The United States has been developing missile defense technologies since the beginning of the Cold War, first with nuclear-tipped interceptors and later with conventional so-called "hit-to-kill" missiles--weapons intended to destroy enemy warheads in flight. As of 2009, the U.S. Missile Defense Agency was testing and maintaining over a dozen interconnected sensors, radars, command-and-control systems, and missile silos. But development has not been without controversy, as this Backgrounder explains. While the Pentagon maintains its testing and deployment schedules are effective, critics argue testing relies on unrealistic battle conditions that would easily be defeated by North Korea, Iran, or other potential adversaries. What follows is an overview of current and planned technologies that make up the program, collectively known as the Ballistic Missile Defense System (BMDS).
Ground-Based Midcourse Defense (GMD): The most complex and costly component of the U.S. government's missile defense system, the GMD, is designed to counter long-range intercontinental ballistic missiles that threaten U.S. territories, deployed forces, and allies. It seeks to detect, track, and destroy missiles in flight by launching a ground-based interceptor missile which releases into space a smaller projectile--the "Exo-atmospheric Kill Vehicle." Once released, the kill vehicle would track and collide with the enemy missile. Currently, twenty-four interceptors are positioned in silos at Fort Greely, Alaska, and Vandenberg Air Force Base, California. The Pentagon had planned for a total of forty-four by 2011, though President Barack Obama's decision to cancel installation of ten interceptor missiles in Poland could alter the total. But hardware problems associated with the kill vehicles halted a pair of planned tests in 2008, raising doubts about the GMD's overall functionality. In total, six of the system's fourteen tests have had some level of failure (PDF) since their inception, according to the Center for Defense Information, which tracks missile defense developments.
Aegis Ballistic Missile Defense: Considered the most effective element of the agency's missile defense system, this sea-based component is designed to intercept short- and medium-range ballistic missiles after takeoff or just before impact. There are currently eighteen Aegis systems deployed on Navy warships, with the majority operating in the Pacific Fleet. Charles E. McQueary, director of the Pentagon's Operational Test and Evaluation command, told lawmakers in February 2009 (PDF) the Aegis system has "demonstrated the capability to detect, track, and engage simple short- and medium-range ballistic missile targets for a variety of mission scenarios." The system utilizes Standard Missile-3 interceptors (SM-3)--a key component of President Obama's revised European missile defense system--to target short- to medium-range missile threats. But Philip E. Coyle III, a senior advisor to the World Security Institute and former assistant secretary of defense in the Clinton administration, tells CFR.org the Aegis system has severe operational limitations, most notably a lack of closing speed. "The interceptors are too slow by half," Coyle says, noting that missiles launched from North Korea, for example, would quickly outmaneuver Aegis interceptors. Since January 2002, four of the twenty test flights have failed, according to missile agency data (PDF).
Terminal High-Altitude Area Defense (THAAD): This is a land-based, highly mobile system capable of intercepting ballistic missiles inside and outside the atmosphere. According to the 2008 Operational Test and Evaluation annual assessment (PDF), 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.
PATRIOT Advanced Capability-3 (PAC-3): Operationally fielded by the U.S. Army, the PAC-3 is the successor to the systems deployed in the 1991 Gulf War and the most mature system in the missile defense arsenal. Rapidly deployable, the system is vehicle mounted and employs sensors to track and intercept incoming threats. The PAC-3 has been used during combat missions in Iraq, with mixed success. On March 20, 2003, a U.S. Patriot missile crew intercepted an Iraqi Ababil-100 missile targeting U.S. and British commanders. It was the first combat-target kill (PDF) by a PAC-3. But two days later, a separate U.S. Patriot missile crew accidentally downed a British Tornado fighter jet, killing two British aviators. The Pentagon deemed it "perceived self-defense."
Other Systems in Development
Airborne Laser: A high-powered chemical laser retrofitted on a jumbo 747-400 aircraft, this system is being designed to detect, track, and destroy enemy missiles in flight. According to the Missile Defense Agency, "its revolutionary use of directed energy makes it unique among the United States' airborne weapon systems, with a potential to attack multiple targets at the speed of light with a range of hundreds of kilometers." Yet critics of the system contend countermeasures could easily beat the laser's kill mechanism, simply by painting missiles with light colors to reflect the laser's energy. No airborne testing has been conducted.
Kinetic Energy Interceptor (KEI): Highly mobile, the kinetic energy interceptor is dubbed the next generation of missile defense due to its ability to deploy within close proximity of enemy launch sites. It is also the first missile to be fully developed following the U.S. withdrawal from the ABM Treaty in June 2002. Testing remains in its early stages.
Multiple Kill Vehicle (MKV): Current capabilities allow for only one kill vehicle per interceptor missile. Research is under way to develop a multiple kill component, which would allow for a single missile to target several incoming enemy weapons or effectively deal with countermeasures, like decoy missiles or fragments. To date only ground testing and computer simulations have been conducted.
Missile Defense Sensors
COBRA DANE radar: Built by Raytheon, this phased-array radar is positioned in Shemya, Alaska. With a range of 3,000 miles, critics contend its usefulness is limited because its field of vision is fixed in a single direction. Facing the Kamchatka Peninsula, the array has for decades collected data on Russian strategic-missile-system tests (PDF) "for the purposes of treaty verification," according to the Missile Defense Agency.
SBX radar: A sea-based, single-face phased array on a moveable mount, this highly advanced X-band radar is able to cover any region of the globe.
AN/TPY-2: This forward-based, Army Navy/Transportable Radar Surveillance system provides advance warning and tracking of enemy missile launches, relaying information to fixed radar sites. The system is operationally deployed at Shariki Air Base in Japan.
Upgraded Early Warning Radars (UEWRs): Three long-range, phased-array radars operated by the Air Force provide early-warning data for ballistic missile tracking. Radars are located in Beale Air Force Base, California; Royal Air Force Fylingdales, Britain; and Thule Air Base, Greenland.
Space Tracking and Surveillance System: Planned as a series of satellites and ground systems to detect and track missiles, allowing for earlier destruction in flight, this network has not moved beyond the initial test phase of development.