Why National Missile Defence Won't Work
By George N. Lewis, Theodore A. Postol and John Pike
(From Scientific American pages 36-41, August 1999)

HIT-TO-KILL INTERCEPTORS are the hallmark of the national missile defense system currently being developed to protect U.S. soil from intercontinental ballistic missiles. The duel between the warhead and the maneuverable kill vehicle (foreground), which is released from the tip of the interceptor missile, would occur high above the earth's atmosphere. The kill vehicle's success would rely on hitting the target dead-on, and critics argue that countermeasures, such as hiding the warhead inside one of a cluster of metal-coated balloons, would likely confuse the vehicle's homing sensors, making a direct hit nearly impossible.

In 1968, with the threat of inter-continental ballistic-missile attacks driving the U.S. toward the development of a national missile defense system, a Scientific American article written by physicists Richard L. Garwin and Hans A. Bethe described how China or the Soviet Union could easily elude the "light" U.S. missile shield then under development [see "Anti-Ballistic-Missile Systems," March 1968]. This argument - that any national defense system would be technologically ineffective - was one reason the U.S. and the Soviet Union signed the Anti-Ballistic Missile (ABM) Treaty in 1972. The fear that such a system would provoke the Soviet Union and escalate the arms race also contributed to the U.S. decision to sign the treaty, considered a landmark of arms control. To this day, the treaty prohibits the U.S. and Russia from deploying nationwide defense systems.

More than 30 years later the U.S. remains without a national missile defense system. The cold war threat of massive Soviet missile strikes has abated, but ballistic-missile technology is rapidly proliferating. U.S. concerns now center on the possibility that a rogue developing state could eventually acquire the ability to threaten or strike the U.S. with long-range missiles. Accidental Russian launches and China's small but potent missile force are considered secondary threats.

Missile defense technology has also advanced. More powerful computers and improved radars and other sensors appear to have created an alternative to the nuclear-tipped interceptors envisioned in the 1960s. These advances offer the possibility that the U.S. could use more politically acceptable "hit-to-kill" missiles designed to destroy their targets by direct high-speed collisions.

Advocates of national missile defenses argue that this combination of missile threats and improved technology makes possible the deployment of an effective "homeland" missile shield, and their efforts to bring this about are bearing fruit. Since taking over the leader-ship of the U.S. Congress in 1994, Republican lawmakers have relentlessly pushed the White House to commit to deployment, and the administration in 1996 announced it would begin to develop a system capable of covering the entire U.S., although it did not name a deployment date.

Intercept failures
INTERCEPT FAILURES have plagued high-altitude, hit-to-kill technology since the first test in 1983. With less than a year until officials decide whether the national defense system is ready, only three of the first 17 intercept tests have hit their targets.

Missile Threats

This fall the Pentagon plans to employ the key components of its national defense in the first test of the system's ability to intercept a long-range missile outside the earth's atmosphere. In June 2000, after only a handful of additional tests, the administration plans to decide whether the technology is ready; if so, a national defense system could be in place by 2003, although the administration says 2005 is a more realistic date.

Whatever the outcome of the June 2000 "deployment readiness review," the U.S. seems more likely than ever to commit to a national missile defense within the next few years. In 1998 then undersecretary of defense for acquisition and technology Jacques Gansler told Congress that the question is no longer whether the U.S. will deploy a national defense, but when. And since then, deployment has become more likely than ever: in January the Pentagon announced the addition of $6.6 billion to future defense budgets for building a national defense, and in March the administration withdrew its opposition to Senate legislation mandating deployment "as soon as technologically feasible." The bill soon passed by a wide margin.

Like the "Safeguard" system Garwin and Bethe analyzed in 1968, however the national missile defense now under consideration could be readily defeated by simple offensive countermeasures. In fact, a system based on hit-to-kill interceptors is more vulnerable to counter-measures than one involving nuclear missiles. Moreover as was feared more than 30 years ago, its deployment is likely to provoke other countries to take actions that lessen U.S. security.

