In the past two weeks Ukraine reportedly intercepted seven Russian Kinzhal missiles – which travel at hypersonic speeds – using its Patriot missile defense system. It is widely believed that Patriot and other current missile defenses could not stop hypersonic weapons, which travel at speeds over Mach five, or five times the speed of sound.

So, what’s going on?

The claim that hypersonic weapons are invincible is one of the many beliefs about these weapons. Here’s why it’s wrong.

Kinzhal is an air-launched ballistic missile with fins that allow it to maneuver as it approaches its target. It is called “hypersonic” since its top speed is reportedly around Mach 10, which would give it a range of somewhat over 1,000 km. This system is not, however, what defense analysts typically mean by the term “hypersonic weapon” since it is not designed to glide over a significant fraction of its trajectory. Its high speed and ability to maneuver, however, mean that it poses a similar challenge as true hypersonic weapons to terminal missile defenses, like Patriot, that are used to defend against weapons of this range.

A maneuvering missile traveling at Mach 10 would be too fast for the U.S. Patriot PAC-3 and similar defense systems to intercept reliably. However, Mach 10 is roughly Kinzhal’s maximum speed, and its speed drops sharply as it reenters and travels through the increasingly dense atmosphere to hit a target on the ground.

Patriot is designed to intercept missiles at low altitudes, and my estimates show that Kinzhal slows enough during its dive that current versions of PAC-3 should be capable of intercepting it. Moreover, reports indicate that at least for the first of the Kinzhal intercepts, Ukraine was not using the most advanced version of the PAC-3 (called MSE, which is 25 percent faster than the previous version).

This analysis has two important implications.

First, Ukraine’s claims that it intercepted Kinzhal missiles are credible, and its defenses may be able to blunt a significant weapon in Russia’s arsenal.

Second and more generally, the medium-range hypersonic glide weapons like those the United States, Russia, and China are currently developing may not be as effective at performing key mission as advocates often claim.

A common argument for building hypersonic weapons is the desire to use them to destroy enemy missile and air defenses early in a conflict, to clear the way for subsequent attacks. Technical modeling, however, shows that the hypersonic weapons the United States has been developing – including the Air-Launched Rapid Response hypersonic Weapon (ARRW), the Army’s Long Range Hypersonic Weapon (LRHW), and the Navy’s Conventional Prompt Strike (CPS) – may also be vulnerable to interception by missile defenses.

In particular, reports about the speeds and ranges of these weapons imply that they begin the glide phase of their trajectory with a speed of about Mach 12 or less. Their speed decreases due to atmospheric drag during the glide phase – especially if they are maneuvering significantly – and will decrease even further as they dive into the thick atmosphere on their way to their targets on the ground. My estimates show that these effects will likely make these systems vulnerable to interception by systems similar to current versions of PAC-3, although intercepting them may require the advanced PAC-3 MSE.

The United States must assume it will face defenses like these in other countries — soon if not now.

To be able to evade such defenses, hypersonic weapons would need to be launched with even higher speeds. Doing so would significantly increase the intense heating they experience during flight, which is a key obstacle to developing these weapons. Increasing their speed also makes them larger and heavier, which reduces the number that aircraft can carry.

Adding propulsion, such as scramjet engines being developed for hypersonic cruise missiles, could help the weapon maintain its speed during the glide phase. But these engines are unlikely to be powerful enough to help much against the exponentially increasing drag encountered during the dive phase, which could leave these weapons vulnerable to interception.

Russian and Chinese hypersonic weapons similar to these U.S. systems (such as the Russian Zircon and Chinese DF-ZF and Starry Sky 2) are also likely to be vulnerable to defenses like PAC-3. In this sense, these weapons do not represent a revolution in threat beyond that posed by medium-range ballistic missiles.

The Ukrainian experience with Kinzhal may be a wake-up call for Russia. It should also be a wake-up call for the United States. Congress and the U.S. military need to think clearly about the missions these weapons can realistically accomplish and whether they justify the high priority and budget share they are getting.

David Wright is a visiting scholar in the MIT Department of Nuclear Science and Engineering’s Laboratory for Nuclear Security and Policy.

More In Opinion