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It used to be that when policymakers talked about "national missile defense," they were referring to systems that would protect U.S. territory from the threat of intercontinental ballistic missiles (ICBMs) armed with nuclear, biological, or chemical warheads. Today, the Bush administration prefers the term "ballistic missile defense." [See below for more on the Bush administration program.]
Although the Pentagon's Missile Defense Agency has various anti-ballistic missile systems in various stages of research and development, the United States currently has no defense against long-range ballistic missiles -- those with a range greater than 5,500 kilometers. Only two countries, Russia and China, are known to possess ICBMs capable of threatening U.S. territory. [See an interactive map for information on countries that are suspected or known to be developing long-range missiles.]
Nor does the U.S. have any defense against short-range ballistic missiles, those with ranges from 100 to 1,000 kilometers, many of which were built using Soviet Scud missile technology. Scuds lobbed by Iraq were a very real threat to U.S. troops on the battlefield, as well as U.S. allies, during the 1991 Gulf War. The Patriot anti-missile system, which was pulled into service then, was an example of a "theater" missile defense; in other words, a defense against short-range missiles. Its successor, in development for more than a decade, is the Patriot Advanced Capability-3 (PAC-3). [Read more on short-range, or "theater," missile defenses.]
 The Flight of an ICBM
To understand how current plans for missile defense systems might work, one needs to understand the three stages -- or phases -- of an ICBM's flight: the initial, or "boost," phase; the "midcourse" phase; and the final, or "terminal," phase:
- Boost phase. When a booster rocket lifts a missile and its warhead into the air, U.S. satellites can easily spot the rocket's hot exhaust plume. But the boost phase lasts, at most, 300 seconds.
- Midcourse phase. The warhead then travels on a "ballistic" course through the vacuum of space at speeds close to 15,000 mph. The midcourse of the flight is the longest phase of an ICBM's trajectory, lasting up to 20 minutes.
- Terminal phase. The warhead re-enters the atmosphere and closes in on its target. This final, and shortest, phase lasts about 40 seconds until impact.
Ground-based Midcourse Defense (GMD)
In theory, there is a chance to shoot down a missile in each of its three phases of flight. In 1996, the Clinton administration chose to develop a system aimed at intercepting a missile in the midcourse phase. This system, which the Pentagon now calls the Ground-based Midcourse Defense (GMD) system, is the only long-range missile defense system that has reached the stage of testing.
Designed only to defend against a limited attack of a few missiles by a "rogue state," the system would consist of interceptor missiles based at Fort Greely, Alaska, and a new X-Band tracking radar at Shemya Island, in the Aleutians.
How Midcourse Defense Is Supposed to Work
To intercept an enemy warhead during the midcourse of its flight, an "Exoatmospheric Kill Vehicle" (EKV) -- a miniaturized package of sensors, computers, and thrusters roughly four and a half feet (1.5 meters) in length -- is launched on top of an interceptor rocket, travels 140 miles up, homes in on the warhead, and destroys it through the sheer force of the collision. [See this diagram of the kill vehicle and booster rocket.] The Pentagon calls this "hit to kill." It has been likened to hitting a bullet with a bullet, though at speeds of 15,000 mph, the kill vehicle and the enemy warhead are traveling much faster than the speed of a bullet.
Once the "kill vehicle" is launched into space, it must be able to distinguish between a warhead and the swarm of decoys almost certain to be launched to disguise it. It is the single most critical technology of midcourse defense, and a task the kill vehicle will have 100 seconds to accomplish.
"We've been trying to develop methods of discriminating decoys in warheads for many years," says Nobel Prize-winning physicist Steven Weinberg. "We may know the nature of the nuclear warhead, but we will not know what other things are sent along with the warhead in the form of decoys that will exhaust our defenses. I don't see how that problem can ever be solved."
The "Hit to Kill" Tests
Since October 1999, the "kill vehicle" component of the Ground-based Midcourse Defense (GMD) system has been tested six times in flight-intercept tests. Four of those six tests have been deemed successful -- meaning that the kill vehicle was able to home in on and destroy the mock warhead as it traveled through space -- but a fuller analysis reveals less encouraging details. In two of the successful tests, for instance, critical elements of the system failed. And, according to former Pentagon director of operational test and evaluation, Philip Coyle, "for all practical purposes, the first five tests have been essentially the same." [See an overview of the "hit to kill" tests conducted between October 1999 and October 2002.]
