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Strategic Defense Initiative and National Missile Defense

Strategic Defense Initiative and National Missile Defense


Since the advent of ballistic missiles at the end of World War II, the United States has considered several anti-ballistic missile (ABM) systems designed to defend against attack by intercontinental ballistic missiles (ICBMs) or, more recently, by shorter-range ballistic missiles. The Strategic Defense Initiative program and its successor, National Missile Defense (NMD), are the two most ambitious ABM schemes proposed to date. SDI sought, according to President Ronald Reagan's original vision (1983), a space-based ballistic-missile defense system that would render the United States safe from even an all-out Soviet attack involving thousands of missiles; NMD, evolved from SDI concepts in the post-Soviet environment, seeks effectiveness against launches of only one or a few missiles, possibly from "rogue states" such as North Korea. NMD thus returns to the limited design goals of the United States earliest ballistic-missile defense concepts of the 1960s.

Research on ABM systems began in the early 1950s. By the late 1960s, a nuclear-tipped interceptor missile dubbed Sentinel had been developed. Sentinel was designed to destroy incoming warheads by detonating near them, and was intended for deployment around major cities to protect them against accidental launch of one or several Soviet missiles or against a limited strike by China. The system was never deployed, however, and in 1969 President Richard Nixon announced that Sentinel would be renamed Safeguard and reassigned to protecting "our land-based retaliatory forces [i.e., nuclear-tipped ICBMs based in the American Midwest] against a direct attack by the Soviet Union." At this time, however, the United States and Soviet Union were nearing agreement that ballistic-missile defense (BMD) is inherently destabilizing.

According to the standard anti-BMD argument, missile defenses encourage their possessor to start a nuclear war strategy because any BMD system will necessarily be more effective against a weakened counterattack than against a first strike; BMD therefore makes it "rational" to attack first. BMD proponents responded that missile defenses would enhance stability by adding to the uncertainties of a first strike. Nobody, during this period, argued that a perfect shield against nuclear attack was technically possible.

The Anti-Ballistic Missile Treaty of 1972 made the anti-BMD point of view official by forbidding the Soviet Union or United States to deploy extensive missile defenses. Each superpower was allowed by the original treaty to build ABM installations at two "widely separated" locations, each with at most 100 interceptor missiles. In 1974, a protocol was added to the ABM Treaty that reduced the number of permitted installations to one per nation. The United States chose to build its permitted ABM system near an ICBM base in Grand Forks, North Dakota; the Soviet Union deployed a system around Moscow. The Soviet system remains operational to this day, but the United States ABM system was shut down after only 9 months of operation in 197475 due to high operating costs and because strategists felt that the system was too small to make any strategic difference.

The ABM treaty's ban on significant defenses left deterrence through "mutually assured destruction" as an undisputed fact of life; that is, strategists hoped that neither superpower would dare start a nuclear war because annihilation of both societies would be the certain result. On March 23, 1983, however, President Reagan made a televised speech in which he declared that this situation was unacceptable. He asked, "Wouldn't it be better to save lives than to avenge them? What if free people could live secure in the knowledge that their security did not rest upon the threat of instant U.S. retaliation to deter a Soviet attack, that we could intercept and destroy strategic ballistic missiles before they reached our soil or that of our allies?" This policyfar more ambitious than any ABM concept that had been contemplated beforewas formalized by Reagan in National Security Decision Directive 85 two days later. Studies of the feasibility of an SDI-type system were made in the coming months, and a Strategic Defense Initiative Organization (SDIO) was chartered by Secretary of Defense Caspar Weinberger in April 1984.

Reagan's original proposal was conceptual, not technically specific. In October 1985, the SDIO released a set of five "architecture" studies describing possible configurations for an SDI system. The favored architecture suggested seven defensive layers, including air-, land-, sea-, and space-based components to track and shoot down ballistic missiles during their boost, cruise, and descent phases of flight. The main emphasis was on space-based defenses. Hundreds of satellites were proposed for command, control, and communications; remote sensing; battle management; and actual shootdown of targets.

