“Right after I graduated, I interned with the Arms Control Association. It was terrific.”

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ABC News
January 1, 2005
Missile Defense Systems at a Glance
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Last Reviewed: 
August 2019

Contact: Kingston Reif, director for disarmament and threat reduction policy, 202-463-8270 x104


For nearly as long as there have been offensive weapons systems, there have also been anti-weapons systems. For years, one of the most dangerous threats to a state was ballistic missiles given the blinding speed with which they could deliver some of the world’s most dangerous weapons: nuclear-armed warheads. As such, some states have made a concentrated effort to build defenses against such weapons, known as ballistic missile defenses. However, during the Cold War, as the United States and Soviet Union experimented with and fielded missile defenses, both sides worried such defenses could prompt an uncontrollable arms race.

These concerns led to the conclusion of the 1972 Anti-Ballistic Missile (ABM) Treaty, which limited each side to 100 strategic missile defense interceptors at one site. The agreement helped to stabilize relations between the two nuclear superpowers and provided a foundation for further agreements limiting strategic offensive forces. However, the abrogation of the ABM treaty in 2002 by the George W. Bush administration—and the development of more advanced cruise and hypersonic missiles—have led to an uptick in funding to attempt to defend against missiles beyond just ballistic missiles and from countries beyond just Russia.

What are missile defense systems specifically trying to defend against?

The main missile threats that missile defense systems have aimed to defend against have been ballistic missiles, but more recently, greater emphasis has been placed on defending against other types of missiles as well.

Ballistic Missile Basics
(Adapted from “Worldwide Ballistic Missile Inventories”)

Ballistic missiles are powered by rockets initially but then follow an unpowered, parabolic, free-falling trajectory toward their targets. They are classified by the maximum distance that they can travel, which is a function of how powerful the missile’s engines (rockets) are and the weight of the missile’s payload, or warhead. To add more distance to a missile’s range, rockets are stacked on top of each other in a configuration referred to as staging.

There are four general classifications of ballistic missiles:

  • Short-range ballistic missiles, traveling less than 1,000 kilometers (approximately 620 miles);
  • Medium-range ballistic missiles, traveling between 1,000–3,000 kilometers (approximately 620-1,860 miles);
  • Intermediate-range ballistic missiles, traveling between 3,000–5,500 kilometers (approximately 1,860-3,410 miles); and
  • Intercontinental ballistic missiles (ICBMs), traveling more than 5,500 kilometers.

Short- and medium-range ballistic missiles are referred to as “theatre” ballistic missiles, whereas ICBMs or long-range ballistic missiles are described as “strategic” ballistic missiles.

Missiles are often classified by fuel-type: liquid or solid propellants. Missiles with solid fuel require less maintenance and preparation time than missiles with liquid fuel because solid-propellants have the fuel and oxidizer together, whereas liquid-fueled missiles must keep the two separated until right before deployment.

Thirty-one countries possess ballistic missiles. Of those, only 9 (China, France, India, Israel, North Korea, Pakistan, Russia, the United Kingdom, and the United States) are known to possess or suspected of possessing nuclear weapons. These 9 states plus Iran have produced or flight-tested missiles with ranges exceeding 1,000 kilometers. China and Russia are the only two states that are not U.S. allies that have a proven capability to launch ballistic missiles from their territories that can strike the continental United States.

Three stages of flight for a ballistic missile:

  1. Boost phase:
    • The boost phase begins at launch and lasts until the rocket engines stop firing and pushing the missile away from Earth.
    • Depending on the missile, it lasts between three and five minutes.
    • Generally, the missile is traveling relatively slowly, although towards the end of this stage, an ICBM can reach speeds of more than 24,000 kilometers per hour. Most of this phase takes place in the atmosphere (endoatmospheric).
  2. Midcourse phase:
    • The midcourse phase begins after the rockets finish firing and when the missile is on a ballistic course toward its target.
    • This is the longest stage of a missile’s flight, lasting up to 20 minutes for ICBMs.
    • During the early part of the midcourse stage, the missile is still ascending toward its apogee, while during the latter part, it is descending toward Earth.
    • During this stage, the missile’s warhead(s), as well as any decoys, separate from the delivery platform, or “bus.” This phase takes place in space (exoatmospheric). The warhead is now called/is on a reentry vehicle (RV).
  3. Terminal phase:
    • The terminal phase begins when the missile’s warhead, or RV, reenters the Earth’s atmosphere (endoatmospheric), and it continues until impact or detonation.
    • This stage takes less than a minute for a strategic warhead, which can be traveling at speeds greater than 3,200 kilometers per hour.

