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​Lt. Gen. Samuel Greaves, director of the Missile Defense Agency, presented this graphic to show a proposed system of space-based sensors to help warn and track both ballistic-missile and hypersonic threats. C-SPAN screenshot.

The US must use space-based sensors to protect itself from the increasing threat of hypersonic weapons, said top Pentagon leaders and the head of the Missile Defense Agency on Tuesday.

The current US missile defense system meets today's threat, but the Missile Defense Agency needs additional developments and investment to "get ahead," MDA boss USAF Lt. Gen. Samuel Greaves said at a Missile Defense Advocacy Alliance event in Washington, D.C. There is developing momentum and interest in a space-based sensor layer as the need to prepare for great power competition under the National Defense Strategy grows, he said.

To protect from hypersonic threats, the US needs a sensor architecture that can watch the Earth in areas that are currently denied, and do so in real-time, Under Secretary of Defense for Research and Engineering Michael Griffin said.

"We do not have systems that give us globally comprehensive, persistent, timely, multi-mode awareness of what is going on on Earth, everywhere all the time," Griffin said. "We don't have that."

The US cannot see a hypersonic threat until it is too late. "To effectively deal with the hypersonic threat, we need to ensure we know what it's doing from birth to death," Greaves said. Unlike a ballistic threat, which can be predicted, hypersonics are maneuverable so determining its path is much more difficult without a sensor tracking it.

And to have an offensive hypersonic strike, the military needs to know where the targets are "right now." The "best hypersonic strike capability in the world … is of no value" unless targets are tracked in real time.

"Those requirements alone … drive me toward a space sensor layer," Griffin said.

Griffin said he is "very tired" about the discussion of space capabilities being cost prohibitive, saying he "can't get" to cost levels in the hundreds of billions. For example, at a cost of $20,000 per kilogram to get equipment to low-Earth orbit and a layer of 1,000 interceptors with each weighing one ton, that entire cost would be $20 billion. Compared to other Pentagon programs, "We've paid a lot more and gotten a lot less," Griffin said.

See also John A. Tirpak's article, The Great Hypersonics Race, from the August issue of Air Force Magazine.