Dr. Greg Canavan’s paper, “Estimates of Performance and Cost for Boost Phase Intercept,” [http://www.marshall.org/article.php?id=262] examines some implications of constellation size and interceptor cost and weight for the total costs and feasibility of a space-based boost-phase interceptor (SBI) system. The paper argues, in general, that a “concentrated” system, that is, one that is tailored to defend against missiles launched from a small geographic area, can be substantially cheaper than is currently believed. North Korea might be considered “small.” The paper states that reductions in expected cost come about from a combination of lower estimates of SBIs mass, lower estimates of individual satellite cost, and a constellation that requires fewer interceptors because it covers only a restricted range of latitudes. We believe that mass and cost estimates are wrong and the simple model of satellite coverage exaggerates the effect of concentration. All the errors together lead to an extreme underestimation of the cost. The paper’s SBI masses are based on unproven and very optimistic estimates of kill vehicle masses; its per satellite costs are based on unrealistic learning curve performance; and a more accurate model of satellite orbits shows that the benefits of concentration are somewhat smaller than the paper’s simple model suggests.