Many more nations possess ballistic missiles today than in 1968. Most of these, however are known as "theater" ballistic missiles because of their shorter range, and no theater missiles are positioned to strike the U.S. What is more, most of the countries possessing these missiles are not hostile to the U.S. Short-range missiles can be used primarily against allies' cities and U.S. forces over-seas, and the U.S. is developing several theater defense systems to defeat them [see below].

Dangers Close at Hand
by Daniel G. Dupont

Debates over missile defenses usually center on national, or "homeland," systems designed to protect the U.S. from inter-continental ballistic missiles. The U.S. is also developing a handful of "theater" systems intended to safeguard troops and assets in 5 other countries from missiles with shorter ranges of 30 to 3,000 kilometers (19 to 1,864 miles). Theater defense is generally considered easier to achieve than national defense because it requires protecting a smaller area from slower missiles. But even shorter-range systems are vulnerable to countermeasures similar to those that make homeland defense tricky. And the U.S. test record of theater defense systems shows that hitting one missile with another missile is far from easy.

The most prominent theater system is the army's Patriot, originally designed to shoot down aircraft and first used in the Persian Gulf War to battle Iraqi Scud missiles. The first generation Patriot - the only theater defense system ever called on in combat - intended to destroy or deflect incoming missiles by exploding an interceptor nearby.

The army claims a 60 percent success rate, but critics counter that the Patriot failed in all its Scud engagements even though the enemy warheads employed no obvious counter-measures. The Patriot system is now being upgraded with a new missile that uses the same "hit-to-kill" concept as the national defense system.

The army's Theater High Altitude Area Defense system, or THAAD, is projected to be the most versatile and sophisticated hit-to-kill system in use. Although it remains less developed than the Patriot, THAAD is intended to intercept the longest-range theater ballistic missiles, both inside and outside the atmosphere. Yet in its short history, THAAD has shown more than any other system the difficulties of developing effective missile defenses: in its first seven intercept tests,which started in 1995, THAAD hit only a single target missile.

Similar in design to THAAD is the navy's Theater Wide system. The navy plans to deploy ships with long-range missile interceptors near countries in which ballistic missiles threaten U.S. troops or allies' cities.The navy is also working on a shorter-range ship-based system known as Area Defense. Less developed programs include a theater defense system that will move with troops on the battlefield.

Beyond the controversial hit-to-kill interceptor technology are laser weapons. The air force's current missile defense plans include the Airborne Laser, which is mounted on a Boeing 747 and designed to intercept ballistic missiles early in flight. The air force is also developing a space-based laser that could one day intercept missiles as their booster rockets propel them into space.

DANIEL G.DUPONT edits the independent newsletter Inside the Pentagon in Washington, D.C.

Unexpected Maneuvers
UNEXPECTED MANEUVERS and breakups of Iraqi Scud missiles in the Persian Gulf War (pictured) thwarted the U.S. Patriot missiles' ability to destroy them. Before the war, Patriot (top) was successful in all tests was against ballistic-missile targets, which flew on stable, smooth trajectories.

Theater ballistic missiles are a far cry from those that could strike the U.S. The latter are known as intercontinental ballistic missiles (ICBMs), and they have always carried nuclear weapons. The U.S. fears that they may one day be armed with other "weapons of mass destruction " - munitions containing deadly chemicals or biological agents. The U.S. national missile defense system is being developed to intercept such ICBMs.

Russia possesses the largest number of such missiles, but advocates of a limited national defense argue that a large Russian attack on the U.S. is highly improbable. The U.S. system is therefore being designed to combat only a handful of ICBMs at a time.

The most commonly cited justification for national missile defense is that ICBMs might be built or obtained by a rogue state, which in the vernacular of the Defense Department could mean Iran, Iraq or North Korea. In July 1998 a commission of experts led by former secretary of defense Donald Rumsfeld concluded that North Korea or Iran could, with little warning, develop an ICBM within five years of deciding to do so. This finding gave a significant boost to national missile defense proponents and was a factor in the administration's decision to add billions of dollars to the Pentagon's budget for the first phases of deployment. Other factors included North Korea's August 1998 launch of a three-stage missile known as the Taepo-dong 1 and reports that a longer-range Taepo-dong 2 missile is being developed. If these reports prove correct, North Korea might one day be able to use the Taepo-dong 2 to strike Alaska or might be able to modify it to deliver small payloads to other parts of the U.S. [see map below].

