U.S. missile defense systems are designed to protect the U.S. homeland, deployed military forces, and allies from limited ballistic missiles attacks. The Pentagon originally sought development of ballistic missile defense (BMD) technology to counter the Soviet nuclear threat during the Cold War, but focus in the twenty-first century has shifted to defending and deterring potential strikes from regional actors, particularly Iran and North Korea. In March 2013, the Pentagon announced it would shore up missile defenses on the U.S. west coast to guard against a growing North Korean threat, while effectively cancelling the final phase of plans to deploy missile interceptors in Europe over the next decade. Proponents of BMD stress its role in the projection of U.S. power abroad and its value as a deterrent, while critics highlight BMD's largely unproven technology and high costs.
How does ballistic missile defense work?
Ballistic missiles can be launched from a variety of platforms, including silos, trucks, trains, submarines, ships, and aircraft. There are four general classifications based on the maximum distance the missile can travel: short-range (less than 1,000 kilometers); medium-range (1,000 – 3,000 kilometers); intermediate range (3,000 – 5,500 kilometers); and intercontinental ballistic missiles, or ICBMs (more than 5,500 kilometers).
Ballistic missiles have three stages of flight: boost phase, which begins at launch and lasts until rocket engines finish; midcourse phase, the longest stage, when the projectile is on its parabolic path to the target; and terminal phase, when the detached warhead reenters the atmosphere, often traveling less than a minute to impact.
Defeating a ballistic missile with a defensive system involves four functions: detection; discrimination (separating the missile from everything else); fire control (determining exactly where to intercept); and killing (hitting the missile with some type of interceptor). However, the effectiveness of BMD systems in test trials has been mixed, and critics continue to question their value in realistic battle conditions.
What is the history of U.S. missile defense?
The first missile defense research effort dates to 1946, when the U.S. Army Air Forces launched two rudimentary antiballistic missile design programs, Wizard and Thumper (PDF). In the mid-1950s, the Pentagon began a concerted undertaking to counter the threat of Soviet intercontinental ballistic missiles (ICBMs). A number of competing programs were stood up by the Army, Air Force, and Navy, including the so-called Nike Zeus program, which sought to use nuclear detonations to destroy incoming missiles. In 1963, however, the Partial Test Ban Treaty between the United States, Britain, and the Soviet Union made such tests illegal. By 1972, arms control had turned its attention to missile defense. With U.S. and Soviet arsenals growing exponentially in size and power, the two signed the Anti-Ballistic Missile Treaty (ABM), limiting the number of missile defense sites each could maintain to two.
In 1981, the Reagan administration announced an expanded research and development effort to include space- and ground-based defensive systems. In a March 1983 speech, Reagan unveiled his plan, the Strategic Defense Initiative, later dubbed "Star Wars." The next year, the U.S. Army successfully tested its Homing Overlay Experiment, the first successful demonstration of a hit-to-kill vehicle.
Meanwhile, tactical or "theater missile defense" continued to develop. American Patriot missile batteries, designed to intercept Soviet intermediate-range ballistic missiles (IRBMs) in Western Europe, were deployed to the Middle East and saw action during the Gulf War, to protect Israel from Iraqi Scud attacks. While they proved ineffective in Scud attacks on Israel and Saudi Arabia in 1991, the concept drew increased attention and funding during the 1990s. By the later part of the decade, advocates pushed for a full-blown national missile defense system, citing nascent North Korean, Iraqi, and Iranian ballistic missile capabilities.
Leading defense officials in President George W. Bush's administration envisioned an integrated, layered defense capable of defeating enemy missiles on a global scale. Within months of assuming office, Bush notified Russia of plans to withdraw from the 1972 ABM Treaty, renamed the Ballistic Missile Defense Organization the Missile Defense Agency (PDF), and ordered the Pentagon to "proceed with fielding an initial set of missile defense capabilities" within two years. In July 2004, the first ground-based missile interceptor was installed at Fort Greely, Alaska, giving the Pentagon limited defensive capabilities on U.S. soil.
What are the primary U.S. missile defense systems?