A few of SDI's many proposals for destroying enemy missiles during each phase of flight, along with some of the countermeasures proposed for each proposal, are described below. Countermeasures are emphasized because many scientists and engineers in government, academia, and industry argued in the 1980s that it would be relatively easy to defeat SDI's proposed defenses using countermeasures, so no SDI system could ever replace deterrence. Today's debate concerning the feasibility of a more limited NMD program revives many of the measure-countermeasure concepts that were discussed during the SDI debate.

Proposals for Boost-Phase Intercept

The SDIO and its critics were agreed that it would be essential to destroy many enemy missiles during boost phase, the period during which a ballistic missile is being accelerated by its rocket engines. With existing missile technology, boost phase lasts for 3 to 5 minutes. Boost-phase intercept is essential to defense against a largescale ballistic missile attack because once the payload of a missile is no longer being accelerated, it can detach from its booster rocket and begin releasing independently targeted warheads (as many as 10 per missile) and hundreds of decoys (i.e., objects designed to confuse sensors). Once a large number of ballistic weapons achieve cruise phase, a "threat cloud" of hundreds of thousands of objects, mostly decoys, could be encountered. It would be impractical, for any defensive system of plausible size, to target every object in such a threat cloud during the few minutes available for cruise-phase defense; therefore, the size of the threat cloud has to be reduced by destroying missiles during boost phase. Some of the methods proposed for boost-phase intercept, along with countermeasures proposed for them, are discussed below.

Space-based directed-energy weapons. For boost-phase intercept the SDIO proposed several hundred satellites armed with powerful (i.e., >100 MW) lasers. Microwaves and neutral-particle beams (beams of hydrogen atoms) were also considered, but lasers were, and remain, the more developed technology. The directed-energy concept was, in essence, simple: lasers would cook boosters. Boosters contain flammable fuel and sensitive electronics and cannot carry armor because it would weigh too much, and so are more vulnerable to laser damage than, say, separated warheads, which are armored against the high heat of atmospheric reentry. Directed-energy weapons have two advantages for boost-phase intercept: First, they reach their targets in effectively zero time (at the speed of light or, in the case of a particle beam, at about half the speed of light). Second, after destroying one booster a directed-energy weapon can be retargeted, so each weapon can destroy a number of boosters. A basic limitation of any directed-energy weapon, however, is that it must illuminate or "dwell" on a booster for some period of time to destroy it. Dwell time for a laser of realistic power is generally estimated at between 1 second and 1 minute. Furthermore, redirection of the beam takes time, as it requires swiveling a mirror or other device. Finite dwell times and retargeting times, combined with the short duration of boost phase, place limits on the number of boosters that each directed-energy weapon can, in theory, destroy.

Space-based directed-energy weapons have the further limitation that beam intensity diminishes approximately with the square of the distance. They would therefore have to be placed in low (i.e., 1203100 mi [2005,000 km]) polar orbits, waiting to destroy boosting missiles not far below them. Low orbits, however, produce "absenteeism." That is, a low-orbit satellite can only see a small portion of the Earth at any one time, and so is absent from the sky over a particular area (e.g., Siberia) most of the time. To provide continuous coverage of a given area therefore requires many satellites. Absenteeism multiplies the number of weapon satellites needed to cover a specific launch zone by a factor of between 6 and 20; that is, for every satellite that happens to be passing over, say, Siberia at a given moment, at least six (or as many as 20) would have to be orbiting elsewhere, waiting their turn.