Other Types of Missiles

Generally, U.S. missile defense systems have been designed to defend against ballistic missiles. However, the Trump administration’s 2019 Missile Defense Review most clearly noted that the United States will be looking for ways to defend against non-ballistic missiles.

Cruise missiles and hypersonic missiles are two additional categories of missiles. Unlike ballistic missiles, cruise missiles remain within the atmosphere for the duration of their flight. Cruise missiles are propelled by jet engines and can be launched from land-, air-, or sea-based platforms. Due to their constant propellants, they are more maneuverable than ballistic missiles, though they are also slower than their ballistic counterparts.

Two types of hypersonic missiles are currently under development. A hypersonic boost-glide vehicle (HGV) is fired by rockets into space and then released to fly to its target along the upper atmosphere. Unlike ballistic missiles, a boost-glide vehicle flies at a lower altitude and can change its intended target and trajectory repeatedly during its flight. The second type, a hypersonic cruise missile, is powered through its entire flight by advanced rockets or high-speed jet engines. It is a faster version of existing cruise missiles.

What makes up a missile defense system?

Satellite Sensors and Ground- or Sea-based Radars

Together, space-based satellites and ground- or sea-based radars create a monitoring system that contribute to offensive missile detection (detecting a missile after it has been launched), discrimination (what is a threat versus a decoy or other countermeasures), and tracking (keeping the missile “in sight” so that an interceptor can find it and eliminate the threat).


Interceptors are the missiles used once a threat has been detected. Missiles carry “kill vehicles,” which detach from the missile (also called the boosters or rockets) and then go to try to eliminate the threat. Today’s kill vehicles are “hit-to-kill,” meaning that they aim to eliminate the threat by actually running into it, or “kinetically” (also called a “kinetic kill”). Due to the speed at which the incoming rival missile and interceptors and kill vehicles are traveling, this has metaphorically been compared to “a bullet hitting another bullet.”

Some interceptors are single pieces (which means that they do not separate from their kill vehicles), such as the Patriot Advanced Capability-3 (PAC-3).

In addition, interceptors need launchers. Some interceptors are launched from in-ground silos, road-mobile trucks, or ships. There currently exist no interceptors in space, though the idea has been proposed. These launchers and interceptors can be carried in a “battery,” which can carry up to a cluster of launchers, interceptors with their kill vehicles, radars, and fire control.

Command and Control

All the data that is being processed by the sensors and radars and then sent to the interceptors and kill vehicles are linked through another network of command and control centers. The centers are located around the entire world and involve several different U.S. military branches and commands working together. Command and control centers also tend to utilize “fire control.”

Working Together

The information from the sensors and interceptors routed through command and control work together similar to the image below, laid out by the Union of Concerned Scientists in order to demonstrate the workings of the Ground-based Midcourse Defense system.

Other FAQs

Are all missile defense systems currently only for ballistic missile defense?

Not exclusively. While most missile defense systems are developed to focus on the blindingly fast speed and specific trajectory of ballistic missiles, some systems could conceivably counter cruise missiles or other shorter-range targets.

Can a missile defense system intercept a threat on any part of the trajectory?

Not yet. Currently, missile defense systems are only developed and designed to carry out an interception at the mid-course (middle) or terminal (final) stage of a missile’s trajectory, even though a missile is slowest during its boost (beginning) phase. The 2019 Missile Defense Review and Congress have both called for further study of “boost-phase intercept” capabilities, proposing the controversial development of interceptors in space or other emerging capabilities, such as drones or lasers. “Left of launch” capabilities have also been proposed, which would aim to counter a missile threat before it is even launched.

What is the difference between a missile defense system (anti-missile system) and other forms of air defense systems?

Generally, missile defense systems are specifically designed to target very fast and very specific threats. However, some forward-based missile defense systems may be able to carry out missions against air-launched cruise missiles and rival aircraft. However, because other forms of air defense systems, mainly anti-aircraft systems, have such smaller areas of defense, they would be unlikely to counter a threat with the speed of a hypersonic or ballistic missile.

What are some criticisms of missile defense systems?

The U.S. pursuit of effective missile defenses has been accompanied by intense debate about the technical capabilities of the system and realism of testing, the scope of the ballistic missile threat, the deterrence and assurance benefits of the defenses, the cost-effectiveness of shooting down relatively inexpensive offensive missiles with expensive defensive ones, and the repercussions for U.S. strategic stability with Russia and China.

