Speed is the new stealth and earlier this week America's top nuclear commander described a grim scenario for U.S. forces facing off against hypersonic weapons.
"We don't have any defense that could deny the employment of such a weapon against us," Air Force Gen. John Hyten, commander of U.S. Strategic Command, told the Senate Armed Services Committee on Tuesday.
"Both Russia and China are aggressively pursuing hypersonic capabilities," Hyten added. "We've watched them test those capabilities."
Researchers and engineers at Rand explain what a hypersonic weapon is, which countries are developing them and how the U.S. could look to defend against them.
What is a hypersonic weapon?
A hypersonic weapon is a missile that travels at Mach 5 or higher, which is at least five times faster than the speed of sound. This means that a hypersonic weapon can travel about one mile per second.
For reference, commercial airliners fly subsonically, just below Mach 1 whereas modern fighter jets can travel supersonically at Mach 2 or Mach 3.
What are some technical requirements needed for hypersonic weapons?
"Once you reach Mach 5 you can't use your traditional jets and just make them go faster," Carrie Lee, a Rand Stanton Nuclear Fellow, explained.
A traditional jet engine could operate up to Mach 3 or Mach 4 and so, anything traveling faster would need an altered system.
"You need a completely different design to unclutter the flow path and sustain combustion of the supersonic airflow inside the engine," said Rand senior engineer George Nacouzi.
The answer would then be a supersonic combustion ramjet (SCRAMJET), which can operate between Mach 5 and Mach 15.
In order to maintain sustained hypersonic flight, a vehicle must also endure the extreme temperatures of flying at such speeds.
"You can think of it as flying into a blowtorch," explained Rich Moore, senior engineer and Rand researcher. "The faster a vehicle flies, the pressure-temperature rises exponentially."
"So you have to have materials that can withstand high temperatures over a long period of time," Nacouzi added.
What types of hypersonic weapons are in development?
There are two types of weapons emerging: hypersonic cruise missiles and hypersonic glide vehicles.
"Hypersonic cruise missiles are powered all the way to their targets using an advanced propulsion system called a SCRAMJET. These are very, very fast. You may have six minutes from the time its launched until the time it strikes," Moore said.
Hypersonic cruise missiles can fly at altitudes up to 100,000 feet whereas hypersonic glide vehicles can fly above 100,000 feet.
Nacouzi said that hypersonic glide vehicles are placed on top of rockets, launched, and then glide on top of the atmosphere.
"It's like a plane with no engine on it. It uses aerodynamic forces to maintain stability to fly along and to maneuver," he added.
What's more, Moore notes that because it's maneuverable "it can keep it's target a secret up until the last few seconds of it's flight."
Which countries are developing hypersonic weapons?
"The U.S., Russia and China are ahead of other nations in developing hypersonic weapons," Richard Speier, adjunct staff with Rand, told CNBC.
Speier, who worked to initiate the Pentagon's Office of Counter-Proliferation Policy, added that France, India, and Australia are also developing military uses of hypersonic technology.
"Japan and various European countries are working on civilian uses of the technology, such as space launch vehicles or civilian airliners, but civilian uses can be adapted for military purposes," Speier noted.
How could the U.S. defend against hypersonic weapons?
"We don't currently have effective defenses against hypersonic weapons because of the way they fly, i.e., they're maneuverable and fly at an altitude our current defense systems are not designed to operate at," he said. "Our whole defensive system is based on the assumption that you're gonna intercept a ballistic object."
Speier explained that ballistic missiles have predictable trajectories akin to a fly ball in baseball.
"A ballistic missile is like a fly ball in baseball, the outfielder knows exactly where to catch it because its path is determined by momentum and gravity."
Nacouzi added that because hypersonic weapons are maneuverable and therefore, unpredictable, they are difficult to defend against.
Looking ahead, Nacouzi said there are potential ways to address hypersonic weapons but that "they will be very expensive."
"As an example, the Missile Defense Agency is proposing developing a space-based sensor system that would be able to track Hypersonic Glide Vehicles globally, this would be one of the first steps in defending against these new missiles."
"Furthermore, the Department of Defense would need additional systems to defeat the missiles," Nacouzi said.