Missile Defense Strategy Not Feasible Against Potential Threats, Shows APS Study

Boost-phase defense (disabling ballistic missiles while they are still under power) has recently received much attention as one possible element of a National Missile Defense system. However, the report shows that issues of timing severely limit the feasibility of this approach. The short time window available for disabling an enemy missile means that interceptor rockets would have to be based close to enemy territory to have a chance of intercepting the missile in time, if it is possible at all.

The study found that defending the United States against solid-propellant ICBMs would be impractical in many cases, because of their short burn times. According to the U.S. intelligence community, countries of concern could deploy such ICBMs within 10 to 15 years, about the same time the study judged would be required for the United States to field a boost-phase defense against ICBMs. Even against the longer burning liquid-propellant ICBMs that North Korea or Iran might initially deploy, a boost-phase defense would have limited use due to the requirement that interceptors be based close to potential missile flight paths.

"Only two to three minutes would be available to achieve a boost-phase intercept, even assuming substantial improvements in systems for detecting and tracking missiles," said Study Group co-chair Frederick Lamb. "Consequently, even fast interceptors could have difficulty catching liquid-propellant ICBMs and would be unable to catch solid-propellant ICBMs in time. In the most optimistic scenarios, the defense would have only seconds to decide whether to fire interceptors and could be required to make this decision before knowing whether a rocket launch were a space mission or a missile attack."

However, boost-phase defense against short- or medium-range missiles launched from ships off U.S. coasts appears technically possible, provided ships carrying interceptors could stay within about 40 kilometers of the threatening ships.

"This study was conducted for the American Physical Society by a group that included recognized experts on missile defense. The group assessed the feasibility of boost-phase intercept in terms of fundamental science and engineering requirements," said APS President Myriam Sarachik. "It is crucial that decisions about large-scale investments in weapons systems take into account their technical feasibility. The APS hopes this report will help in evaluating whether to build boost-phase defense systems."

The APS Study Group looked at boost-phase defense systems utilizing land-, sea, or air-based interceptors, space-based interceptors, or the Airborne Laser.

The effectiveness of interceptor rockets would be limited by the short time window for intercept, which requires interceptors to be based within 400 to 1,000 kilometers of the possible boost-phase flight paths of attacking missiles. In some cases this is closer than political geography allows. Even interceptors that were very large and fast and that pushed the state of the art would in most cases be unable to intercept solid-propellant ICBMs before they released their warheads.

A system of space-based interceptors, also constrained by the short time window for intercept, would require a fleet of a thousand or more orbiting satellites just to intercept a single missile. Deploying such a fleet would require a five- to tenfold increase in the United States' annual space-launch capabilities.

The Airborne Laser currently in development has the potential to intercept liquid-propellant ICBMs, but its range would be limited and it would therefore be vulnerable to counterattack. The Airborne Laser would not be able to disable solid-propellant ICBMs at ranges useful for defending the United States.

"Few of the components exist for deploying an effective boost-phase defense against liquid-propellant ICBMs and some essential components would take at least 10 years to develop," said Study Group co-chair Daniel Kleppner. "According to U.S. intelligence estimates, North Korea and Iran could develop or acquire solid-propellant ICBMs within the next 10 to 15 years. Consequently, a boost-phase defense effective only against liquid-propellant ICBMs would risk being obsolete when deployed."

Although a successful intercept would prevent munitions from reaching their target, live nuclear, biological, or chemical warheads could strike populated areas short of the target in the United States or in other countries, shows the study. This "shortfall problem" is inherent in any boost-phase defense and difficult to avoid.