A POTENTIAL MISSILE ATTACK by "rogue" nations, such as North Korea, is a driving force behind current U.S. defense plans. In 1998 North Korea flight-tested its Taepo-dong 1 missile, which could presumably haul a 1,000-kilogram nuclear bomb about 2,500 kilometers. The same missile might carry a lighter biological or chemical warhead 4,100 kilometers - just shy of the two closest parts of the U.S. (The tip of Alaska's Aleutian Islands and the western end of the Hawaiian Islands lie about 4,500 kilometers away.) The untested Taep-dong 2 missile is thought to have a range of up to 6,000 kilometers. North Korea could also launch shorter-range missiles from ships-a tactic that would render the current U.S. defense plan worthless.

A secondary justification for a limited national defense is the possibility of an accidental or unauthorized Russian missile launch, which might involve only one or a few warheads. Because of the way in which Russia's nuclear missile forces are organized, however; a break in Russia's chain of command would more likely involve all the warheads of a ballistic missile submarine - up to 200 - or a large part of Russia's ICBM force. And proponents say that China, which possesses no more than two dozen ICBMs capable of reaching the U.S., also provides a justification for a limited national defense.

Designing a Defense

The particulars of the U.S. national missile defense system are not yet fully decided, but most key components are well along in development, and the general details of how it would operate are well known. An ICBM fired at the U.S. would be detected first by infrared early-warning satellites and then by one or more of the large phased-array early-warning radars, which are positioned in Massachusetts, California, Alaska, Britain and Greenland. These radars operate at relatively low frequencies, and although their range and angle resolution are poor, they can provide trajectory data on a small number of well-separated ballistic targets.

Data on the missile's path from satellites and early-warning radars would be used to cue the primary sensor of the national defense system, the ground-based radar; enabling it to increase its detection range by concentrating its search for the missile on a smaller area. This X-band phased-array radar is designed to provide long-range detection and tracking of ballistic-missile targets. A prototype is al-ready in use at the U.S. Army's Kwajalein Atoll missile range in the Pacific.

The radar and sensor data would then be passed on to a battle management center which would determine possible intercept points and issue launch and guidance commands to a ground-based interceptor missile. Each interceptor consists of a rocket booster and what is known as an exoatrnospheric kill vehicle, which does the intercepting in space once it separates from the booster stack.

To maximize the defended area and the number of opportunities to strike the incoming missile, the interceptor would have to be launched soon after an attack was detected. Extremely fast, with a burnout speed in excess of seven kilometers per second (about four miles per second), the interceptor would receive guidance updates during flight based on data from various sensors. To increase the probability of destroying a target, several interceptors could be fired at a single missile. Current plans call for up to 100 interceptors at a single site.

The kill vehicle is designed to intercept incoming warheads well above the earth's atmosphere. (Enemy missiles are launched from too far away for this system to intercept them earlier.) Using its own infrared seeker and data from the ground-based radar and other sensors, the kill vehicle would attempt to discriminate between the attacking war-head and any missile debris or decoys employed to confuse it. It would then use thrusters to maneuver into a high-speed collision with the warhead. Ideally, an intercept would totally destroy both kill vehicle and target.

Several new or improved sensors would also play key roles in an expanded national defense. Existing early-warning radars will be enhanced so they can better track targets and guide interceptors. New X-band phased-array radars, similar to the main ground-based radar will be installed, some of them along-side the early-warning radars. Finally, a space-based missile-tracking system known as SBIRS-Low (space-based infrared system-low earth orbit) is in the works. This satellite system, formerly called Brilliant Eyes, is designed to track missiles and their warheads from early in flight using short-, medium-and long-wavelength infrared sensors as well as visible-light sensors.