According to the Defense Department’s independent testing office, existing U.S. missile defenses have “demonstrated capability” to defend the U.S. homeland against a small number of intercontinental ballistic missile (ICBM) threats that employ “simple countermeasures.” The testing office assesses that defenses to protect allies and U.S. troops abroad possess only a “limited capability” to defend against small numbers of intermediate-range ballistic missiles (IRBMs) and medium-range ballistic missiles (MRBMs). The capability of defenses against short-range ballistic missiles is labeled as “fair.” Apart from the point-defense Patriot system, no systems in the U.S. BMD arsenal have been used in combat.

Leaders of the U.S. missile defense enterprise have increasingly voiced concerns that the current U.S. approach to national and regional missile defense is unsustainable and that existing defenses must be augmented with emerging capabilities to reduce the cost of missile defense and keep pace with advancing adversary missile threats.

Current and Under Development U.S. Missile Defense Components and Equipment

Homeland “Strategic” Defense Systems

  • Ground-based Midcourse Defense System

Regional “Theater/Tactical” Defense Systems

  • Aegis BMD system
    • Aegis BMD System (Part of the Aegis Combat System, aka Aegis Afloat; Sea-Based BMD)
    • Aegis Ashore (Land-based variant of Afloat)
  • Terminal High Altitude Area Defense (THAAD)
    • (Emerging) THAAD Extended Range
  • PAC-3

For more detail on current day programs and next generation efforts, visit: “Current U.S. Missile Defense Programs at a Glance.”

Each system has a combination of the previously mentioned sensors, radars, interceptors, kill vehicles, and largely use the networked command and control. The above systems rely on the below equipment and components:


Air- and Space-Based Sensors Used:

  • Space Tracking and Surveillance System (STSS) and Space Tracking and Surveillance System-Demonstrators (STSS-D) constellation operated by the Missile Defense Agency
  • Space-based Kill Assessment (SKA) hosted on commercial satellites
  • Near Field Infrared Experiment (NFIRE) technology project, operated by the Missile Defense Agency
  • Defense Support Program (DSP), constellation of satellites operated by the U.S. Air Force Space Command
  • (Under Development) Space-based Infrared System (SBIRS), constellation of integrated satellites operated by the U.S. Air Force Space Command
    • SBIRS-LEO (Low Earth Orbit), incorporated into the STSS program in 2001 with the Missile Defense Agency
    • (Under Development) SBIRS-GEO (Geosynchronous orbit), intended to replace Defense Support Program (DSP)
    • (Under Development) SBIRS-HEO (High Elliptical orbit), intended to replace DSP


  • Ground-Based Interceptors (GBI), for the GMD System
  • SM-2
  • SM-3 (RIM-161 Standard Missile-3)
    • 3 variations: Block IA, Block IB, Block IIA
  • SM-6 (RIM 174 Standard Missile-6)
  • (Under Development) Boost Phase Laser Defenses
  • Evolved Seasparrow Missile (ESSM), NATO Interceptor
  • Space-Based Intercept (SBI) Layer

Kill Vehicles:

  • Exo-atmospheric kill vehicle (EKV)
  • (Terminated August 2019) Redesigned kill vehicle (RKV)
  • (Under Development) Multi-Object Kill Vehicle (MOKV)

Command and Control Centers:

For more detail on how the above components fit together in each separate missile defense program and next generation efforts, visit: “Current U.S. Missile Defense Programs at a Glance.”

History of U.S. Missile Defense Systems

Brief History of U.S. Missile Defense Systems

After the end of World War II, U.S. military planners began to weigh the need to be able counter ballistic missile threats before they reached their targets. During the war, German V-2s were particularly concerning, and in 1946, the U.S. Army Air Forces (USAAF) embarked on the Projects Wizard and Thumper study programs to develop an anti-ballistic missile (ABM).

Recognizing the complexities of what would be a multi-year study, the Air Force focused on Project Wizard as a long-term study. In 1949, the Army began to develop its own Project Plato, the services’ first effort to develop a theatre ABM system. As the Cold War began to ramp up during the 1950s and the Soviet Union continued their ICBM development, the Army and Air Force began to compete for a role in strategic missile defense, which led to the 1957 initiation of the Army’s nuclear-capable Nike Zeus ABM interceptor. The program's high costs and shortcomings spurred criticism of the ABM system concept. Meanwhile, to settle the Air Force and Army dispute over who should pursue the strategic missile defense initiative, then Defense Secretary Neil H. McElroy assigned the mission to the Army and established the Advanced Research Projects Agency (ARPA).

