Hypersonic missiles bring a new element to the ever-evolving competition between missile offense and defense. As former State Department official Christian Brose notes in his book “The Kill Chain,” ballistic missiles travel fast but in a predictable parabolic path. Cruise missiles, on the other hand, travel relatively slowly but are maneuverable and therefore unpredictable.

Hypersonic missiles are both unpredictable and fast — six times faster than a Tomahawk missile. That means they are uniquely suited to defeat our missile defenses.

Not only can’t we counter them, but we also can’t track them adequately at the moment. Over the summer, the head of North American Aerospace Defense Command — yes, that’s NORAD of Cold War fame — said the new missiles would pose “significant challenges to my NORAD capability to provide threat warning and attack assessment.”

The FT reports that the test missile missed its target by roughly two dozen miles. That’s a significant miss but wouldn’t necessarily be that much comfort if the missile were carrying a nuclear payload. And the targeting will presumably be improved — that’s one reason to carry out the test in the first place.