Possible attack by ballistic missiles has been a concern to U.S. military planners ever since Germany struck Great Britain with V-2 rockets 60 years ago. Unlike powered cruise missiles (such as the Tomahawk) or winged glide missiles (such as the Joint Standoff Weapon), ballistic missiles are unpowered and unassisted by aerodynamic lift forces for most of their trajectory. Much as a fly ball in baseball is only under power while it is in contact with the bat, a pure ballistic missile is only under power while its booster burns at the beginning of its flight.⁽¹⁾ Nevertheless, intercontinental ballistic missiles (ICBMs) can travel more than 10,000 kilometers (6,000 miles).

Ballistic missiles' flight can be divided into distinct phases:

• The boost phase, from launch until the missile's booster burns out;



• The midcourse phase, as the warhead coasts toward its target;⁽²⁾ and



• The terminal phase, the warhead's final descent to its target.

In the past, U.S. work on missile defenses has focused mainly on the midcourse and terminal phases. But today, efforts to defend against ballistic missiles emphasize building layered defenses, with each layer targeting missiles in a different phase of their flight. Thus, interest has grown in developing systems to intercept ICBMs in the boost phase, during the first few minutes after they are launched.
 

A Brief History of U.S. Ballistic Missile Defenses

Efforts to defend against attacks by ballistic missile are nearly as old as ballistic missiles themselves. As early as 1945, for example, the Army's Project Thumper (which sought to develop a high-altitude defense against aircraft) examined how Allied forces could defend against Germany's new V-2 rockets. (Project reports concluded that such defenses were beyond the capability of existing technology.) By the time the Soviet Union deployed its first intercontinental ballistic missiles in 1960, the Army, Navy, and Air Force were pursuing a variety of ballistic missile defense (BMD) programs. Much of that research was consolidated in Project Defender under the newly established Advanced Research Projects Agency. In 1962, President Kennedy assigned Project Defender to the highest category of national priorities for research and development.⁽³⁾

Throughout the 1960s and early 1970s, BMD efforts progressed amid debate about their technical feasibility, their affordability, and whether ballistic missile defense fit into the nation's strategic defense posture. The Army's first BMD program, called Nike-Zeus, went through several iterations: Nike-X, Sentinel, and finally Safeguard, a system that was briefly operational in 1976 to defend the ICBM fields at Grand Forks Air Force Base in North Dakota. That deployment complied with a 1974 protocol to the Anti-Ballistic Missile (ABM) Treaty, which limited ballistic missile defenses to a single fixed site for defending either the national capital or an ICBM field.

After the ABM treaty was ratified in 1972, the United States expended little effort on developing ballistic missile defenses until the early 1980s, when concern grew about a Soviet first-strike capability that might be able to attack both U.S. strategic forces and most metropolitan areas in the United States. In 1983, the Joint Chiefs of Staff recommended that the United States put more emphasis on strategic missile defense. Soon after, the Strategic Defense Initiative Organization was established to pursue an expanded program of missile defense research.

Earlier BMD systems such as Safeguard relied on nuclear warheads to destroy incoming ICBMs. By the early 1980s, however, technological improvements in sensors, guidance systems, boosters, and command-and-control systems had revived interest in "hit to kill," a concept that had been explored by Project Defender as early as 1960. A hit-to-kill interceptor is itself a missile. But instead of having an explosive warhead, it uses precise homing to fly a "kill vehicle" into a target (akin to a bullet hitting a bullet). Relative velocities at impact can reach many kilometers per second, which means that the kinetic energy of such a collision can be much greater than the chemical energy of a similarly sized explosive warhead. The feasibility of hit to kill was demonstrated in 1984 by the Army's Homing Overlay Experiment when a kill vehicle launched from Kwajalein Atoll in the Pacific intercepted a dummy ICBM warhead launched from Vandenberg Air Force Base in California.

The Strategic Defense Initiative was originally focused on defending against a large-scale Soviet attack, so concepts for BMD systems included large numbers of interceptors. For example, scientists and engineers at Lawrence Livermore National Laboratory proposed a constellation of small, smart, space-based interceptors--called Brilliant Pebbles--that were intended to destroy target ICBMs during their boost phase. By attacking an ICBM in that phase, before its multiple independently targeted reentry vehicles could be deployed, a single Brilliant Pebbles interceptor could potentially destroy as many as 10 Soviet warheads. Although early concepts to defend against worst-case scenarios envisioned deploying as many as 100,000 Brilliant Pebbles, later estimates were reduced to around 7,000.⁽⁴⁾

The end of the Cold War brought fundamental changes in the rationale for a strategic missile defense system. The focus shifted from countering a large-scale Soviet attack to two other objectives: defending the United States against accidental or limited ICBM strikes and defending deployed U.S. forces against attacks by theater (shorter-range) ballistic missiles. Some defense planners argued that limited strikes against the United States could be a threat if rogue elements in the former Soviet Union seized control of strategic nuclear systems or if ICBM technology proliferated to other hostile countries. The threat posed to deployed forces by theater ballistic missiles was demonstrated during Operation Desert Storm when an Iraqi Scud missile killed 28 soldiers in Al Khobar, Saudi Arabia.

