DARPA calls it "counter-hypersonics."

The rest of us would call it a way -- or a prayer -- to stop nuclear warheads coming down on our heads at 20 times the speed of sound.

DARPA, the Pentagon's pet research agency, wants an interceptor that can stop weapons that are hypersonic (travel faster than Mach 5). The agency has begun soliciting proposals for Glide Breaker , its project to stop boost-glide vehicles that are lofted high into the atmosphere atop a ballistic missile, and then glide down to Earth. The current exemplar is Russia's Avangard, touted by President Vladimir Putin as unstoppable by anti-missile defenses. The Avangard is lofted by a giant RS-28 Sarmat ICBM, and then glides down to its target at Mach 20. But China and the U.S. are also developing boost-glide vehicles.

DARPA seeks to "develop and demonstrate a technology that is critical for enabling an advanced interceptor capable of engaging maneuvering hypersonic threats in the upper atmosphere." And it wants this technology in a hurry: Glide Breaker should be tested in 2020.  Meanwhile, the Missile Defense Agency -- the Pentagon organization charged with stopping ballistic missiles -- also has its program to develop defenses against hypersonic weapons.

There's a reason for the rush. Hypersonic weapons may be able to penetrate U.S. missile defenses or streak past the defenses of U.S. aircraft carriers. Even more worrisome, they might be armed with conventional warheads to destroy targets -- notably ICBMs in hardened silos -- once thought invulnerable to anything but nuclear weapons.

DARPA's solicitation is light on unclassified details, though it says it wants "innovative solutions" to stop boost-glide vehicles. That's putting it mildly. If shooting down ballistic missiles is hard, then boost-glide vehicles, also known as hypersonic glide vehicles (HGVs), is even harder.

For starters, the gliders don't traverse outer space like an ICBM, but instead soar through the thin upper atmosphere, where they can achieve extremely high speeds while flying too low to be easily detected by early warning radars designed to track ballistic missiles arcing through outer space. For another, while an ICBM warhead follows a predictable (and Mach 23) path as it descends through the atmosphere, a boost-glide vehicle -- like a hobby glider -- can maneuver, which make it much harder to an interceptor to hit.

Intercepting a ballistic missile with an anti-missile has been likened to "hitting a bullet with a bullet." Imagine if the bullet were taking evasive action.

Or put another way, counter-hypersonics encounters all the difficulties of ballistic missile defense against ICBMs, and then some. "The most obvious challenge is the maneuverability of HGVs, which makes it very difficult to maintain track on the vehicle and plan an intercept course using our current capabilities," George Nacouzi, an engineer at the RAND Corp. think tank, told the National Interest . "Flight altitude is also challenging for our current systems. The HGV may fly too high for many endo-atmospheric interceptors and too low to be detected and tracked early by long range radars."

Nacouzi believes there are ways to shoot down HGVs, "but they would involve using a nearly ubiquitous surveillance and tracking system accompanied by strategically positioned very high performance interceptors or, possibly in the future, directed energy weapons." The U.S. is developing these solutions for intercepting ballistic missiles, but they all have drawbacks: directed energy weapons such as lasers can be affected by weather, while having armed drones or aircraft constantly hovering over North Korean missile sites could trigger a war.

James Acton, an arms control expert at the Carnegie Endowment for International Peace, argues that despite their speed, hypersonic weapons can be destroyed by some ballistic missile defense systems such as the Terminal High Altitude Area Defense (THAAD) system. The problem is that THAAD is a point-defense weapon designed to protect a small area: covering the entire United States with THAAD-like defenses would be prohibitively expensive.

So, given current technology, a leak-proof shield that can reliably stop a massed salvo of hypersonic glide vehicles seems doomed to failure.

But maybe the value of counter-hypersonics isn't shooting down these lethal gliders?

DARPA may have captured the real value of counter-hypersonic defenses in a notice for a July 2018 Proposer's Day , where industry had a chance to learn about the project. The notice stated that "a key figure of merit is deterrence: the ability to create large uncertainty for the adversary’s projected probability of mission success and effective raid size."

Note the significance of that phrasing: anti-hypersonic defense is successful not by necessarily destroying every incoming boost-glide vehicle, but by making a potential adversary uncertain of which hypersonic vehicles will get through. It's the equivalent of body armor that will stop only 50 percent of bullets fired at it -- but the attacker can't be sure of whether a particular bullet aimed at a vital spot will hit its target.

That's been the whole basis of nuclear deterrence since the early days of the Cold War. Even if a first strike could destroy much of the enemy's nuclear missiles and bombers, an attacker couldn't be sure that enough nukes would be left over to mount a devastating retaliation.

However, the Achilles heel of ballistic missile defense has been that it's cheaper for an attacker to build an overwhelming mass of missiles and warheads than it is for the defender to build interceptors to stop them. It remains to be seen whether the economics of hypersonic missile defense will be the same.

Michael Peck is a contributing writer for Warrior Maven and The National Interest.

This first appeared in Warrior Maven here.