A key criticism of the Pentagon’s ground-based midcourse missile defense system scheduled for deployment next fall is that an attacker could employ countermeasures, such as decoys that look like a warhead, to fool the interceptor into missing its target.
The Pentagon contends that near-term foes are not likely to be able to build effective decoys. It also claims that the decoy and countermeasure problem will be addressed by fielding additional defenses designed to destroy ballistic missiles shortly after launch and before any decoy warheads could be deployed—a period known as the boost phase.
But a July report by the American Physical Society (APS)—the largest U.S. society of professional physicists—raised questions about the Pentagon’s strategy. It estimated that boost-phase defenses are a decade away and might only work in the most limited circumstances.
The APS study, conducted by 12 experts, assessed whether land-, air-, sea-, or space-based systems could intercept a long-range ballistic missile three to four minutes after its launch. Though the experts said they made optimistic assumptions about the future performance of possible defenses, the study members told reporters July 17 that carrying out such intercepts would test the bounds of what is physically, technologically, and operationally feasible.
During a missile’s boost phase, it presents a larger, more visible, and slower-moving target than during later flight stages. The missile remains in one piece, its rocket engines are still burning hotly and brightly, and it is still accelerating. After the boost phase ends, the missile’s payload separates from the burned-out engines, leaving smaller, faster, colder, and possibly multiple targets hurtling through space. Temperature matters because proposed U.S. defenses rely on infrared sensors to home in on the target.
The primary constraint on intercepting ballistic missiles during the boost phase is time. Detecting a launch and then formulating an intercept plan would require at least 45 to 65 seconds, leaving less than 200 seconds at most for an interceptor to be fired and to reach its target before the boost phase ended. Field commanders would have little or no time to consult with superiors before firing their interceptors.
The time available for an intercept is constrained further by the fact that currently envisioned boost-phase defenses would destroy an enemy missile’s body but potentially leave its payload untouched. A surviving payload would almost never drop onto where it was launched from but fall somewhere between its launch point and intended target. Therefore, an enemy missile would have to be destroyed early enough in order to stop it from attaining a speed and trajectory that momentum would carry its payload to U.S. territory. The APS experts calculated this requirement could cut as much as 40 seconds from the potential intercept time.
Calculating an intercept so a destroyed missile’s payload would not fall on a U.S. ally, neutral country, or any populated area would be very difficult. Intercept windows to avoid such a possibility range between five to 20 seconds, the study reported. For example, preventing a destroyed, Iranian-launched missile’s payload from landing in western Europe would require the missile to be intercepted within a 10-to 20-second window. A North Korean missile fired at the U.S. interior would have to be shot down within a 10-second time frame to avoid having its payload inadvertently fall on Russia.
Due to the brief time frames for a boost-phase intercept, air-, sea-, and land-based interceptors would generally need to be deployed within 400 to 1,000 kilometers of the projected intercept point, according to the APS study. Consequently, boost-phase defenses would be most viable against relatively small countries, such as North Korea, and offer no protection against missiles launched from deep within the borders of large countries such as China and Russia.
Iran, which Washington charges is seeking nuclear weapons and long-range ballistic missiles, presents a more ambiguous case. The APS study concluded that a terrestrial-based boost-phase defense could potentially shoot down a liquid-fueled Iranian missile, but not one powered by faster-burning solid-fuel engines because its boost phase is shorter.
Intercepting long-range missiles fired from a small country or a liquid-fueled missile from Iran would still require the development of interceptors more powerful and larger than any models the United States has built, according to the report. Ideally, interceptors would need to reach speeds up to six to 10 kilometers per second. Long-range ballistic missiles travel seven to eight kilometers per second.
The new interceptors would need to be based on land or ships bordering the potential missile launch point. In the cases of North Korea and Iran, this would require convincing countries that are not close U.S. military allies, such as China or Turkmenistan, to host U.S. interceptors on their territories. Ships could be stationed in nearby seas, though they would likely need protective escorts.
Another proposed alternative is to use a plane armed with a laser to knock out missiles rising toward space. The APS study judged that the Pentagon’s current Airborne Laser (ABL), which has seen its first intercept test slip by at least two years to 2005 because of problems coupling the laser and the plane, could counter liquid-fuel missiles—but not more heat-resistant solid-fuel missiles—launched by small countries. Because the ABL, which is initially designed to counter short- and medium-range ballistic missiles, has to be relatively close to its target to be effective, the APS experts deemed that it would be too vulnerable to enemy attack to be of any use against larger countries.
A possible solution to overcoming problems posed by geography is to place interceptors in space. Yet, the APS study estimated that approximately 1,000 space-based interceptors would need to be deployed to guard against a single missile launch because each individual interceptor would only cover a certain spot on Earth for a short period.
Boost-phase defenses themselves are not immune from countermeasures, the report found. A missile can be programmed to execute evasive maneuvers early in its flight. Moreover, a missile’s acceleration rate can vary unpredictably, making it difficult for a missile defense system to predict a missile’s course and pinpoint when it will reach a certain location.
The Pentagon’s Missile Defense Agency has stood by the boost-phase concept. “There’s no reason to believe we can’t develop an interceptor fast enough to perform boost-phase intercept,” agency spokesman Rick Lehner said Aug. 7.