In 1991, lawmakers enacted the Missile Defense Act, which defined the goal of deploying a system to defend against limited attacks by ballistic missiles while still complying with the ABM treaty. With that treaty's restriction on developing defenses against ICBMs, and in the wake of the Scud attacks during Desert Storm, BMD efforts focused on developing theater-level defenses against missiles in their terminal phase. Those efforts resulted in systems such as Patriot Advanced Capability-3 (or PAC-3) and the Theater High-Altitude Air Defense (known as THAAD).⁽⁵⁾ Those systems were permitted under a 1997 agreement among the parties to the ABM treaty because they lacked the performance necessary to defeat long-range ballistic missiles.

By the mid-1990s, intelligence estimates of threats to the United States prompted greater interest in national missile defense (NMD). The Department of Defense announced a program in 1996 that called for three years of development of an NMD system and then--if the system's components had been tested successfully and threats to the United States warranted its use--three more years to deploy an operational system. That system as envisioned in the late 1990s would have included a new tracking radar and 20 midcourse interceptors based in Alaska, upgrades to existing missile defense radars, space-based sensors, and a command-and-control system. Officials recognized that pursuing that system would eventually require additional modifications to or withdrawal from the ABM treaty. However, in September 2000, President Clinton decided against deploying an NMD system.

The Bush Administration subsequently withdrew the United States from the ABM treaty and broadened BMD efforts to develop and deploy integrated systems to defend against ballistic missiles of all ranges in all phases of flight. To meet that goal, the Missile Defense Agency (MDA) is working on a variety of sensors, weapons, and command-and-control infrastructure that will be integrated into a layered ballistic missile defense system (BMDS).
 

Layered Defenses and Boost-Phase Intercept

Defending against ballistic missile attacks is a challenging technical undertaking. In the case of ICBMs, a defensive system may need to hit a warhead smaller than an oil drum that is traveling above the atmosphere at speeds greater than 13,000 miles per hour. Countermeasures such as decoy warheads that may be carried by ICBMs further complicate the problem of intercepting targets. To achieve a high probability of destroying ballistic missiles in flight, MDA is pursuing a layered defensive approach. Each layer is designed to exploit the particular vulnerabilities and overcome the particular challenges that a ballistic missile presents during a phase of its flight: boost phase, midcourse phase, or terminal phase (see Table 1-1). Layered defenses are built on the premise that although technological limitations might keep any one layer from having an adequate chance of successfully intercepting its target, multiple layers could together provide an effective defense.

Table 1-1.

For the past several years, midcourse intercept has been the primary focus of efforts to defend the United States against attack by ICBMs. The Department of Defense plans to field the initial elements of a midcourse defense by the end of 2004, including the Ground-Based Midcourse Defense system and portions of the sea-based Aegis BMD system. Boost-phase and terminal-phase efforts--such as the Airborne Laser and THAAD, respectively--have focused more on shorter-range ballistic missiles, such as Scuds.

Recently, however, MDA initiated a new effort to develop hit-to-kill interceptors capable of engaging ICBMs in the boost phase. Interest in boost-phase intercept of ICBMs dates to the 1950s, when the Air Force's Ballistic Missile Boost Intercept program looked at using a system of space-based interceptors to deploy large wire-mesh structures that would destroy ICBMs in the boost phase. The Brilliant Pebbles and Global Protection Against Limited Strikes programs of the late 1980s and early 1990s were also primarily envisioned as boost-phase intercept (BPI) systems.

The previous focus on space-based BPI was necessitated by the large size of the Soviet Union. Only an orbiting platform would have access to the interior of that country, the area from which ICBMs would be launched. Because today's concerns center around smaller countries, attention has shifted to terrestrial BPI systems--either land-based, sea-based, or airborne--that could be deployed to the borders of a nation considered a threat. Such systems have become a potentially attractive means of providing a boost-phase defense layer against ICBMs fired at the United States. In December 2003, MDA awarded a contract to Northrop Grumman to develop an initial (or Block 10) surface-based BPI system.

