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National Missile Defenses (Åñòü ïîñòñêðèïò ñ êàðòèíêàìè)
The Continuing Debate on National
Missile Defenses
Concerns about "emerging missile states" have spurred development of a system to
defend the US from small-scale ballistic missile attacks. But the planned system could
be compromised by simple countermeasures, and the security costs of deployment
could be high.
Lisbeth Gronlund, George N. Lewis, and David C. Wright
Over the past several years, the Clinton administration has developed a national
missile defense (NMD) system, the first phase of which could be operational
sometime in the second half of this decade. President Clinton was scheduled to
decide this fall whether to begin deployment of the system next spring; on 1
September he announced that he did not have "enough confidence in the technology,
and the operational effectiveness of the entire NMD system, to move forward to
deployment," and would leave that decision to his successor. Whether or not to go
forward with deployment of this system--or any other one, perhaps based on
different technologies--will be a central policy concern for the next administration.
The proposed NMD system is intended to defend all 50 states from small-scale
attacks (up to several tens of missiles) by intercontinental-range ballistic missiles
(ICBMs) armed with nuclear, chemical, or biological warheads. (See Physics
Today, June 2000, page 46.) NMD supporters argue that such a system is needed
to deal with the potential future threat from emerging missile states such as North
Korea and Iran and to guard against an accidental or unauthorized Russian missile
launch. Some also argue that it is needed to counter the Chinese missile arsenal.
Opponents say that it will not work against a real-world attack, that it will interfere
with USRussian nuclear arms reductions and provoke adverse reactions from
China, and that it will complicate efforts to limit the proliferation of ballistic missiles
and weapons of mass destruction, consequently reducing US security.
This is the third time that NMD has become a national controversy, and physicists
have been involved from the beginning. Not only did physicists help develop the
missile defense technologies, but they have played a key role in past debates by
pointing out technical limitations in the proposed systems. Through their involvement
in these debates, scientists--both in and outside the government--have helped avoid
the deployment of expensive, counterproductive, and ineffective NMD systems.
Particularly in the current debate and in the earlier one over the Strategic Defense
Initiative (SDI) in the 1980s, independent scientists have been able to provide peer
review of military and government experts, who would otherwise have been the only
authoritative voices. The involvement of independent experts has also helped prevent
security classification from limiting public debate on these issues.
In the current debate, the key technical question remains the same: Will the
proposed NMD system work against an attack when realistic and practical steps
are taken to defeat it?
Past debates
During the late 1950s and the 1960s, both the US and the Soviet Union worked
vigorously to develop NMD systems--then called anti-ballistic missile (ABM)
systems--that used nuclear-tipped defensive missiles to intercept incoming ICBMs.
The first debate in the United States over ABM systems occured largely out of
public view. In the early 1960s, government scientists (such as Herbert York and
Jerome Wiesner) and other insiders were able to prevent deployment of an ABM
system based on the NikeZeus interceptor. In the mid-1960s, the Soviet Union
began building a missile defense system around Moscow, a version of which remains
in place today. In 1967, the US announced that it would begin deploying a
nationwide antimissile system, then called Sentinel. However, in 1969, the incoming
Nixon administration renamed the program Safeguard and restructured it to
emphasize defense of missile silos and bomber bases. A single Safeguard site was
eventually constructed, but it was shut down within a few months of becoming
operational.
Physicists played a key role in the very public debates over deployment of the
Sentinel and Safeguard systems. Particularly influential was the 1968 Scientific
American article by Richard Garwin and Hans Bethe,1 in which they discussed
countermeasures such as decoys, chaff (to confuse radars), and the use of nuclear
detonations to black out defense system sensors. US scientists also influenced
Soviet thinking by discussing the technical and arms control issues with their Soviet
colleagues at Pugwash and other international meetings. And scientists, including a
group of physicists at the Argonne National Laboratory, played a high-profile role in
public protests against deployment of Sentinel's nuclear-tipped interceptors near
cities.2
In 1972, the US and Soviet Union signed the ABM Treaty, which prohibits the
deployment of nationwide missile defenses. The treaty reflected the shared
understanding that building more missiles would be a straightforward and relatively
inexpensive way to overwhelm a defense, and therefore deployment of an NMD
system would block reductions in nuclear weapons and could even lead to an
offensedefense arms race.
