Aug 6th 2025|3 min read
For decades, the state of the art in long-distance warfare was the ballistic missile. Fast and capable of intercontinental ranges, it remains a mainstay of national arsenals. But the predictability of these weapons’ high, arcing flightpaths makes them vulnerable to detection and interception. In recent years America, China and Russia have begun developing hypersonic missiles as an alternative. These fly inside Earth’s atmosphere, below the coverage of long-range radar, and can manoeuvre unpredictably. That makes them trickier to spot coming.
But not impossible. The Pentagon, for example, is pursuing a multipronged approach towards detecting them. One of the approaches being explored involves cameras that use visible and infrared light to pick up the telltale hot glow caused by air friction. Though satellite-borne cameras would have the best coverage, they would miss hypersonic missiles flying beneath cloud cover. Full coverage may therefore require a network of EO/IR (electro-optical/infrared) sensors, as they are known, lower down, on aircraft, airships or floating platforms. America’s Navy has commissioned Surface Optics of San Diego to develop new EO/IR sensors with resolutions and refresh rates capable of tracking hypersonic targets, a task beyond the ability of existing systems.
Another Navy contract, with HyperKelp of San Clemente, California, takes a more innovative approach. This involves buoys equipped with microphones in the deep ocean. The reason, says Graeme Rae, HyperKelp’s boss, is that hypersonic missiles produce a sonic boom audible over great distances. The buoys are also equipped with hardware that can run artificial-intelligence models capable of analysing sound in real time. Even so, the signal processing is still challenging: the faint boom of a hypersonic missile can be difficult to pick out from the background noise of waves, shipping and aircraft.
Acoustic detection of air vehicles predates radar. In the 1930s the Royal Air Force deployed parabolic concrete “sound mirrors” along the British coast (pictured on previous page) so observers could hear incoming bombers, providing detection at night and in bad weather. Faster aircraft and radar rendered these obsolete. But a similar principle has been applied elsewhere to tackle other low-speed intruders. Ukraine’s Sky Fortress has thousands of microphones mounted on poles across the country to track Shahed drones by their distinctive “moped” engine sound.
Tracking hypersonic missiles in this way may seem of limited use; after all, a projectile travelling at Mach 5 will be long past by the time it is heard. But HyperKelp’s plan is to fit thousands of low-cost buoys with microphones both above and below the surface, allowing them to triangulate the position of the source. Even more accurate locations can be obtained by combining data from multiple buoys. If the buoys are located far enough away from American territory, the system could function as a tripwire, providing an initial detection so that other sensors can be pointed towards the threat.
Earlier research has already shown that such a system works. Researchers at Sandia National Laboratories in New Mexico have used data from the International Monitoring System, a network of microphones designed to detect nuclear tests, to successfully track spacecraft re-entering Earth’s atmosphere.
Dr Rae notes that whereas space-based detection systems are visible and vulnerable to enemy action, the buoys are hard to take out. In addition, they could be deployed in sufficient numbers—and over so great an area—that it would be hard to destroy enough to disrupt the network. They are also far more affordable than satellites.
This is, however, likely to be a temporary solution to the challenge of hypersonics. Things rarely stay still for long in an arms race, and the next counter-move is never far away. Hypersonic missiles were developed to evade the radar that spotted ballistic missiles. As they become visible in turn, it is just a matter of time before new means of evasion emerge. ■
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