The Bush Administration's "Layered" Defense
In January 2002, Secretary of Defense Donald Rumsfeld announced the reorganization of what had been the Pentagon's Ballistic Missile Defense Organization, giving it a new name -- the Missile Defense Agency -- and a new mission. Its orders were: "First, to defend the United States, deployed forces, allies and friends from ballistic missile attack. Second, to employ a Ballistic Missile Defense System (BMDS) that layers defenses to intercept missiles in all phases of their flight (i.e. boost, midcourse, and terminal) against all ranges of threats. Third, to enable the Services to field elements of the overall BMDS as soon as practicable."
Unlike previous administrations, which organized and funded missile defense research and development along the lines of "national missile defense" (designed to defend U.S. territory against long-range missiles) and "theater missile defense" (to defend U.S. troops and allies against short-range missiles) the Bush administration has combined all of its efforts under a single heading: "ballistic missile defense."
"What we're trying to do with our system is shoot it in the boost phase, shoot it in the midcourse phase, and shoot it, if we can, in the terminal phase. [It's] a layered defense with multiple shots," explains Lt. Gen. Ronald Kadish, director of the Missile Defense Agency.
For every missile defense system being contemplated by the Bush administration, at least 20 developmental tests must succeed before a move to operational testing -- that is, testing conducted by military service personnel instead of industry contractors and performed in realistic operational environments: at night, in bad weather, in conditions that would approximate battle.
If each test takes a year, then that means 20 years before moving to realistic operational testing. Two tests a year, 10 years to operational testing. The Missile Defense Agency has promised one test every three months. But the most recent test, which took place on Oct. 14, 2002, was delayed by three months.
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Animation: How It's Supposed to Work
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This Defense Dept. animation illustrates the basic concept of the Ground-based Midcourse Defense (GMD) system's "hit to kill" strategy, showing the key components of the system architecture, the flight of an interceptor rocket, the deployment of the "kill vehicle," and the interception of a mock warhead.
 
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Video Excerpt:
Obstacles to Success
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In this section of "Missile Wars," scientific and military experts discuss the artificialities in the early "hit to kill" tests conducted in 1999 and 2000, and the inherent technological difficulties presented by simple countermeasures such as decoy balloons.
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Related Links
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+ Missile Defense Agency
This is the official site of the Pentagon's Missile Defense Agency, the organization within the Defense Department which oversees all missile defense programs, with information on recent developments and an overview of missile defense basics. Also see this special report on missile defense from the Defense Department's American Forces Information Service (AFIF).
+ Union of Concerned Scientists: Missile Defense
This section of the Union of Concerned Scientists' website offers a critique of national missile defense. The section on "Countermeasures" includes a four-minute animation video illustrating the inherent technological hurdles presented by decoys.
+ BBC: U.S. Missile Defence
BBC News Online offers a simple illustration of how the ground-based midcourse defense system might work (in Flash and non-Flash versions).
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Articles
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+ "Can Missile Defense Work?" by Steven Weinberg
"Would a missile defense system actually protect the US against even the sort of attack that might be launched by rogue states like North Korea or Iraq?" (The New York Review of Books, Feb. 14, 2002)
+ "Why Missile Defense Won't Work," by Theodore A. Postol
"Whether or not one believes there is any threat serious enough to require deployment of a national missile defense, it makes no sense to advocate a concept that will not work." (Technology Review, April 2002)
+ "Rhetoric or Reality? Missile Defense Under Bush," by Philip Coyle
"A lot has happened in missile defense in the first year or so of the Bush administration. But have these actions brought the United States any closer to realizing its missile defense goals, especially deployment of a national missile defense?" (Arms Control Today, May 2002)
+ "Missile Defense After the ABM Treaty," by James M. Lindsay and Michael E. O'Hanlon
"Although Bush's commitment to proceeding with missile defense development is beyond doubt, his precise plans remain unclear. The administration has not settled on a specific missile defense architecture, and its public statements about future deployments are sketchy at best." (The Washington Quarterly, Summer 2002)
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