Joshua Pollack writes: Perhaps no aspect of national defence is as poorly understood as ballistic missile defence. After North Korea’s shot over Japan last week with an intermediate-range ballistic missile, many people wanted to know why it wasn’t shot down. The answers may be disappointing – but hopefully they will also be enlightening.

Focus on missile defence capabilities will only increase after Pyongyang’s claims on Sunday that it had tested a hydrogen bomb that can be loaded on to an intercontinental ballistic missile.

The first and most fundamental issue to understand is that developing and operating ballistic missile defence, or BMD, is an extremely challenging undertaking. Some are better than others, but the resulting systems are inherently limited in their capabilities and roles.

Perhaps the most attractive sort of defences simply do not exist today, and quite probably never will. So-called boost-phase systems are designed to stop ballistic missiles early in flight, while their engines are still firing and they are ascending into the upper atmosphere and beyond. At times, the US has contemplated a global network of boost-phase interceptors that would whirl around the planet in low-Earth orbit, but the complexity and the economics of the idea are forbidding. [Continue reading…]