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.