The Missile Defense Agency is developing a number of systems that offer multiple opportunities to defeat limited ballistic missile attacks. These systems are not designed to shield against large-scale nuclear attacks from Russia and China. MDA has spent approximately $90 billion on missile defense systems since 2002, and plans to spend roughly $8 billion per year through 2017--or approximately 2 percent of the Pentagon's baseline budget.
Critics say BMD technologies remain mostly unproven to date, often run behind schedule, have significant cost overruns, and would be of limited value in a real-world attack. In 2009, President Obama ordered the first comprehensive review of BMD (PDF), which, in addition to laying out threats and strategy, sought to improve the BMD testing program, oversight, and cost-effectiveness. The new administration also scrapped three BMD programs: the Multiple Kill Vehicle (April 2009), the Kinetic Energy Interceptor (May 2009) and Airborne Laser (February 2012).
The four existing BMD programs include:
Ground-Based Midcourse Defense (GMD): The most complex and costly component of the U.S. missile defense system, the GMD, is designed to destroy long-range, strategic ballistic missiles in the midcourse stage with ground-based interceptors. As of spring 2013, thirty interceptors were positioned in silos at Fort Greely, Alaska, and Vandenberg Air Force Base, California, with plans to increase this number to forty-four by 2017. To date, the MDA says there have been seven successful intercept tests out of fourteen. However, all "have involved substitute components in highly-scripted scenarios," according to the nonpartisan Arms Control Association. GMD has not successfully hit a test target since 2008.
Aegis Ballistic Missile Defense: Considered the most effective element of U.S. missile defense, this traditionally sea-based component is designed to intercept short- and medium-range ballistic missiles after takeoff or just before impact. As of spring 2013, there were 24 BMD-capable Aegis systems deployed on Navy warships, with the majority operating in the Pacific Fleet. Plans call for thirty-eight such ships by 2015. As of February 2013, the Pentagon says the program has had twenty-four successful intercepts out of thirty tests.
The Aegis BMD system is the linchpin of the Obama administration's plan for a missile defense umbrella in Europe intended to protect U.S. forces and NATO allies from Iranian short- and medium-range ballistic missiles. While the Pentagon effectively canceled the final phase of this plan in March 2013, it says it remains committed to the first three phased deployments, which include land-based "Aegis Ashore" sites in Poland and Romania by 2018.
Terminal High-Altitude Area Defense (THAAD): This land-based, highly mobile system is capable of intercepting short- and medium-range ballistic missiles inside and outside the atmosphere. THAAD missiles, which contain eight interceptors each, are fired from a truck-mounted launcher. According to the 2008 Operational Test and Evaluation annual assessment, U.S. Strategic Command intends to deploy THAAD systems "to protect critical assets worldwide." All five of the system's tests since 2006 have been deemed successful by the missile agency, though cost overruns and design flaws have slowed development. In April 2013, the Pentagon announced plans to deploy one of its three THAAD batteries to Guam to help defend U.S. forces on the Pacific island territory.
PATRIOT Advanced Capability-3 (PAC-3): The PAC-3 is the successor to the systems deployed in the 1991 Gulf War and the most mature system in the U.S. missile defense arsenal. Rapidly deployable, the system is vehicle-mounted and employs sensors to track and intercept incoming missiles in their terminal phase, at lower altitudes than THAAD systems. The PAC-3 was used during combat missions in Iraq in 2003 with mixed success. PAC-3 batteries have been deployed to several nations including South Korea, Afghanistan, and Turkey, among others, and more than a dozen nations have purchased variants of the system.
Source: GAO Missile Defense Report: Opportunity to Refocus on Strengthening Acquisition Management; April 2013
What is the ballistic missile threat from North Korea?
North Korea has several hundred short (Scud) and medium-range (Musudan and Nodong) ballistic missiles, and is developing an intermediate-range missile (Taepo Dong-2) that analysts say would be able to hit the continental United States should it become operational. The TD-2 reportedly succeeded in placing a satellite in orbit in December 2012. However, experts say this feat does not translate into a reliable missile, and that further testing would be needed. North Korea's missiles are capable of delivering conventional warheads, and potentially biological and chemical munitions. U.S. intelligence officials said in 2008 that TD-2 missiles could, in theory, deliver a nuclear payload to the United States, but noted that the potential for this was low without further testing. While Pyongyang held its third nuclear test in February 2013, which was roundly condemned by the international community, analysts continue to speculate whether the regime has mastered the technology required to shrink and mount a nuclear warhead on its missiles.
