ON DECEMBER 26TH a hatch at the Dombarovskiy missile base in the Ural Mountains flipped open and sent a missile streaking into the Russian sky. But the missile’s warhead did not swing back down to earth with a clean, predictable arc. Instead, a re-entry vehicle detached and steered itself, unpowered, across the sky at whopping speed onto a target in Kamchatka, several thousand miles away. The test of Avangard, a hypersonic boost-glide weapon, was a “perfect New Year’s gift for the country”, said President Vladimir Putin. Russia’s test highlights the early stages of what could become a hypersonic arms race between America, Russia and China, as all three countries prepare for a new era of faster, smarter and more nimble missiles. What are hypersonic weapons and will they change the character of war?

Hypersonic weapons are those that can travel more than five times the speed of sound, or around one mile (1.6km) per second. They come in two flavours. Hypersonic cruise missiles are powered by rockets or jets throughout their flight. They are simply faster versions of existing cruise missiles, like the Tomahawk. Hypersonic boost-glide weapons are different. They are launched into the upper atmosphere in the normal fashion atop existing ballistic missiles, but then release hypersonic glide vehicles (HGVs) which fly lower, faster and—to an adversary—much more unpredictably than old-fashioned re-entry vehicles. Though some, like the Avangard, are intended to carry nuclear warheads, others can use their high speed and accuracy to destroy targets with the kinetic energy of impact alone. At ten times the speed of sound, a kilogram of anything has more kinetic energy than you get from exploding a kilogram of TNT. Current ballistic weapons are very fast, but not as manoeuvrable; current cruise missiles are very manoeuvrable, but not as fast. Hypersonic cruise missiles and HGVs are novel because they fuse these qualities of speed and agility.

Russia’s test of the Avangard, which followed the test of another hypersonic missile in March 2018, is likely to spur on its rivals. China has been testing prototypes for years. America more than doubled its budget for hypersonics between 2018 and 2019, building on an earlier initiative, referred to as conventional prompt global strike, aimed at allowing it to strike anywhere on Earth in under an hour. It hopes to field the first weapons in the early to mid 2020s, and to churn them out on a large scale. France, India, Australia and Japan are all working on hypersonic technologies of their own.

Hypersonic missiles built in large numbers could pose a serious challenge to missile defences. The low-altitude path of HGVs combined with the curvature of the Earth helps them hide from radar. Their speed gives adversaries less time to respond. And their manoeuvrability makes them harder to intercept. “Point” defence systems like America’s Terminal High-Altitude Area Defence, which protects fixed sites from incoming missiles, might remain effective. But the unpredictable trajectory of HGVs for most of their flight allows them to hold a huge area at risk, even switching target midcourse. It will “completely change the balance between offence and defence”, says a French official. A missile travelling 1,000km at ten times the speed of sound would reduce response times to six minutes, notes a RAND study. It could be mere seconds between the time the target is known for certain and the moment of impact. That could force jittery leaders to devolve control of their own weapons to military commanders or even to launch on the mere warning of an attack. And since HGVs are so much harder to spot, a larger number of lower-altitude satellites will have to be deployed; they, in turn, will become juicy targets in wartime.