The signing of the ABM Treaty effectively removed NMD from the national scene
for the next decade. Then, in March 1983, President Ronald Reagan announced the
SDI program. As initially conceived, the SDI was to create an impenetrable shield
against a massive Soviet attack, using multiple layers of ground- and space-based
weapons, some of which relied on technology that was yet to be developed. SDI
inspired a vigorous public debate, from which a consensus emerged that its
ambitious early goals were unachievable on both technical and financial grounds.
Particularly influential was a study by the American Physical Society (APS) on
directed energy weapons,3 which found that the advanced lasers and charged
particle beams that were a centerpiece of the SDI program were so far from
realization that their feasibility could not even be assessed until after many more
years of research.
SDI's objectives were gradually scaled back, first to enhancing deterrence and then
to countering small-scale attacks. By 1993, when President Clinton took office,
NMD had once again faded from national attention, with efforts focused solely on
technology development. US missile defense activity shifted instead toward theater
missile defenses, which gained support after the Iraqi Scud missile attacks on Israel
and Saudi Arabia during the 1991 Gulf War.
NMD returned to national attention in 1995, when the newly elected Republican
majority in Congress pushed for NMD deployment. The Clinton administration
initially opposed these efforts, arguing that the threat did not justify deployment.
However, continuing congressional pressure led the administration to announce an
NMD development program in 1996, with the goal of designing a system by 2000
that could be deployed by 2003.
Two events in 1998 gave greater political urgency to NMD development efforts.
The first was publication in July 1998 of the report4 of the congressionally chartered
Commission to Assess the Ballistic Missile Threat to the US (called the Rumsfeld
Report after the commission's chair, former Secretary of Defense Donald Rumsfeld).
The report concluded that a threat of attack from long-range missiles could emerge
with little warning, and such a threat could emerge sooner than earlier estimates had
suggested. The second event was the August 1998 launch by North Korea of a
rocket that overflew Japan. In January 1999, the administration announced it would
make an NMD deployment decision in the summer of 2000. But the target
deployment date of 2003 no longer seemed feasible, and was postponed to 2005.
The proposed NMD system
There are many ways an NMD system could be constructed. It could use
interceptor missiles tipped with conventional explosives or nuclear warheads (as was
Safeguard) or equipped with homing "kill vehicles" that destroy an incoming warhead
by directly colliding with it. It could use directed-energy weapons like those
envisioned for the SDI program, including the space-based laser still under
development. Interceptors or directed-energy weapons could be based on the
ground, at sea, in the air, or in space. For target acquisition, tracking, and
discrimination, an NMD system could use radar, infrared, or visible-light sensors. It
could attempt to intercept missiles in their boost phase or to intercept warheads
either as they coast through space (midcourse phase) or as they begin to reenter the
atmosphere (terminal phase). These many possibilities cannot be fully assessed in the
abstract; each possible approach raises unique political and technical issues that
depend on the details of the system design. However, because the Clinton
administration has laid out in detail how its proposed NMD system would
operate,5,6 it is both possible and appropriate to subject this system to technical
review.
The centerpiece of the planned NMD system is the
"exoatmospheric kill vehicle" (EKV), shown in
figure 1, which would be launched by the
interceptor missile. The EKV is designed to destroy
an incoming warhead by colliding with it above the
atmosphere during the midcourse phase of the
warhead's trajectory, as shown schematically in
figure 2. Using a passive infrared seeker (supported by a visible-light sensor that is
not currently configured for homing), the EKV would fire small thruster rockets to
maneuver into the warhead's path. To increase the probability of an intercept, the
defense could launch multiple EKVs at each incoming warhead.
For detection and tracking, the NMD system will
incorporate two types of ground-based sensors:
X-band phased-array tracking radars and
upgraded early warning radars. The X-band radar
was developed specifically for the NMD system; it
is named for its operating frequency range (around
10 GHz). High operating frequency gives X-band
radars the high resolution required for precise
tracking and for discrimination of incoming
warheads from other objects. The US presently has early warning radars deployed
at five locations worldwide; these have much lower resolution and thus less ability to
discriminate, but could be given software upgrades that would enable them to guide
NMD interceptors.
The deployment plan for the NMD system is given in the table on page 39. Initial
deployment would include 100 interceptors in central Alaska and one X-band radar
on Shemya, a small island at the far western end of the Alaskan Aleutians that is well
situated for observing missiles launched from North Korea or China. Coverage of
other avenues of attack would be provided by upgrades of the existing early warning
radars in central Alaska, California, Massachusetts, Greenland, and the UK. The
initial deployment is intended to protect all 50 states against a few tens of simple
warheads--with either no countermeasures or ineffective countermeasures--from
North Korea or about five simple warheads from the Middle East. (Initial system
capabilities are limited by lack of X-band radar coverage for the Middle East.) The
full NMD system, to be deployed after 2010, would include eight additional X-band
radars and 250 interceptors at two sites, one in Alaska and one in North Dakota.