In recent threat assessments, U.S. officials have characterized North Korea's nuclear aspirations (PDF) as focused more on "deterrence, international prestige, and coercive diplomacy than for war fighting," according to a 2013 report by the nonpartisan Congressional Research Service. However, heightened belligerent rhetoric from Pyongyang in early 2013 prompted the Pentagon to shift U.S. BMD assets to the region.
What is the ballistic missile threat from Iran?
Experts say Iran has "the largest and most diverse" ballistic missile arsenal in the Mideast, having obtained most of this from allies abroad, particularly from North Korea. The majority of Iran's ballistic missile inventory consists of short-range Scud missiles with a top range of approximately 500 kilometers. While Tehran views these missiles as important tactical weapons, experts say their ability to strike U.S. and allied targets in the region are limited because they would need to be launched from vulnerable positions along Iran's Persian Gulf coastline. Furthermore, analysts say these ballistic missiles are not very accurate, and serve more as a psychological threat to large urban and economic centers in the region than as decisive military weapons.
The Islamic Republic also has a stockpile of about 100 longer-range ballistic missiles (Shahab-3 and Ghadr-1) that are able to hit any target in the Mideast, including Israel; but analysts say these weapons suffer from significant inaccuracy. Michael Elleman, a missile defense expert at the International Institute for Strategic Studies, estimates that Iran should not be able to strike Western Europe with ballistic missiles before 2014, and the United States before 2020.
U.S. intelligence officials have warned in recent years of Iran's potential to deliver weapons of mass destruction with these weapons. In the 2012 U.S. Worldwide Threat Assessment (PDF), Director of National Intelligence James Clapper said Iran "continues to expand the scale, reach and sophistication of its ballistic missile forces—many of which are inherently capable of carrying a nuclear payload." Tehran claims its controversial nuclear program is intended solely for peaceful applications, but Western analysts continue to speculate whether the regime has decided to pursue a nuclear weapon.
How does the U.S. collaborate with its allies on missile defense?
A major component of U.S. BMD strategy is partnering with allies around the globe to expand their own ballistic missile defenses and deterrence capabilities. The U.S. BMD commitment to European allies is provided within the NATO security context.
In Asia, Japan is one of the Pentagon's closest collaborators in this arena. Tokyo has procured a layered missile defense system from Washington, including Aegis-equipped destroyers and Patriot missile batteries. The two longtime allies "cooperate in a way that is highly interoperable, and the nations are working together to jointly develop a future system," says the 2010 BMD review. The United States has similarly provided BMD technology to South Korea, which also bought Aegis warships and Patriot missile batteries. But while all three nations are wary of the looming ballistic missile threat from North Korea, analysts note that long-standing tensions between Tokyo and Seoul have kept them from cooperating on BMD and other military matters.
In the Mideast, BMD cooperation (PDF) has been "a vital component of Israel's strategic relationship with the United States," according to CRS. In recent years, Washington has co-funded and co-developed several rocket and missile defense systems with Israel, including the so-called Iron Dome (deployed in 2011), intended to guard against short-range rocket attacks, David's Sling, and the Arrow I, II, and III systems, designed to counter regional threats, including Iran. Israeli officials touted Iron Dome's high success rate in shooting down Hamas rockets during the November 2012 Gaza conflict, but some weapons experts are skeptical of the system's effectiveness.
Completed in February 2010, the Pentagon's Ballistic Missile Defense Review Report provides a broad overview of the emerging global missile threats and U.S. strategy to defend against them.
This report from the U.S. Institute of Peace, authored by Michael Elleman, provides an outline of Iran's ballistic missile program and the implications for U.S. security policy.
This profile from the nonpartisan Arms Control Association examines North Korea's main weapons programs as well as the major arms control agreements the Kim regime subscribes to.