Hypersonic speed is not what makes these missiles hard to stop

In short: Ballistic warheads have flown far faster than Mach 5 for decades, and defenses were built around them. What makes a hypersonic glide vehicle difficult is that it maneuvers and flies low, which shortens warning time and breaks trajectory prediction. In its final seconds it is often slower, and easier to hit, than the hype suggests.

Every new weapon announcement now comes with the same word attached. Hypersonic. The implication is that anything moving above Mach 5 simply outruns air defense. That is not how the physics, or the history, works.

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Mach 5 is old news

The warhead of a long-range ballistic missile reenters the atmosphere at many times the speed of sound. Russia's Avangard glide vehicle is credited with Mach 20, a figure in the same range as intercontinental warheads that have existed since the Cold War. Speed alone was never the barrier.

Ballistic warheads are interceptable for one main reason: they are predictable. Once the boost phase ends, a warhead follows a curve that radars can compute. The interceptor does not chase it. It flies to where the warhead will be.

The real trick is the bend in the path

A hypersonic glide vehicle rides a rocket up, separates, and then glides through the upper atmosphere instead of arcing through space. It can turn. That kills the predicted intercept point, which is the foundation of most missile defense geometry.

It also flies lower. A ground radar sees a ballistic warhead coming over the horizon from far away, but a glider stays under the radar's horizon for much longer. Warning time shrinks. That is the actual threat.

Gliding costs speed

Here is the part that rarely makes the headlines. Every maneuver in the atmosphere bleeds energy. By the terminal phase, a glider is often moving slower than a short-range ballistic warhead, and its evasive tricks work mostly in the thin upper air.

Down low, it becomes a target for systems built for lower-tier threats. The Kinzhal is a useful reality check. Russia marketed the air-launched ballistic missile with a reported top speed of Mach 10, and Ukraine reported the first interception of one in May 2023, about a month after receiving Patriot batteries.

What defenders actually need

The answer is not one faster interceptor. It is a longer sensor chain and a defense that layers several engagement chances on top of each other.

• Space-based tracking, so the glider is followed from launch instead of picked up at the horizon.
• Fire control that keeps updating the intercept point while the target turns.
• Lower-tier interceptors positioned for the terminal phase, where the glider is slowest.
• A command network fast enough to hand the track from one sensor to the next without a gap.

The last item matters more than it sounds. As with any defended airspace, the system behind the interceptor decides whether there is a shot at all.

China, Russia, the United States, France, North Korea and Iran all have glide vehicle programs listed at various stages, from operational to experimental. A summary of the projects and of known countermeasures is kept in the Wikipedia entry on hypersonic glide vehicles.

Next time a launch is described as "unstoppable because it is hypersonic," ask two questions instead. How low does it fly, and how much does it maneuver in the last 50 kilometers?