US Missile Defense: Closing the Gap

In March 2013, US Secretary of Defense Chuck Hagel, citing the progress of North Korea’s nuclear program, announced that the United States would be bolstering its missile defenses. Fourteen new ground-based interceptor missiles, known as GBIs, would be deployed to Alaska, augmenting the thirty already in silos there and in California. The Pentagon would develop a new two-stage GBI, as well as a more advanced version of the “kill vehicle,” which interceptors carry to smash into adversary warheads (“hit-to-kill”). The Obama administration would be deploying a second advanced mobile radar system, the Army Navy/Transportable Radar Surveillance 2, in Japan. And Hagel even indicated that the administration would restructure its plans for US missile defenses in Europe, canceling the SM-3 IIB interceptor, the cornerstone of the fourth and final phase of its European Phased Adaptive Approach (EPAA), which had been announced in 2009 as a means of deploying more-advanced interceptors in Eastern and Central Europe specifically designed to defend the US homeland from intercontinental ballistic missiles launched from Europe, Eurasia, or the Middle East.

Hagel’s announcement came in the middle of a steady buildup of the size and capability of ballistic missiles among states antagonistic to the US and its allies. More than thirty countries already have acquired, or are acquiring, short- and medium-range missiles able to deliver conventional payloads at great speed and distance. Some are trying to develop longer-range missiles that can carry warheads armed with various weapons of mass destruction (nuclear, chemical, and biological), seeing these as cheaper, easier to maintain, and often more effective strike weapons than manned aircraft. The most immediate missile threats to the West come from Iran and North Korea. US government experts believe that either or both of these countries might have an intercontinental-range ballistic missile (ICBM) capable of hitting a target in North America within the next few years.

Under both the Bush and Obama administrations, the United States has employed a variety of tools to address this possibility—multilateral diplomacy and bilateral negotiations, economic sanctions and inducements, warnings against developing these capabilities, and threats of retaliation for their use. It has reinforced these measures by constructing missile defense architectures around the world, including short-range missile defense systems such as surface-to-air batteries, theater defenses such as Aegis-equipped naval vessels and ground-based systems capable of targeting midrange missiles, and the ground-based midcourse interceptors based in Alaska and California.

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The main difference between Obama’s policies and those of Bush is that the current administration has deemphasized near-term efforts to develop and apply new ballistic missile defense technologies, such as space- or airborne-based lasers and two-stage GBIs that could conduct an early intercept of an Iranian ICBM, and instead has sought to build on proven existing technologies such as the Patriot Air and Missile Defense System, designed for countering attacking missiles in their terminal (or final attack) phase, and the family of combination Aegis SM-3 midcourse interceptors currently used by the US Navy. Of the six main ballistic missile defense programs inherited from its predecessor, the Obama administration has expanded some and restructured or cut others. It cancelled the Kinetic Energy Interceptor program because of high costs and the Airborne Laser program in February because of technological challenges as well as cost. (Yet while the US is no longer developing these directed-energy weapons, which use lasers to heat the metal skin of a long-range ballistic missile until it ruptures and disintegrates, it has built systems that can use lasers against unmanned aerial vehicles and cruise missiles, while researching how direct energy could assist with discriminating between warheads and decoys, a difficult discrimination problem.)

Another weapon the United States lacks at the moment is an operational early interceptor, designed to attack an enemy missile in its boost or ascent phrase, when the rocket has ceased burning but the missile is still gaining altitude and when it is most vulnerable to attack. Previous attempts to build such weapons interceptors failed because of their immature technologies (lasers and rapidly accelerating interceptors that can compute an interception solution even before the missile goes ballistic), impractical operational concepts (the interceptors would have to be located on land near the enemy launch site), and exorbitant costs. Some defense strategists have suggested the development of a high-speed, two-stage, hit-to-kill interceptor missile, launched from a high-altitude, long-endurance unmanned aerial vehicles, but for now the main protection of the US homeland from missile attack comes from the GBI and Aegis SM-3 systems.

