Hypersonic

Hypersonic describes speeds of Mach 5 or greater, five times the speed of sound, and in defense the word attaches to a class of weapons that combine such speeds with the ability to maneuver, which is what makes them militarily significant. The distinction matters, because ballistic missile warheads have flown at hypersonic speeds for decades, so raw speed alone is not new. What is new about the weapons everyone means when they say hypersonic is the pairing of extreme speed with maneuverability and a relatively low, unpredictable flight path, which together are meant to defeat the missile defenses built to intercept the predictable arcs of traditional ballistic missiles.

The two kinds

Modern hypersonic weapons come in two main forms. A hypersonic glide vehicle is boosted high by a rocket, like a ballistic missile, but instead of arcing predictably through space and falling on its target, it releases a glider that skims along the upper atmosphere at hypersonic speed, maneuvering as it goes, following a flatter, less predictable path that is harder to track and intercept than a ballistic arc. A hypersonic cruise missile is powered throughout its flight by an advanced air-breathing engine, a scramjet, sustaining hypersonic speed at lower altitude, more like an extremely fast cruise missile than a boosted glider. Both aim at the same effect: arriving fast, maneuvering unpredictably, and flying a path that the sensors and interceptors of existing missile defense were not designed to handle, so the defender has less warning, less time to react, and a target that does not follow the trajectory the defense expects.

The strategic anxiety these weapons generated was intense, and it had a specific trigger: Russia and China fielded hypersonic weapons before the United States, which reversed the usual technological order and produced considerable alarm that America had fallen behind in a critical technology. Russia deployed hypersonic systems and used some in Ukraine, China tested advanced hypersonic glide vehicles, and the US accelerated its own programs in response, so hypersonics became a symbol-laden arena of great-power competition, freighted with prestige and fear beyond their purely military effect. Much of the alarm, though, deserves a reality check, because hypersonic weapons also have real limitations that the breathless coverage often omitted.

The reality check

Hypersonic weapons are formidable but not magic, and sober analysis has tempered the early alarm. They are extremely expensive, so no one will field them in the numbers of ordinary missiles, and the same flight through the atmosphere at hypersonic speed that gives them maneuverability also creates challenges, intense heat, communication difficulties, and physical stresses that constrain the design and can degrade accuracy. Their combat use in Ukraine was less dramatic than feared, with some intercepted by existing defenses, suggesting the weapons are not the unstoppable game-changers they were sometimes portrayed as. And they occupy an awkward middle ground, more expensive than the cruise and ballistic missiles that can do many of the same jobs, while offering advantages that matter mainly against the most capable missile defenses, which raises the question of whether their cost is justified for many targets. The honest assessment is that hypersonic weapons are a real and significant capability, genuinely harder to defend against than traditional missiles and worth the great powers' investment, but not the revolution in warfare that the initial alarm suggested, and the defensive response, sensors and interceptors designed for their flight profiles, is developing in turn, continuing the endless measure-countermeasure contest that no weapon escapes.

Why are hypersonic weapons so hard to defend against?

Because they break the assumptions that existing missile defense was built on. Traditional missile defense against ballistic missiles works by detecting the launch, calculating the predictable arc the warhead will follow, and positioning an interceptor to meet it, a solvable problem precisely because the trajectory is predictable. A hypersonic glide vehicle flies lower, so it is detected later and there is less time to react, and it maneuvers, so the defender cannot simply calculate where it will be and send an interceptor there, since the weapon can change course. This combination, later detection, less reaction time, and an unpredictable maneuvering path, defeats the logic of conventional interception, requiring new sensors that can track a maneuvering target through the upper atmosphere and new interceptors fast and agile enough to catch it, capabilities that are being developed but that do not yet exist at the scale needed. That said, defense is not impossible, hypersonic weapons have been intercepted, and the contest between hypersonic offense and the defenses being built to counter it is unfolding as one more round in the perpetual competition between weapons and the means to stop them.