After the 1962 Cuban Missile Crisis, using the justification that the crisis caused the Soviets to aggressively ramp up their ICBM program, the U.S. military also reoriented its ABM efforts to create an improved system called Nike-X. News also reached the U.S. military that the Soviets were developing their own ABM capabilities. U.S. leaders felt that in order to overcome the Soviet ABM system, they would either need an overwhelming offensive force or arms control agreements—so they resisted calls to deploy the Nike-X ABM system until China conducted its first nuclear test. The Chinese test meant that proponents of the Nike-X ABM system could now argue that a limited ABM deployment which could counter China would be better than a heavy ABM deployment to counter the Soviets. The United States deployed the Nike-X ABM in 1967 and renamed the ABM system the Sentinel. The Navy and Air Force also began to develop their own ABM system concepts.

In 1968, the Johnson administration began to shift the limited mission of the Sentinel system from against China towards a heavier defense mission against a large-scale Soviet attack. Though this may have been done in part to use the system as a “bargaining chip” as the Soviets had just agreed to begin long-sought arms control negotiations, the shift caused debate, confusion, and criticism over the purpose of the controversial Sentinel system.

In 1969, the Nixon administration re-oriented the U.S. ABM system again so that instead of protecting urban areas, it would now be used to protect the nation’s strategic deterrent: the silo-based Minuteman ICBMs. President Nixon renamed the system “Safeguard.” The system was still used as a bargaining chip as the United States and Russia continued with the Strategic Arms Limitation Talks, which eventually led to the 1972 Anti-Ballistic Missile Treaty.

The ABM Treaty initially limited each side’s ABM deployments to only two locations with no more than 100 interceptors total. After a 1974 protocol was negotiated, each side was allowed only one site. The Safeguard site was closed in 1976 because it could be easily overwhelmed by a Soviet attack and because detonation of its nuclear-armed warheads would blind its own radars.

In 1983, President Ronald Reagan launched the Strategic Defense Initiative (SDI) to revisit the issue of the feasibility of missile defense. The day after his announcement, Senator Edward Kennedy (D-Mass.) called the president’s speech “reckless Star Wars Schemes”—a phrase that had previously been used to also reference exotic Pentagon space weaponry projects, but now was the new nickname of SDI. Around this time, the Army had begun working on developing a nonnuclear hit-to-kill interceptor and, in 1984, was able to intercept a dummy warhead outside of the atmosphere in space.

Meanwhile, ARPA’s successor, the Defense Advanced Research Projects Agency (DARPA), began developing laser and particle beam technologies for application that included ballistic missile defense and space defense. The Reagan administration highlighted that SDI would not jeopardize U.S. compliance with the ABM Treaty because of SDI’s focus at the time was as a research- and development-based project, not deployment. The Department of Defense then chartered the Strategic Defense Initiative Organization (SDIO) in 1984.

Toward the end of the 1980s, SDI—which had developed a broad and costly space- and ground-based defense concept—reoriented its focus to the “Brilliant Pebbles” (BP) program, which used autonomous, small-scale, space-launched interceptors. In 1990, BP was introduced as an affordable hit-to-kill system that skirted concerns about the exposure of large-scale space systems. However, in light of the fall of the Soviet Union, under the directive of the George H. W. Bush administration, SDI was overhauled to address limited nuclear strikes in 1991. Bush announced a new system, the Global Protection Against Limited Strikes (GPALS).

When President Bill Clinton entered office, he shifted focus on theatre missile defense instead of national missile defense. To reflect this, he canceled the BP program and changed the name of SDIO to the Ballistic Missile Defense Organization (BMDO). He also broke up the Bush GPALS program into several Army, Navy, and Air Force programs, introducing what is now the PATRIOT Advanced Capability-3 (PAC-3) program, the Theatre High Altitude Area Defense (THAAD) system, the ship-borne Aegis air defense system and Standard Missile (SM) interceptor, and the Air Force’s Airborne Laser Project. However, during his administration, President Clinton was pressured by Congress to pursue national missile defense that would have consequences for U.S. obligations towards the ABM Treaty. President Clinton signed the 1999 National Missile Defense Act, which made it “the policy of the United States to deploy as soon as is technologically possible an effective National Missile Defense (NMD) system capable of defending the territory of the United States against limited ballistic missile attack.” However, in 2000, President Clinton announced that he would leave the final decision of pursing a national missile defense system to his successor.