According to a recent U.S. General Accounting Office estimate, the deployment and operation of a limited national defense system would cost between $18 billion and $28 billion. But costs are likely to exceed these estimates, and the program's schedule is optimistic when compared with those of past major weapon systems. The administration's planned defensive system is also designed to he expandable; once in place it is likely to he augmented with more interceptors or launch sites, which would increase the system's capability and cost.

The U.S. success rate in tests of high-altitude hit-to-kill systems is dismal, with only three successes in the first 17 tries. This test record illustrates the challenge of hit-to-kill intercepts and suggests that the technology is not yet ready for use. Yet even if all the tests had been successful, they would not have established that the defense would work in the real world. Why? Consider the Patriot missile system, the only missile defense weapon ever used in combat. Patriot, a theater defense system, had a perfect test record before the Persian Gulf War in 1991, with 17 successes in 17 intercept tests. Yet contrary to most media reports, it failed in most or all 44 of its attempts to destroy Iraqi Scud missiles, which behaved differently from test-range targets.

Beating the System

Assuming its basic components can be made to work, the real-world effectiveness of the national missile defense system will depend primarily on its ability to cope with similar unexpected circumstances and, in particular with measures taken by adversaries to defeat it Isee box]. One way to foil the system would be to launch enough ICBMs to overwhelm it. A less expensive and more feasible option would be to devote some of each missile's payload to lightweight countermeasures designed to confound defensive missile systems.

From the beginning, the U.S. has developed countermeasures that can be used with its strategic missiles and any country capable of producing or obtaining both ICBMs and weapons of mass destruction would be able to produce or obtain effective countermeasures. Thus, if the U.S. deploys a national defense system, it must anticipate that any ICBM launched against the U.S. will be equipped with countermeasures.

In space, where the U.S. system is de-signed to intercept incoming missiles, many types of countermeasures could be used. Because objects travel on identical trajectories in space regardless of their weight, for example, an ICBM could be designed to disperse a light-weight decoy warhead alongside the real thing, and a U.S. kill vehicle would have to decide which to pursue. Once decoy and warhead hit the atmosphere, of course, the lighter of the two would travel more slowly, and sensors could discriminate between them, but by then it would be too late for an intercept.

Three types of simple countermeasures are especially worthy of note:

Submunitions. An attacker intent on reaching the U.S. with chemical or biological weapons could pack an ICBM with dozens or even hundreds of small submunitions containing deadly gases or biological agents. Each submunition would be designed to withstand re-entry into the atmosphere, and combined they would thwart a U.S. defense simply by overwhelming it - there would be too many targets to intercept. This method is also more effective for dispersing chem-ical or biological agents than packing them in a single warhead.

Decoys. An attacking missile could be made to release dozens of lightweight decoys to overwhelm a U.S. defense. Replica decoys, which closely resemble actual warheads, could make discrimination by U.S. radars difficult if not impossible. Far easier and more effective, however are antisimulation methods - making warheads look like decoys. Warheads wrapped in metal-coated Mylar balloons, for example, could be launched along with a large number of empty balloons. The thin metallic layer covering each balloon would reflect radar beams, preventing detection of the warheads, and each balloon could be equipped with a small heater to pre-vent discrimination by infrared sensors.

Alternatively, rather than making each balloon identical, the attacker could use different sizes and shapes and equip them with heaters of varying strengths. The U.S. defensive system would then face the daunting task of deciding which is the real thing among a large number of different targets - none of which would look like a warhead.

Cooled shrouds. An ICBM warhead with a shroud cooled by liquid nitrogen would be effectively invisible to an infrared horning interceptor Such a shroud could be made of aluminum alloy and thermally isolated from the warhead by a multilayer insulator [see illustration below]. A shroud weighing 15 to 20 kilograms (33 to 44 pounds) would require a roughly equal weight of coolant to reach liquid-nitrogen temperature and about 300 grams of coolant per minute to maintain this temperature. The total weight of the shroud and coolant would be 40 to 50 kilograms, a small fraction of that of a 1,000-kilogram first-generation nuclear warhead. Assuming some care was taken in shaping and orienting the warhead to avoid reflecting light back to the interceptor such a shroud would make the warhead invisible to the infrared sensor guiding the interceptor.