The full system is meant to defend all 50 states against attacks involving a few tens of
missiles with "complex" countermeasures. (There is no publicly available definition of
what "complex" means.)
The full NMD configuration would also include a system of observation satellites that
would track suspected warheads and help discriminate them from other objects.
This Space-Based Infrared SystemLow Earth Orbit (SBIRSLow) would consist
of some 24 satellites with infrared and visible-light sensors that are intended to
follow missile targets throughout their entire flight with sufficient accuracy to guide
interceptors.
Countermeasures
The planned NMD system involves many technological and engineering challenges,
as reflected by the interceptor test record to date: In two of the three tests, the
interceptor has failed to hit its target.7 Still, it seems reasonable to assume that, with
enough time and effort, NMD interceptors can be made to reliably hit cooperative
targets on the test range. However, clearly this hurdle does not mean the system
would work in the real world, against a realistic threat. Ultimately, the effectiveness
of the NMD system will depend primarily on its ability to deal with the
countermeasures that an attacker might take to defeat the defense.
It may seem surprising that an emerging missile state such as North Korea could
overcome the huge technological and financial advantages of the US to successfully
counter an NMD system. However, countermeasures require far simpler technology
than does a missile defense. Furthermore, an attacker has inherent advantages in
confronting any NMD system. An attacker can design countermeasures to defeat a
specific system, because the defender must commit in advance to a specific
architecture, the construction of which will take many years. In contrast, the
defender may know little about the countermeasures of potential attackers, because
countermeasures programs are easily concealed. In addition, an attacker with limited
objectives, such as an emerging missile state, needs only a low level of effectiveness
to pose a credible threat, whereas any useful NMD system must achieve very high
levels of effectiveness and reliability.
The technical issue of countermeasures is thus at the heart of the NMD debate.
Given the expressed goals of the NMD system, the key questions to be addressed
are 1) What kinds of countermeasures could an emerging missile state deploy and
when could it deploy them? 2) How effective would such countermeasures be
against the planned NMD system?
The Rumsfeld Report, in its analysis of the missile threat, employed two principles
that might also be applied to an assessment of possible countermeasure capabilities
in emerging missile states. First, the absence of evidence is not evidence of
absence--that is, a failure to detect such programs does not necessarily mean that
they don't exist. Second, because such programs could exist without being
observed, any threat analysis should include consideration of what systems a
potential adversary is capable of building, based on its technical capabilities.
Subsequent to the Rumsfeld Report, a September 1999 consensus report of the US
intelligence agencies8 stated that emerging missile states could use "readily available"
technology to develop countermeasures and could do so "by the time they flight test
their missiles." The report listed a number of such countermeasure technologies.
However, this important statement has had limited effect on the debate because the
report contained few details.
The issue of countermeasures was addressed in detail by a recent panel formed for
this purpose by the Union of Concerned Scientists (UCS) and the Security Studies
Program at MIT. The panel, in which we participated, was chaired by APS past
president Andrew Sessler and included 11 physicists and engineers, some with
direct experience in ballistic missile defense and countermeasures issues. The panel
assessed the effectiveness of countermeasures against an ideal version of the planned
NMD system--one limited only by the laws of physics--thereby sidestepping the
difficulty of obtaining the often-classified details of real-system performance. In its
April 2000 report,9 the panel evaluated three particular countermeasures that could
be implemented by an attacker with limited technical resources and could possibly
defeat the planned NMD system:
Biological weapons in submunitions. A lethal biological agent delivered by
a ballistic missile could be divided into 100
or more small bomblets, or submunitions,
which would be released shortly after the
boost phase. This strategy would overwhelm
the planned NMD system with far too many
targets to intercept. It would also be an
effective way of dispersing the agent over a
wide area, and so would likely be adopted
regardless of NMD concerns. In its analysis,
the panel found that such technical issues as dispersal of the bomblets, reentry
heating, and the release of the biological agent did not present serious
difficulties. (See figure 3.)