Surface-based directed-energy weapons. Two other SDI proposals for boost-phase intercept using directed-energy weapons were made. First was the use of fixed, ground-based optical lasers stationed in the continental United States. These lasers would send their beams up to orbital "fighting mirrors" which would reflect their energy back down over the horizon to enemy ballistic missiles in boost phase. The fighting mirrors would swivel rapidly to aim the laser light at the boosters. The lack of maneuverable mirrors that could reflect so much power without being destroyed by it was a major obstacle to this concept. Another technology, intensively urged by the SDIO for several years, was the nuclear-pumped x-ray laser. By surrounding a nuclear bomb with appropriate materials, it is possible in theory to cause those materials to lase (emit laser radiation, in this case in the x-ray part of the spectrum) briefly when the bomb explodes. If even a small fraction of the bomb's explosive energy is converted into x-ray laser energy, and this energy can be precisely aimed and focused, pulses powerful enough to destroy distant missiles could be generated. A basic limitation on this concept is that x rays cannot travel very far through the atmosphere; like space-based directed energy weapons (with the possible exception of optical-frequency lasers), nuclear-pumped x-ray lasers can only attack boosters during the portion of the boost phase that is above the densest part of the atmosphere, that is, above 50 to 56 miles (8090 km). It was therefore proposed that nuclear-pumped x-ray lasers be deployed in a "pop-up" system. That is, they would be mounted on missiles deployed on land or at sea not far from the Soviet Union (or other potential enemy). When orbiting detectors observed the infrared signatures of ballistic missile launches (i.e., the infrared glow of hot rocket exhausts), the missiles bearing the nuclear-pumped x-ray laser devices would be launched within a few seconds (i.e., would "pop up"). They would race to the edge of space and there explode, destroying their targets before they could finish boost phase.

Proposals for Boost-Phase Countermeasures

Fast-burn boosters. As described above, boost-phase intercept must by definition gain access to target missiles during their boost phase, which lasts only 3 to 5 minutes. What is more, pop-up x-ray lasers and most proposed space-based directed-energy weapons can reach their targets only during that fraction of the boost phase which takes place in near-vacuum, because lasers and particle beams tend to be scattered and absorbed by the atmosphere. Therefore, an important boost-phase countermeasure would be to build boosters that accelerate rapidly ("fast burn" boosters). With fast-burn boosters, boost phase would take place entirely within the atmosphere, reducing or eliminating the defense's chances for boost-phase interception using space-based or pop-up directed-energy weapons. Fast-burn boosters would in any case give boost-phase intercept less time to operate, which would require the defense to build more satellites, which could eventually become prohibitively expensive.

Booster hardening. Boosters could be coated with a material that ablates, or vaporizes, when illuminated by laser light. Such a coating could increase the dwell time needed to destroy a booster, again forcing the defense to build more satellites in order to cope with a given number of boosters in the time available.

Rotation. Boosters could be designed to spin as they fly. This would spread energy from an attacking laser over a larger surface area, increasing dwell time.

Decoys. Cheap rockets that simulate the infrared signature of real, weapons-carrying boosters could be deployed alongside real boosters. Such decoys could be made so numerous that no affordable defensive system could attack them all. Although decoy rockets might stagger and veer after launch, unlike real boosters, the real boosters might be deliberately programmed to stagger and veer like the cheap imitations, further confounding the defense. The later technique is termed "antisimulation": making a real weapon behave like a cheap decoy, rather than trying to make a cheap decoy that behaves like a real weapon.

Space mines. All space-based components of any SDI or NMD system would be vulnerable to space mines, which are simply bombs (possibly nuclear) orbiting near the defensive system's satellites. Before launching its ICBMs, the attacker would detonate its space mines. Since only one space mine is needed per battle station, and bombs are simpler than megawatt lasers, space mining would be intrinsically cheaper than defense building.

Kinetic weapons. Kinetic weapons (also termed "kinetic-kill weapons") destroy by virtue of their kinetic energy, that is, by colliding with their targets at high speed. Clouds of pellets orbited in the opposite direction to defensive satellitesreleased, probably, by space mines in appropriate orbitscould strike their targets at tens of thousands of miles per hour.

Directed-energy weapons. All the devices proposed for boost-phase intercept, including nuclear-pumped xray lasers, would be effective antisatellite weapons, and could therefore be used against an opponent's defensive satellites even more readily than they could be used against an opponent's missiles.

Nonballistic weapons. An enemy might employ weapons that do not have a boost phase at all. Cruise missiles (which fly at very low altitudes and can be made stealthy to both radar and infrared tracking), crewed aircraft, and bombs smuggled aboard ships or across land borders are all possible methods of making a nuclear attack that would not be vulnerable to boost-phase intercept.

