Hypersonic missiles are weapons that fly inside Earth's atmosphere at speeds above Mach 5. Unlike ballistic missiles, whose high, arcing flightpaths are predictable, hypersonic missiles fly below the coverage of long-range radar and can manoeuvre unpredictably, making them harder to detect and intercept. America, China and Russia have all been developing them.
The Pentagon is pursuing several approaches to detecting hypersonic missiles:
Electro-optical/infrared sensors. Cameras using visible and infrared light can pick up the hot glow caused by air friction. Satellite-borne cameras would have the best coverage but miss missiles flying beneath cloud cover, so a network of lower-altitude sensors on aircraft, airships or floating platforms may be needed. America's Navy has commissioned Surface Optics of San Diego to develop EO/IR sensors with resolutions and refresh rates capable of tracking hypersonic targets.
Acoustic detection. The Navy has also contracted HyperKelp of San Clemente, California, to develop ocean buoys equipped with microphones and AI hardware for real-time sound analysis. Hypersonic missiles produce a sonic boom audible over great distances. Thousands of low-cost buoys could triangulate a missile's position and serve as a tripwire for other sensors. Researchers at Sandia National Laboratories in New Mexico have already used data from the International Monitoring System—a microphone network designed to detect nuclear tests—to track spacecraft re-entering Earth's atmosphere. The buoys have the advantage of being hard to destroy, cheap to deploy and less vulnerable to enemy action than space-based systems.
Acoustic detection of aircraft predates radar. In the 1930s the Royal Air Force deployed parabolic concrete "sound mirrors" along the British coast so observers could hear incoming bombers. A similar principle underpins Ukraine's Sky Fortress, which uses thousands of pole-mounted microphones to track Shahed drones by their distinctive engine sound.
The development of hypersonic missiles and their countermeasures forms part of an ongoing arms race. Long-range hypersonic missiles and "fractional orbital" systems that partly encircle the Earth take more unpredictable routes than traditional ballistic missiles, complicating existing defences. Golden Dome, Donald Trump's proposed missile-defence shield, is partly a response to this threat.
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