The authors of the Report of the APS Study Group on Boost-Phase Intercept Systems for National Missile Defense are to be commended on the most in-depth public examination of boost-phase missile defense to date. However, their study and the Physics Today article based on it are marred by overstatement. I particularly address the analysis of ground-based interceptors.

The study’s crucial calculation of the earliest possible time to launch an interceptor appears to be flawed. The study authors claim that the defender must wait until a fairly precise track on the offensive missile has been established before launching an interceptor—that wait is a major factor in the firing delay. In a sense, that claim is true: If a defender fires its interceptor too far away from the threat missile’s actual track, the interceptor will be unable to correct course and destroy the threat missile.

The defender can compensate, however, by firing multiple interceptors, each in a direction predicated on a different potential threat-missile trajectory; that option was not considered in the study. Using only a few interceptors, the defender can bracket the range of possible offensive trajectories thoroughly enough that at least one interceptor will always be able to correct course and intercept the threat missile—assuming, of course, that the threat missile can be reached in time. Thus, the defender could possibly shave 15–20 seconds off the launch delay by firing when the enemy missile is detected, rather than waiting to establish its trajectory.

The second problem with the study’s launch-delay calculations, as reported in the Physics Today article, concerns the assessment of cloud cover. The article authors, Daniel Kleppner, Frederick Lamb, and David Mosher, “assumed that a modern system would first see a bright spot when a missile reaches [7 km].” They noted that even “state-of-the-art sensors would not detect a rocket until it has risen above any dense clouds. But at mid-latitudes, dense clouds are relatively rare above 7 km.” Of course, that only shows that detectors would likely see a rocket when or before it reached 7 km; a more careful analysis would have to show that 7 km is both the lower and the upper bound on detection altitude. The study does not contain such an analysis. Because the expectation of lower cloud cover would imply a shorter launch-delay time, it is essential to tighten the 7-km estimate before making such certain conclusions. Estimates should also be made on a more careful regional basis—a general mid-latitudes average might differ significantly from a specific estimate for Iran or North Korea.

Even if one were to accept the article authors’ basic technical analysis, there is still reason to question how they translate it into judgments about North Korea and Iran. Consider the authors’ first conclusion on North Korea: “Using terrestrial-based interceptor rockets to defend the 50 states against liquid-propellant ICBMs [intercontinental ballistic missiles] launched from North Korea may be feasible, but that would push the limits of what is possible physically, technically, and operationally.” Presumably, that statement refers to the 10-km/s I-5 interceptor, since only that interceptor is characterized in the article as “a surface-based interceptor with a performance at the limit of what might be practical.” Yet the study refutes their conclusion—twice! In its executive summary, the study states that defending against North Korea “would require interceptors with speeds of 6.5 km/s”—those would be the less capable I-4, not the envelope-pushing I-5. And further on, in the detailed discussion of North Korea, the study says “To defend [the US], the 5-km/s interceptor would have to be fired with zero decision time.” That the study authors offer no technical reason to reject the zero-decision-time option presumably implies that even the rudimentary 5-km/s I-2 interceptor could do the job.

The article authors assert, “Taking all relevant factors into account, … we reached the conclusion that defending the 50 states against solid-propellant ICBMs, from either North Korea or Iran, would not be feasible.” But again, the study refutes this claim. Regarding North Korea, a statement on page 124 says, “Even the 6.5-km/s interceptor could be used to defend” the US. And farther down that same page, “The giant 10-km/s interceptor … could be used with about 30 s of decision time.” Neither statement agrees with the article’s conclusion that no defense is feasible.

The article’s pessimism with respect to solid-propellant ICBMs isn’t justified for Iran, either. According to the APS study, “It appears that by basing a 10-km/s interceptor in the Caspian Sea and a second one in Afghanistan or Turkmenistan, all 50 states could be defended” (page 94) against missiles launched from central Iran. However, the study further states, “If the launching site for solid-propellant missiles destined for [Washington, DC] … were moved about 200 km to the southeast, this defense would be precluded.” That statement is transparently incorrect. If one extrapolates from the study’s figure 5.17, the base in western Afghanistan would clearly be able to intercept effectively. Thus, the study should not be read as implying that defense against Iran is impossible.

These are technical, rather than political, reasons to question the study’s overall pessimism. Still, because the study contains some optimistic assumptions that might yet be tightened, even a revised version would not necessarily conclude that boost-phase defense is possible. These persistent ambiguities warrant further study.