MDA's five-year budget plan envisions that funding for kinetic-energy boost- and ascent-phase intercept systems--currently called BMDS Interceptors--will grow from $118 million in 2004 to $511 million in 2005 and reach nearly $2.2 billion by 2009 (see Table 1-2). Although the BMDS Interceptors program accounts for only about 6 percent of MDA's proposed 2005 budget, that share grows to nearly 28 percent by 2009. Over the 2004-2009 period, that program averages 15 percent of the agency's budget, or a total of about $7.6 billion. Most of the program's current funding is focused on mobile terrestrial systems, although about $10 million of the 2005 budget request is slated for initial analysis of a space-based system. Through 2009, MDA plans to allocate a total of about $700 million for work on a space-based BPI system.

Table 1-2.

Characteristics of ICBMs Important for BPI

A ballistic missile is composed of a guidance system and one or more warheads mounted on a rocket booster. To achieve intercontinental range--approximately 10,000 kilometers (km) or more--ICBM boosters typically accelerate their payloads to a speed of about 6 to 7 km per second.

Boosters can be grouped into two types: liquid-fuel and solid-fuel. Liquid-fuel boosters are the older and simpler technology. A liquid propellant and a liquid oxidizer are used to fuel the missile's rocket motor. Although liquid-fuel missiles are easier to build than solid-fuel missiles, they require complicated and potentially dangerous fuel-handling activities and thus can be more difficult to operate and maintain.

Solid-fuel boosters use a propellant and an oxidizer that are molded into a solid motor core with a binding agent. The solid-fuel motor, which is ignited in its hollow interior and burns from the inside out, generates thrust by expelling the combustion products from its nozzle. Producing large solid-fuel boosters is challenging both technologically and industrially, but such boosters are relatively easy to handle after production.

The type of booster used in an ICBM is particularly important to designers of boost-phase intercept systems. Solid-fuel ICBMs usually have shorter boost phases than liquid-fuel ICBMs do. Thus, a BPI system designed to counter solid-fuel ICBMs will need higher performance because its interceptors will have less time to reach their targets. (Performance requirements for BPI systems are discussed in Chapter 2.)
 

Representative Threats for Comparing BPI Systems

As many as 35 countries are thought to possess ballistic missiles, although only 13 of them have missiles with ranges greater than 500 kilometers (see Table 1-3). Just four nations--China, Russia, the United States, and the United Kingdom--are known to possess ICBMs.⁽⁶⁾ But other countries, including some that have had less-than-friendly relations with the United States over the years, are believed to be pursuing ICBM capability. For example, North Korea and Iran could have such capability by 2015, according to the December 2001 National Intelligence Estimate.⁽⁷⁾ Specific intelligence estimates are subject to debate, but if a nation is developing space-launch capability, it is in effect gaining the ability to field ICBMs. Both North Korea and Iran have said they plan to develop space-launch systems.⁽⁸⁾

Table 1-3.

To compare the effectiveness of alternative designs for BPI systems, this analysis uses North Korea and Iran as representative threats. North Korea is believed to be developing two long-range ballistic missiles--the Taep'o-dong 1 SLV (space-launch vehicle), with a range of about 5,000 km, and the Taep'o-dong 2, with a range of about 6,000 km, according to unclassified estimates (see Table 1-4).⁽⁹⁾ In August 1998, North Korea launched a Taep'o-dong 1 SLV with the stated goal of orbiting a small test satellite. Although the launch was unsuccessful, it clearly demonstrated North Korea's pursuit of long-range-missile capability. Iran has Shahab 3 missiles with an estimated range of about 1,300 km. It is thought to be developing a Shahab 4 with a range of about 2,000 km.⁽¹⁰⁾

Table 1-4.

Those North Korean and Iranian ballistic missiles have much longer ranges than do tactical ballistic missiles, such as the Scud. But their ranges fall far short of the more than 10,000 km distance typical of ICBMs that have been fielded in the past (see Table 1-4).

Besides the characteristics of missiles that might be targeted, geography is an important factor with respect to the performance needed from a BPI system. Iran and North Korea are also good representative threats for assessing the implications of geography. Surface-based BPI interceptors must fly farther to reach ICBMs fired from the interior of large countries and thus usually need higher speeds if deployed against such countries. Iran is the world's 16th largest country by area, so it provides a case study that could stress surface-based systems. Geography plays a role in space-based systems as well. Orbital dynamics require that the higher the latitude of the country to be covered, the more interceptors must be deployed. A space-based system with orbits that provided coverage of North Korea (the northern tip of which is located above 42 degrees latitude) could cover about 75 percent of the world's countries--and about 90 percent of the countries that, at least currently, might be considered potentially hostile to the United States.

The next chapter assesses the performance characteristics that a BPI system would need to defend the United States against ICBMs fired from those two representative countries. Chapter 3 examines alternative BPI designs that would meet the needed performance, and Chapter 4 compares each alternative's strengths, weaknesses, and costs.