The main problem is coverage. While the military and the defense industry possess the know-how to build a missile-armed satellite, the sheer number of satellites and missiles that a reliable shield would require makes it impractical.

A satellite in low orbit is constantly moving relative to Earth. “This means an interceptor that is within range of a missile launch site at one moment will quickly move out of range,” David Wright, a physicist with the Massachusetts-based Union of Concerned Scientists, explained in a series of blog posts.

Round-the-clock coverage of all the potential launch sites in North Korea alone could require hundreds, possibly thousands, of armed satellites, Wright estimated. A 2012 study by the National Academies of Science and Engineering projected that such as system would cost at least $300 billion, or roughly half of the military’s entire annual budget.