Nuclear warheads with antisimulation balloon decoys. A nuclear
warhead could be hidden within an
aluminum-coated mylar balloon and released
together with a large number of empty
balloons, as illustrated in figure 4. Such
"antisimulation"--making a warhead look like
a decoy--could be easier and more effective
than making decoys look like warheads. The
technique is particularly useful against the
exoatmospheric interceptors planned for the
NMD system: Because light and heavy objects travel on the same trajectories
above the atmosphere, large numbers of effective decoys could be added to a
missile without a prohibitive weight penalty. Simple techniques can be used to
deny the defense system sensors any distinguishing physical signal that would
show which balloon contains a warhead. For example, balloons could be
given slightly different temperatures, either passively, by using surface coatings
with different emissivities (figure 5), or actively, by using small
battery-powered heaters.
Nuclear warheads with cooled shrouds. An attacker could enclose a
nuclear warhead within a double-walled
cone containing liquid nitrogen to hide it from
the EKV's infrared sensors (see figure 6).
Cooling the outer cone to 77 K would
reduce the infrared radiation emitted by the
shrouded warhead by a factor of at least a
million. While the shrouded warhead would
still be seen by the NMD system's X-band
radars, the kill vehicle would be unable to
detect the warhead in time to maneuver to hit
it.
The panel concluded that these three countermeasures could either completely
defeat or seriously degrade the effectiveness of the planned NMD system.
Furthermore, it found that such countermeasures were within the capability of
emerging missile states such as North Korea or Iran, and could be built by the time
the planned NMD system was operational.
Critics of the UCS/MIT countermeasures report say that the study underestimates
the difficulty of developing and deploying countermeasures and that an emerging
missile state will take many years to do so. They also argue that
counter-countermeasures could eventually be developed by the US. However, a
counter-countermeasure program would require extensive development and flight
testing, giving potential attackers the opportunity to adjust their countermeasure
strategies. For the same reasons discussed earlier, the attacker would have a good
chance of maintaining its ability to defeat the defense.
Resolving the debate
The dispute over operational effectiveness in the face of countermeasures has led to
a refocusing of the NMD debate. In the past nine months or so, the technical
feasibility of the system has been widely questioned for the first time. But how can
the issue of real-world effectiveness of the NMD system be resolved in the
politically charged and divisive setting of the current deployment debate?
One solution would be to rely on objective and realistic testing, as advocated by the
statement on NMD adopted on 29 April 2000 by the APS.10 The statement begins,
"The United States should not make a deployment decision relative to the planned
National Missile Defense (NMD) system unless that system is shown--through
analysis and through intercept tests--to be effective against the types of offensive
countermeasures that an attacker could reasonably be expected to deploy with its
long-range missiles." However, there is no agreement--and even less clarity--on
what the role of testing should be in the NMD program, what degree of confidence
in the effectiveness of the NMD system should be required, or what degree of
confidence is achievable under a realistic testing program.
The planned NMD system is, in fact, being tested
against countermeasures, but these test
countermeasures are quite different from the
real-world countermeasures just discussed. The
three intercept tests to date have each included two
potential targets, a mock conical warhead and a
large spherical balloon decoy, as shown in figure 7.
For these targets, warheaddecoy discrimination is
not difficult because the test decoy is very different
from the mock warhead in physical appearance, infrared signature, and radar cross
section. Furthermore, the defense had complete advance knowledge of the
characteristics of both warhead and decoy. None of the 19 intercept tests planned
before the first phase of the NMD system would be deployed will include warheads
that are in any way disguised.
One way to ensure that tests are done against realistic countermeasures would be to
establish an independent group (a so-called "red team") to develop, build, and test
countermeasures using only technology available to emerging missile states. Such a
program might help answer the contentious questions of what countermeasures
emerging missile states could reasonably be expected to deploy and when they
could do so. There is a precedent for such an effort: The Ballistic Missile Defense
Organization (BMDO), the successor to the SDI program office, oversees a similar
countermeasure prototype program, the Countermeasures Hands-On Program
(CHOP), for theater missile defenses. The program involves young scientists,
engineers, and military officers not specifically trained in missile defense or
countermeasures who are given access only to the open literature and commercial
off-the-shelf technology. The CHOP program is not oriented to the NMD system,
and it is not independent (its funding, staff, and direction are under BMDO control),
but CHOP shows that this kind of activity can be done.
After countermeasures are developed by such a program, the planned NMD system
needs to be tested against them. It is important, too, that there be independent
oversight of the NMD testing program.
Although the APS position of advocating no deployment decision until the NMD
system has been realistically tested may seem like common sense--and is standard
fare for other weapons programs--the present plan is for NMD deployment