Proposals for Cruise-Phase Intercept

SDI envisioned using the same orbital directed-energy stations described above for both boost-phase intercept and for cruise-phase intercept. However, SDI's designers anticipated that during cruise phase the task of distinguishing between actual warheads and the hundreds of thousands of radar reflectors, decoys, and other objects released after boost phase would become paramount. There would simply be too many objects to attack if one could not tell the warheads from the chaff and decoys. It was therefore proposed that "tapping" each object with a laser pulse and measuring its change in velocity might be used to determine which objects were heavy enough to be warheads; directed-energy weapons would then be used to destroy the real warheads.

Countermeasures for Cruise Phase

The primary cruise-phase countermeasure would be the release of large numbers of decoys. Atmospheric nuclear explosions could also be used to confuse or blind infrared sensors by providing a glowing background (as seen from space), and all methods mentioned above for destroying defensive satellitesorbital or ground-based directed-energy weapons, space mines, kinetic weapons, and so onwould also be threats to cruise-phase defense. Reactive decoy balloons that sensed a laser tap and accelerated themselves to mimic the mass of a real warhead were proposed as a countermeasure to "weighing" using laser pulses.

Proposals for Descent-Phase Intercept

Descent phase (also known as "terminal phase") is the period during which a warhead is falling through the atmosphere toward its target. Descent-phase intercept has the advantage that atmospheric friction will strip away all decoys released during cruise phase, sifting out the real warheads. Descent-phase intercept was the traditional, pre-SDI focus of ballistic-missile defense; the Sentinel system, for example, was designed to use high-altitude nuclear explosions to knock out enemy warheads in descent phase, and the primary focus of post-SDI ballistic-missile defense concepts has also been on descent-phase intercept. Since electromagnetic pulse (EMP) from high-altitude nuclear explosions could cripple communications and electrical systems over a continent-sized area, descent-phase intercept concepts since Sentinel have focused on kinetic-kill weapons that would actually strike their targets.

Countermeasures for Descent Phase

Possible countermeasures for descent phase are few, but stealth technology could make warheads harder to track; furthermore, the attacker is favored in descent phase by the great speed and small size (>1.5 m long, >.5 m wide at the base) of each cone-shaped warhead. To strike even a single incoming warhead moving at some 10,000 miles per hour, much less hundreds or thousands of them simultaneously, is an extremely difficult rocketry problem, often compared to hitting a bullet with a bullet in midair. Since the inception of the SDI program a number of tests have been conducted in which kinetic weapons (also termed "kinetic kill vehicles") have intercepted, or sought to intercept, incoming missiles or warhead-like objects, but most tests have failed or produced ambiguous results. Nevertheless, kinetic descent-phase intercept is not impossible, and continuing technological advances may render it more reliable.

The measures-countermeasures debate was vigorous during the 1980s because both sides agreed that a defensive system that allowed even 1% of the Soviet Union's 8,000 or so strategic warheads to reach the United States80 thermonuclear weaponscould not protect United States society from destruction. The technical side of the SDI debate therefore revolved around the question of whether a BMD system providing better than 99% defensive coverage was buildable or not, and if so, whether it could be built within a plausible budget. In general, the countermeasures school prevailed. SDI funding was cut back in the late 1980s and the SDIO retreated from its original goal of "render[ing] nuclear weapons impotent and obsolete" (in President Reagan's 1983 words) to the traditional concept of defense against limited ballistic-missile attack. The SDIO was renamed the Ballistic-Missile Defense Organization (BMDO) in May, 1993, and its official emphasis was shifted away from space-based defenses, marking the end of the SDI period.

After SDI: GPALS and NMD

Three years before the term "strategic defense initiative" was abandoned, SDI officially gave up on being the total nuclear umbrella proposed by President Reagan in 1983. At the order of President George H. Bush, the program assumed in 1990 a more limited mission: Global Protection against Limited Strikes (GPALS). GPALS resembled Sentinel, in seeking to defend only against accidental or small-scale ballistic attacks, rather than massive launches of thousands of missiles. It differed from pre-SDI ballistic-missile defense in that it envisioned global protection, especially "theatre" defense against ballistic missiles fired in extended combat zones far from the continental United States.

