US Air Force says it will test bizarre 'hypersonic' weapon this month

At some point in the next few weeks, a B-52H bomber will carry a missile high into the air and launch it at an unprecedented velocity toward its target, according to the U.S. Air Force. If everything goes according to plan, that missile will accelerate to more than five times the speed of sound before deploying a dummy second stage that will promptly "disintegrate" somewhere in the atmosphere.

The missile, known as AGM-183A, is supposed to be the first hypersonic weapon — or Air-launched Rapid Response Weapon (ARRW) — in the U.S. arsenal. It should move so quickly through the atmosphere — about 20 times the speed of sound — at such low altitudes that it's impossible for enemy missile defense systems to shoot out of the air. And its speed means that it can be useful for destroying "high-value, time-sensitive targets," the Air Force said in a statement.

Hypersonic missile designs, including this one, typically involve two stages.

First, a rocket accelerates the weapon to many times the speed of sound, while remaining at a much lower altitude than Intercontinental Ballistic Missiles (ICBMs) that arc high above the atmosphere before delivering their nuclear payloads.

Second, it releases a glider that carries the weapon for the final leg of its journey to the target, riding the atmosphere like a surfer bobbing and weaving over waves — adding another wrinkle to any attempt to shoot it down.

That lower altitude, in theory, makes a hypersonic weapon harder to detect and more difficult to destroy:  It's harder to detect for the same reason that it's harder to see an airplane when you’re standing on the ground at an airport 5 miles away than an airplane 10 miles away in the air approaching that airport to land; the closer an object is to the ground, the more stuff — from trees to buildings to another airplane — gets in the way. And a hypersonic missile is theoretically harder to shoot down for more or less the same reason; most missile defense technology is designed to intercept an ICBM close to the peak of its arc through space. Up there, a missile defense system has a clearer line of sight to the target and the ICBM itself moves in a more predictable way. 

A Mach 20 hypersonic glider would actually move at about the same speed as a decades-old ICBM, which can accelerate to similar velocities during its space travel but must cover a much longer distance to reach the same target. (It's the difference between driving in a straight line from New York to San Francisco and driving between the two cities with a stopover in the Arctic Circle.) 

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The U.S. isn't the only country working on hypersonic weapons technology. As Live Science previously reported, Russian President Vladimir Putin first announced his country's own hypersonic weapons program in 2018, promising the nation's hypersonic weapon would reach Mach 20.

Pavel Podvig, a military analyst, told Live Science at the time that such weapons likely won't be useful..

"It has been described as a weapon in search of a mission," he said. "My take is, you don't really need this kind of capability. It doesn't really change much in terms of ability to hit targets."

That's because ICBMs are already perfectly capable of evading missile defense systems. The U.S. has the most advanced missile defense technology in the world; and according to Union of Concerned Scientists physicist Laura Grego and many other analysts it simply does not work. So it's not clear why a hypersonic missile would be necessary for striking any other country. The Air Force does emphasize the idea that an ARRW might be useful against "time sensitive" targets, because of its high speed (at least compared to non-ICBM missiles typically used to deliver non-nuclear weapons).

The danger of hypersonic weapons, Podvig said, is that they aren't covered by existing treaties designed to prevent arms races. 

And there's still a great deal of uncertainty around the technology. "These systems create greater risks of [strategic] miscalculation," Podvig said, "and it's not clear if we can effectively deal with those risks."

Meanwhile, there are questions about whether hypersonic tech will work at all.

The upcoming test will only demonstrate the missile itself, not the glider, which is the more cutting-edge technology. (Rockets that go very fast have existed for a long time. Gliders that fly many times faster than an F-16 have not.) And, as The Drive pointed out, this test has also been delayed. The missile arrived at Edwards Air Force Base in California on March 1, and the service had originally said the test would take place by March 6. Then, the March 5 statement extended that timeline to "the next 30 days" without explanation.

Meanwhile, an independent analysis published in 2020 in the journal Science and Global Security argued that "fundamental physics" puts hard limits on the usefulness of these weapons. They showed that the physics of atmospheric flight prevent these weapons from ever going fast enough to substantially outpace ICBMs, and that it would be relatively easy to detect a hypersonic missile launch with the right satellite. The idea that hypersonic missiles would offer a revolutionary upgrade over ICBMs, the researchers argued, is a "social" phenomenon, not a scientific one.

Originally published on Live Science.