In 2001, the new George W. Bush administration announced that it was giving its six-month notice of its withdrawal from the ABM Treaty, which took effect in 2002. Also in 2002, President Bush changed the name of BMDO to the Missile Defense Agency (MDA). The military began to reorient the missile defense program to be an integrated, layered, and nationwide defense system.

The Obama Administration

Upon taking office in 2009, the Obama administration took steps to curtail the Bush administration’s rush to expand the U.S. homeland missile defense footprint and instead place greater emphasis on regional defense, particularly in Europe. The Obama administration decided to alter its predecessor’s plans for missile defense in Europe, announcing Sept. 17, 2009, that the United States would adopt a European “Phased Adaptive Approach” (EPAA) to missile defense. This approach primarily uses the Aegis Ballistic Missile Defense system to address the threat posed by short- and intermediate-range ballistic missiles from Iran. The Aegis system uses the Standard Missile-3 (SM-3) interceptors, which are deployed on Arleigh-Burke class destroyers in the Baltic Sea (Aegis Afloat), as well as on land in Romania and Poland (Aegis Ashore).

President Obama's first Secretary of Defense, Robert Gates, also canceled a number of next generation programs, including two designed to intercept missiles during their boost phase, due to “escalating costs, operational problems, and technical challenges.”

However, while continuing to invest in regional defense, the Obama administration also made substantial investments in homeland defense largely in response to North Korea. The Ground-based Midcourse Defense (GMD) system comprises missile fields in Ft. Greely, Alaska, and Vandenberg Air Force Base, California, and is designed to protect the United States against limited, long-range missile strikes from North Korea and Iran. Despite concerns about the system’s technical viability, from 2013 to 2017, the Obama administration expanded the number of ground-based interceptors (GBIs) in these fields from 30 to 44.

The Obama administration also oversaw the deployment of additional regional missile interceptor and sensor capabilities to allies in Northeast Asia in response to North Korea, including the deployment of the THAAD system to Guam and South Korea and two advanced radars to Japan.

To view the history in a timeline form, visit the Union of Concerned Scientists.

For current day programs since the beginning of the Trump administration, visit: “Current U.S. Missile Defense Programs at a Glance.”

Recently Canceled Programs

A number of high-profile missile defense efforts that began during the George W. Bush administration were canceled by President Bush’s last Defense Secretary, Robert Gates, under President Barack Obama. Below is a summary of some of these programs, the reason they were canceled, and the amount of money that was spent to develop them.

[Previously known as Space-based Infrared System-low (SBIRS-low)]

Program Elements

The program was a planned network of 9-12 satellites which were expected to support U.S. missile defense systems by providing tracking data from space on missiles during their entire flight.

Dates of Program

October 2009 – April 2013

Money Spent

More than $230 million

Major Issues

As reported by the LA Times, outside experts found that the satellites would not have been able to detect warheads flying over the arctic. In order to provide continuous tracking of the missiles, MDA would have actually needed at least 24 satellites. An independent cost assessment projected the total cost of the system to be $24 billion over 20 years instead of the $10 billion MDA projected.


Program Elements

The original program included a modified Boeing 747 plane equipped with a chemical oxygen-iodine laser (COIL) and two tracking lasers. The laser beam would be produced by a chemical reaction. The objective was to shoot down ballistic missiles during their boost phase right after launch, but the system could also be used for other missions.

Dates of Program

November 1996 – February 2012

Money Spent

$5.3 billion

Major Issues

The laser would have had a limited range, which meant the 747 would have been vulnerable to anti-aircraft missiles. To increase the range, the laser would have needed to be 20-30 times more powerful than planned.


Program Elements

KEI was to be comprised of three powerful boosters and a separating kill vehicle. The booster was expected to travel at least six kilometers per second, which is comparable to an ICBM. The kill vehicle was not designed to carry an explosive warhead but to destroy its target through the force of a collision.

Dates of Program

March 2003 – June 2009

Money Spent

$1.7 billion

Major Issues

In order to carry the KEI, Navy ships would have needed to be retrofitted. The range was not great enough to be land-based.


Program Elements

The program was designed to launch multiple kill vehicles from a single booster in order to increase the odds of destroying an incoming missile. It was designed to destroy both missiles and decoys.

Dates of Program

January 2004 – April 2009

Money Spent

~$700 million

Major Issues

The program was canceled by the Obama administration in order to focus on “proven, near-term missile defense programs that can provide more immediate defenses of the United States.”