Thus the new moves Hagel announced may provide the incremental enhancements needed to match a potential adversary’s missile capabilities, notably those of North Korea, but the margin for error is still small—adversaries may make unexpectedly rapid progress or the defenses may suffer unanticipated technological setbacks. To reduce the risks of an offense-defense gap arising, America’s best option for the next few years would be to continue improving its two midcourse defense systems—ground-based interceptors on US territory and mobile, sea-based ones—and make greater use of enhanced variants of the sea-based interceptor, the SM-3, as a hedge against the continued development and reliability problems with the GBIs in Alaska and California. This structure, supplemented by the construction of an advanced missile defense radar in the northeast United States, would create a layered defense to better protect the US homeland, especially in light of the cancellation of the final phase of EPAA. The US should also continue to pursue as well as fund new technologies (such as direct-energy weapons) that can broaden its defensive capabilities.


The GBIs deployed in Alaska and California are three-stage, solid-fueled boosters with an exoatmospheric kill vehicle (EKV), which separates from its booster and uses the kinetic energy from colliding with the re-entry vehicle or warhead to destroy it. Once released from the booster, the EKV uses guidance transmitted to it and from its own onboard sensors to collide with the target above the Earth’s atmosphere.

The Bush administration had intended to deploy GBIs in Poland to attempt an early intercept of Iranian ICBMs launched toward the United States. The Obama administration canceled this move in 2009, when intelligence concluded that Iran was making less than expected progress in developing ICBMs, but the Defense Department’s Missile Defense Agency has recently resumed flight-testing a two-stage GBI that could be deployed closer to potential missile threats from Northeast Asia or the Middle East.

The other US midcourse interceptor is the Navy’s ship-based Aegis SM-3 combination, which builds on the Navy’s Standard Missile, the Aegis radar system, and the Pentagon’s command, communication, and control systems. The Aegis Ballistic Missile Defense System (BMD) can defeat short- to intermediate-range, midcourse-phase ballistic missile threats, either unitary or separating, as well as short-range ballistic missiles in the terminal phase. The SM-3 consists of a kill vehicle attached to a three-stage booster. It intercepts missiles, warheads, and other targets above the atmosphere in their midcourse phase of flight using a “hit-to-kill” collision in which the target is destroyed through the kinetic energy released when the EKV collides with it. The Obama administration has expanded the Aegis SM-3 system to make it the main BMD effort, especially for intercepting long-range ballistic missiles in their midcourse phase. Unlike ground-based midcourse defense weapons (GMD), the ship-based Aegis BMD system can be maneuvered and reconfigured more easily to respond to changes in the threat environment.

Missile Defense Agency and Navy plans call for deploying increasingly capable versions of the SM-3 on US warships in coming years. The next version is scheduled for deployment in 2015. It has an improved two-color infrared seeker to better identify the target (despite adversary debris and other countermeasures), an advanced signal processor to take advantage of recent upgrades to the Aegis system’s capabilities, and an enhanced control system that employs short, precise propulsion bursts to adjust course as it approaches the target. It has a greater maximum velocity, a longer flight time, and is capable of attacking targets at a greater distance.

The United States and Japan have cooperated in developing certain technologies for the third SM-3 variant, with Japan funding a significant share of the effort and leading the development of the second- and third-stage rocket motors and nose cone. Its initial flight test is scheduled for 2015, with a transition for full production around 2018. In addition to the expected deployment on US and Japanese warships, the United States plans to place some of the new SM-3s in Poland and perhaps elsewhere.

The United States currently has two terminal-phase, land-based interceptors, the PAC-3 and the THAAD. The Patriot Advanced Capability-3 missile interceptor battery, which is used by the US Army and many US partners, builds on the proven Patriot air and missile defense infrastructure, such as the PAC-2 that the United States used during the Gulf War against Iraqi SCUDs. Whereas the PAC-2 employs blast fragmentation warheads to destroy nearby missiles and warheads, the PAC-3 relies on the more sophisticated “hit-to-kill” technology, which destroys the target through the kinetic energy generated by a midair collision.

The PAC-3 can work with the Terminal High Altitude Area Defense (THAAD) system now under development, which can intercept short- and medium-range missiles in their terminal phase using a hit-to-kill approach. It is presently the only interceptor in the US arsenal with the operational flexibility to intercept missiles both inside and outside of Earth’s atmosphere. A THAAD battery consists of four main components: a truck-mounted launcher, eight interceptors, a radar, and a fire control system, which serves as the communication and data-management backbone linking the THAAD components together as well as to the rest of the ballistic missile defense system. The first THAAD battery was activated in 2008, the second a year later. The Missile Defense Agency hopes the third and fourth batteries will be fielded in the next two years.


