pulse. Other than t h i s , the equipment may
either encased i n metal
Faraday Shield) or simply taken d e e p underground.
New e q u i p m e n t , s t o r e s , a n d
spares could b e protected by t h e use of a continuous metal c o v e r i n g , possibly
a f o i l , applied a s part of t h e packaging process.
S o m e protection for large open networks such a s electrical power a n d
telephone g r i d s could be achieved by replacing the systems n o w i n u s e with
underground networks or by
the networks a b o v e ground.
procedures a r e generally considered too costly for practical
Advancing technology may eventually reduce t h e EMP a s a threat to the
telephone a s copper conductors a r e replaced by o p t i c a l fibers.
optical f i b e r systems have terminals and repeaters which must a l s o b e
Military hardening began in aircraft strategically i m p o r t a n t i n conducting
a retaliatory strike or serving a s a n a e r i a l command post.
most aircraft i n the Strategic Air Command.
Unhardened aircraft in which t h e
pilot's i n p u t to the control surfaces i s electronic
disabled by t h e EMP, but there a r e only a f e w of these.
Further hardening i s n o w underway to protect
surface installations and
military communication networks.
Continuity of electric power
i n missile
launching f a c i l i t i e s i s being assured by the installation of f u e l c e l l s a s
well a s auxiliary generators.
conventional telephone networks for l o c a l communication.
Information o n the
hardening s t a t u s of long distance military
communication networks i s n o t
Since t o t a l hardening of civil and military resources against EMP
i s not
possible, d e c i s i o n s on protection must weigh
threat a s s e s s m e n t , c o s t , a n d
In the c a s e of the military, t h e deterrent value of a n i m m u n e
retaliatory f o r c e i s a l s o a consideration.
1. T h e n u m b e r of space-capable l a u n c h e r s increases with time and some may
become commercially available during this decade.
Can a threat a s s e s s m e n t be
made of t h e prospect of a hostile nation or g r o u p inflicting a n EMP i n the
manner of a terrorist blow, not a s a n opening t o war?
2. Should governing bodies be encouraged to protect their o w n computer
capabilities a n d magnetically recorded data? Should f i n a n c i a l institutions
and communication .networks?
S h o u l d systems of protective
developed a n d their use encouraged?
4. Should industrial a n d public educational programs be
encourage hardening of electronic circuitry i n vehicles,
and possibly i n t h e home?
5. When a power plant generator i s s t a r t e d , it will
n o t produce current
until a n o u t s i d e electrical source i s applied to i n i t i a t e the f o r m a t i o n of
i t s magnetic field. Once formed the generator sustains i t s own field.
means that i n a total shut d o w n , the plant remains off-line until electricity
can be brought i n from a n o t h e r plant o r generator.
H a v e utilities made
adequate provision to reduce down time in the event of widespread
shut-downs (such a s by keeping small auxiliary generators on hand)?
6. Should the replacement of copper telephone lines with cables of optical
fiber, at least over trunk l i n e s , be encouraged or somehow accelerated?
7. To what extent should orbiting equipment, civil a s , w e l l a s military, be
protected against SGEMP?
ADDITIONAL REFERENCE SOURCES
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Nuclear pulse (11) ensuring the delivery of the
Science, June 5 , 1981: 1116.
Nuclear pulse (111) playing a wild card.
June 1 2 , 1981: 1248.
a sleeping electronic dragon.
news, May 9 , 1981: 300.
Lemer, E. J.
IEEE s p e c t r u m , May 1981: 41.
Longmire, C. L.
On the electromagnetic pulse produced by
IEEE transactions on electromagnetic
compatibiiity, v. EMC-20, no. 1 , Feb. 1978: 3.
Dept. of Energy.
Feasibility of isolating vulnerable
equipment of the electric power s y s t e ~ .from sources of
Glasstone, Samuel and Philip Dolan.
T h e effects of nuclear
Chapter 1 1 , the electromagnetic pulse and its effects:
Third edition. Washington, U.S. Govt. Print. Off., 1977.