Consider the split-second timing needed to hit a fastball from a major league pitcher, traveling something like 90 miles per hour. The batter must swing at precisely the right moment, within a few milliseconds. Too soon, or too late, and the ball may go foul, too far left or right.
Now consider the split-second timing needed to hit a ballistic missile warhead in space with an interceptor rocket. In order to stop the warhead, the interceptor's "kill vehicle" must be accurate within a few inches, while closing in on the moving target at some 6,710 miles per hour, according to a recent article in Survival by Dean A. Wilkening. If the kill vehicle misses, the warhead gets through. And if the warhead is nuclear, even a small number getting through could be a calamity.
Such daunting physics problems haunt missile defense. The difficulty is even greater if the interceptor must distinguish the real warhead from decoys.
The Cold War showed that building an effective missile defense is exceedingly hard. In 1972, both the United States and Soviet Union signed the ABM treaty to limit defenses. The United States built one system called Safeguard to defend a U.S. missile field in North Dakota.
Safeguard interceptors carried nuclear weapons that would explode in an attempt to block incoming warheads. But it was decommissioned in 1976 after studies showed that it could be simply overwhelmed if the Soviets added more missiles and warheads. The Soviet Union built a system around Moscow with interceptors that were also nuclear armed.
When President Reagan revived the idea of national missile defense in 1983, he appealed to the best scientific minds to solve the physics problems. Reagan's vision put a lot of faith on American superiority in technology, and since then, computing power has indeed taken great leaps forward. The Soviet Union, lacking the technology, responded by planning relatively cheap decoys and countermeasures that could have been used to fool a missile defense system.
At one point, Soviet leader Mikhail Gorbachev was told by his own specialists that Reagan's defensive shield could be easily overwhelmed by simply building more offensive missiles and warheads. Fortunately, Gorbachev didn't want another arms race and didn't go down that road.
Since Reagan's time, missile defense has seized the imagination of many conservatives in the United States. Reagan did not want to build a nuclear-armed missile defense, and today's systems are intended to collide directly with the warhead in order to stop it-- a kinetic "hit-to-kill" that is technically challenging. Congress has approved billions of dollars for missile defense, and President Obama is moving ahead with a limited system for protection of Europe. There's also a ground-based system in California and Alaska. The president's Ballistic Missile Defense Review in 2010 concluded that "the United States is currently protected against limited ICBM attacks." Rather, potential threats are regional, such as Iran and North Korea. Both nations have long-range missile ambitions, but it requires years of research, development and testing; failures like the one North Korea experienced recently are common.
At the same time, some fundamental problems remain unresolved with missile defenses. On April 21, the Associated Press carried a story about the latest difficulties, including questions about the adequacy of radars. The article and others have focused on a little-noticed report from the Defense Science Board last autumn which raised the issue of fake warheads and other distractions.
The DSB is a 50-year-old advisory body intended to give the Pentagon guidance about science and technology. In their report, the board said the success of missile defense systems outside the earth's atmosphere is "predicated on an ability to discriminate…the missile warhead(s) from other pieces of the offensive missile complex, such as rocket bodies, miscellaneous hardware, and intentional countermeasures." The importance of doing this reliably, the experts said, "cannot be overemphasized."
They added that detecting a warhead from decoys and junk "is still not a completely solved problem."
They also declared: "If the defense should find itself in a situation where it is shooting at missile junk or decoys, the impact on the regional interceptor inventory would be dramatic and devastating!" In other words, you might run out of interceptors and not stop the warhead. (The exclamation point is included in the original.)
The U.S. Missile Defense Agency responded to the report by saying it has "a program in place to incrementally introduce discrimination capabilities over time," including both radar and infrared color sensors.
So far, there has not been a successful missile defense test against realistic countermeasures.
Professor Ted Postol of the Massachusetts Institute of Technology, who in the past has cast a critical eye on many aspects of U.S. missile defense, described the problem this way:
The basic problem with discrimination is that the characteristics of objects that can be measured with radars and infrared sensors are not unique and can easily be modified. In order to recognize a warhead you not only need to know what it looks like, but it also needs to look uniquely different from other objects that are not warheads. Making matters yet more complicated, a warhead and other objects can and will look different to a radar or infrared sensor due to changes in orientation relative to the sensor. This results in an array of characteristics that do not result in mathematically unique characteristics that can be used to identify each object, but also results in different estimates which objects are warheads and which are not at different times. It is simply ridiculous to turn these facts of physics on their head to claim that discrimination might be possible at some future time."
A generation ago, Reagan promised that missile defense could make nuclear weapons impotent and obsolete. It was a captivating idea, but did not happen then and is not happening today. In fact, an impasse with Russia over possible cooperation on European missile defense has become an unnecessary stumbling block to the next stage of negotiations to reduce our remaining nuclear arsenals. Certainly, the quest for workable missile defense will go on. But it is time to be realistic -- and not romantic -- about the technology and its limitations.
David E. Hoffman is a Pulitzer Prize-winning author and a contributing editor to Foreign Policy.