During the 1990s, the term National Missile Defense (NMD) replaced GPALS, and the program re-evolved many features of SDI, albeit on a smaller scale. Today, NMD proposes a system with boost-phase, cruise-phase, and descent-phase intercept layers, much like SDI, only not intended to cope with the simultaneous launch of thousands of attacking missiles.

For boost-phase intercept NMD proposes lasers, both airborne and space-based. The airborne laseralready built and test-flown, and scheduled to attempt its first missile shoot-down test in 2003 or 2004would be flown on a modified Boeing 747, use hydrogen fluoride lasing in the infrared part of the spectrum, have a range of several hundred kilometers, and be directed against theatre (shortand medium-range) ballistic missiles. Adaptive optics that measure atmospheric distortion between the weapon and the target would be an essential component of such a system. Having measured the atmospheric distortion in real time, the weapon imparts an inverse distortion to the laser beam as it fires; the predistortion and the actual atmospheric distortion cancel each other out, allowing a focused beam to dwell on the missile. (A similar technique is widely applied in ground-based astronomy.) NMD also proposes to eventually use space-based lasers for boost-phase intercept. The weapons proposed, a product of research begun under SDI, would consist of infrared (hydrogen fluoride) lasers in low orbits. However, the technical hurdles to such a system are numerous, even disregarding possible countermeasures, and funding for the space-based boost-intercept component of NMD has lately been reduced.

For both cruise and descent (midcourse and terminal) defense, NMD proposes to use kinetic-kill warheads that would destroy by collision. Both land-based and sealaunched kinetic-kill missiles are under development for these phases of defense.

NMD architecture. The detailed structure of the proposed NMD shift frequently, depending on technological and political factors. One typical, recently proposed NMD architecture consists of six essential elements:

1) A satellite system for the detection and tracking of missile launches, the first components to be launched in 2006 or 2007. The system consist of six geosynchronous satellites designed to observe the infrared emissions of booster rockets.

2) Approximately five ground-based early-warning radars to project the approximate trajectories of missiles detected by the infrared satellite system.

3) A number of high-frequency ground-based radars in the United States, United Kingdom, Greenland (Denmark), South Korea, and perhaps other locations, designed to discriminate between warheads and decoys during the cruise phase.

3) A midcourse (exoatmospheric) kinetic-kill interceptor missile, to be guided by information received from ground radars. The kinetic-kill warhead or vehicle is thrown by its booster as nearly as possible toward its target, then guides itself during final approach using onboard sensors, computers, and steering rockets.

4) A "Battle Management, Command, Control, and Communications" network of ground stations where computers will integrate information from sensors, fire and guide interceptors, and assess success and failure in real time so that multiple attempts can be made on a given target if necessary.

The United States withdrew from the ABM Treaty in 2002, allowing it to begin construction on a preliminary cruise-phase kinetic-kill interceptor site in June 2002, in Fort Greely, Alaska. The 16-missile complex is scheduled for completion in 2004.



Causewell, Erin V. National Missile Defense: Issues and Developments. New York: Novinka Books, 2002.

Drell, Sidney D., Philip J. Farley, and David Holloway. The Reagan Strategic Defense Initiative: A Technical, Political, and Arms Control Assessment. Cambridge, MA: Ballinger Publishing Co., 1990.


Bethe, Hans A., et al. "Space-Based Ballistic-Missile Defense." Scientific American. (October, 1984).

Fowler, Charles. "National Missile Defense (NMD)." IEEE Aerospace and Electronic Systems Society (AESS) Systems Magazine (January, 2002):412.

Lewis, George N., and Theodore A. Postol. "Future Challenges to Ballistic-Missile Defense." IEEE Spectrum (September, 1997): 668.


Ballistic Missiles
Cold War (19451950), The Start of the Atomic Age
Cold War (19501972)
Cold War (19721989): The Collapse of the Soviet Union

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