Potential Missile Defence Countermeasures

Overwhelm the defense
  • Build more missiles than the defense can intercept
  • Put multiple nuclear warheads on each missile
  • Deploy chemical or biological agents in many small submunitions

Hinder warhead identification

  • Deploy replica or traffic decoys
  • Hide warhead in one of many metal-coated balloons
  • Surround warhead with thousands of tiny radar-reflecting wires called chaff
  • Disguise warhead among debris from exploded booster rockets

Hinder warhead detection

  • Jamradars
  • Lead attack with nuclear explosions to blind infrared detectors
  • Encase warhead in cooled shroud so it is invisible to infrared detectors
  • Shape the warhead or the shroud soit reflects less radar energy
  • Cover warhead with radar absorbing materials
  • Attack missile-tracking satellites and coastal radars

Prevent the interceptor from hitting the warhead

  • Hide warhead behind screens or large balloons
  • Launch low-flying cruise missiles and shorter-range ballistic missiles from ships
  • Add thrusters to warhead to enable maneuvers

SHROUDED WARHEADS are one way an attacker might "blind" a missile defense system. Interceptors use an array of infrared sensors to target room-temperature warheads (300 kelvins; 80 degrees Fahrenheit) as far as a few hundred kilometers away. A warhead shrouded in cold liquid nitrogen (77 kelvins) would radiate an infrared signal less than one millionth as intense, making it invisible until it came within a few hundred meters of the interceptor.

Any of these countermeasures could devastate a U.S. defense, and many more possibilities exist: radar jammers or other electronic countermeasures, warhead maneuvers, chaff or the use of shaping and radar-absorbing materials to reduce the visibility of the warhead to the defense's radars. Such counter-measures could be used singly or in many combinations.

LIMITED RESOLUTION of the kill vehicle's homing sensors could make choosing the proper target difficult. A warhead, booster rocket and heated balloon decoy tumbling through space (left) could appear nearly indistinguishable to the kill vehicle about one second before impact (right).

Because of the open nature of the U.S. political system and the ongoing debate over national defenses, any adversary will know the general properties of a national missile defense system. Although only one effective countermeasure would be needed to defeat a U.S. defense, that defense must be able to defeat every plausible combination of countermeasures. Moreover if it is to be effective in countering weapons of mass destruction, the U.S. system must work the first time it is used. The proposed system does not appear even close to capable of meeting these goals.

Arms-Control Concerns

Technology concerns aside, setting up a limited U.S. national missile defense system would give Russia and China something new to think about. The administration readily acknowledges the possibility of adding more interceptors and sites. And although the U.S. says the national missile defense system is needed only for accidental launches or rogue-state attacks, it would feature much of the infrastructure necessary for a more robust defense. In particular once SBIRS-Low or the forward-based X-band radars are deployed, sensors that could support an expansion would be in place. Because of the time it takes to develop them, sensors are key to the rapid building or expansion of strategic defense systems, which is precisely why the ABM Treaty so sharply limits them.

Moreover; the U.S. is also currently developing two advanced, high-altitude theater missile defense systems whose interceptors are likely to have at least limited strategic capabilities if guided by sensors like SBIRS-Low. In short order, the U.S. could link these interceptors to the national missile defense system and have at its disposal 1,000 or more interceptors. Many Republican law-makers, in fact, are campaigning to upgrade the navy's ship-based theater defense system and make it part of a homeland defense system; offensive force planners in Russia or China would have to take this possibility into account.

How are Russia and China likely to respond to a U.S. decision to establish a national missile defense? Although technically informed Russians may under-stand that effective countermeasures are available, Russian political leaders may not. And the idea that the U.S. would spend many billions of dollars to set up a defense that can be easily countered may not strike Russian leaders as credible. In fact, Russian policymakers have said they oppose both a U.S. national defense and the suggestion that the ABM Treaty should be modified to permit such a system.