A serious gap in current missile defense planning would occur if the threat of an attack against the US homeland developed more rapidly than the US technology advances needed to defend against it. The best strategy to deal with these gaps would be to focus more on improving and multiplying existing capabilities in the near term while preparing to develop significantly more capable systems over the long term. In particular, an increased reliance on Aegis and newer SM-3s would yield considerable advantages in the protection of the US homeland as well. This option would cost significantly less than developing a new system, can be made available more rapidly, uses proven technology, and does not depend on making improbable revolutionary breakthroughs.

Relying only on the US-based GBIs would be risky. This system has had a modest fifty-percent success rate in its interception tests, having destroyed incoming missiles in only seven of fourteen tests. The last time the GBIs in Alaska and California destroyed a target was in 2008. The latest version of the EVK (the “kill vehicle”) failed its first two tests in January and December 2010. The Pentagon believes it has identified the reason for the failure—a fault in the guidance system—but demonstration tests are still needed to verify that the problem has been resolved. In addition to the low success rate, another concern is that most tests have not employed decoy missiles, which would likely be present in genuine attacks (either as deliberate penetration aides or simply as pieces of the rocket or other debris). In a September 2012 report, the National Research Council said that the ground-based midcourse defense system has fundamental weaknesses, including a vulnerability to counter-measures, meaning that “it will not be able to work against any but the most primitive attacks” without further upgrades and testing.

Besides technical impediments, lack of funding remains a major constraint on the development of US ballistic missile defense (BMD) capabilities. The decisions announced by Defense Secretary Hagel in March 2013 will not add new any money to US BMD programs, but will instead redirect what has already been allocated. The Missile Defense Agency is now struggling to meet the military’s demands for more assets, even as the agency tries to develop new technologies to stay ahead of advances being made by likely adversaries. The number of potential hostile missiles continues to increase, as do their capabilities.

As the Obama administration emphasizes a shift toward a sea-based missile defense platform, and as China continues to develop its own anti-ship missile capabilities, the Navy also faces an ever-increasing strain. The current US Navy fleet consists of some two dozen Aegis-equipped cruisers and destroyers. These vessels have many important missions in addition to their vital defense role. The expected demand for Aegis ships will rise to more than forty in coming years, which will require the Navy to increase its baseline surface combatant inventory to more than eighty-eight vessels or reduce support for other missions. The need to increase the number of air and missile defense ships will be met by modernizing existing destroyers and cruisers, as well as restarting the production of Arleigh Burke class DDG 51 Flight IIA destroyers with BMD capabilities. Naval BMD capabilities will also be reinforced by the FY2016 construction of Flight III Arleigh Burke class destroyers. These ships will potentially have new hybrid electric drive systems, electromagnetic rail guns, and solid-state air defense lasers. But until these BMD vessel capabilities are expanded, current BMD ships may experience longer deployments and shorter inter-deployment periods.

The Missile Defense Agency must also improve the United States’ space-based missile-tracking capacity to defend against long-range ICBMs. And allies require BMD interceptors, thus driving up the demand for additional ordnance. The Aegis air and missile defense systems (the AN/SPY-1 and the MK99) can be found on more than one hundred warships, mostly on US Navy cruisers and destroyers, but also on seventeen allied foreign vessels. Greater efforts will be needed to integrate US and foreign missile defense capabilities in coming years.

Filling the gaps in US ballistic missile defense capabilities will inevitably force painful trade-offs between national homeland and regional defenses. The timely identification of threats is essential for their neutralization. This requires persistent intelligence, surveillance, and reconnaissance capabilities at both the global and regional level. Even so, these measures can have only a limited impact. Fundamentally, numbers matter, a fact of life to bear in mind as the United States and its allies continue to develop a flexible missile defense architecture with mobile assets that can be deployed in other theaters in an emergency, while also providing a homeland defense network that can be augmented and upgraded as the threat evolves.

Richard Weitz is the director of the Hudson Institute’s Center for Political-Military Analysis.

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