Should the U.S. move ahead with its plans, Russia might refuse to make negotiated reductions to its nuclear forces. Russia has linked its implementation of the START I and START II nuclear-reduction treaties to continued U.S. compliance with the ABM Treaty. Economic difficulties make it unlikely that the country will keep more than 2,000 intercontinental warheads in place anyway, but a U.S. national defense system may complicate efforts to reduce nuclear stock-piles further. In addition, Russia might keep more of its nuclear forces ready for rapid launch to increase the number that would survive an attack and could retaliate. This strategy, however, would also increase the risk of inadvertent launches against the U.S. - one of the key reasons behind the push for a national defense.

China's response to a U.S. national defense may also be problematic. To date, China has been content to maintain a very small deterrent force of ICBMs capable of reaching the U.S.. China, however, could view even a very limited or ineffective U.S. defense system as a threat to its small ICBM force, so the country might feel motivated to improve its long-range missile capabilities. And any expansion of China's ICBM force would increase the threat to U.S. security.

So long as Russia and China seek to maintain relationships with the U.S. based on the concept of nuclear deterrence, a U.S. national missile defense system most likely will impede efforts to reduce nuclear forces. A U.S. deployment could also hinder U.S.-Russian co-operation on efforts to reduce the dangers of accidental launches-removing missiles from alert status and warheads from launchers, co-operation on early-warning and installing destruct-after4aunch de-vices. Deployment will also make more difficult U.S. attempts to secure Russian and Chinese co-operation on other vital issues, such as limiting the transfer of weapons materials and technology to other countries and permitting enhanced controls on Russian fissile material.

Arms-control concerns, technological doubts, enormous price tags - these and other problems have dogged U.S. attempts to establish nationwide defenses for more than three decades. And today as much as ever the problem of simple but effective countermeasures looms as the most daunting challenge. As Garwin and Bethe pointed out in 1968, a country that takes the time and risk to develop a costly capability to strike the U.S. with ICBMs armed with weapons of mass destruction cannot be expected to sit by and watch this capability be nullified by a national defense system if there are steps it can easily take to defeat it.

Although proponents continue to argue that the possibility of even one missile striking the U.S. is reason enough to push for a national missile defense system, a limited system with major technological shortcomings would do little to increase national security. In fact, it would have the opposite effect. Only a national defense that can reliably counter a real threat to U.S. security should be pursued: the system the U.S. is pre-paring to put in place will do neither.

The Authors

GEORGE N. LEWIS, THEODORE A. POSTOL and JOHN PIKE are longtime analysts of missile defense programs. Lewis is associate director of the Security Studies Program at the Massachusetts Institute of Technology. For the past 12 years, his research has focused on technical analyses of arms control and international security issues. Postol is professor of science, technology and national security policy at M.I.T. Formerly with the Congressional Office of Technology Assessment, he has also worked as a scientific adviser to the chief of naval operations. His research interests include ballistic-missile defense, cruise missiles and the Anti-Ballistic Missile Treaty. Pike is director of the Federation of American Scientists's Space Policy Project, begun in 1983 in response to President Ronald Reagan's Strategic Defense Initiative.

Further Reading

FUTURE CHALLENGES TO BALLISTIC MISSILE DEFENSES, George N. Lewis and Theodore A. Postol in IEEE Spectrum, Vol.34, No.9, pages 60-68; September 1997.

More information is available on the World Wide Web:
The Ballistic Missile Defense Organization: www.acq.osd..mil/bmdo/bmdolink.html
The Carnegie Endowment for International Peace Non-Proliferation Project: www.ceip.org/programs/npp/missiledefense.htm
The Coalition to Reduce Nuclear Dangers: clw.org/coalition/libbmd.htm
Union of Concerned Scientists: http://www.ucsusa.org/arms
Federation of American Scientists: http://www.fas.org/spp/starwars

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