‘Once the rockets are up, who cares where they come down?/ That’s not my department!’ says Wernher von Braun.
TOM LEHRER’S satirical ditty on the Nazi-turned-American rocketeer was faithful to the essence of early missile development, whose principal challenge was hoisting the weapons into the sky. Gravity did most of the rest. The first warheads capable of steering on descent did not arrive until the 1980s. Even they were limited in how much they could move around, making it pretty easy to predict their target area.
A new generation of hypersonic missiles is changing all that. Some might be capable of gliding across continents at great speed, their target unpredictable until seconds before impact. Russia claims to have a hypersonic glider on the cusp of deployment; others are redoubling their efforts. Many are likely to start entering service in the 2020s. All this opens up new military possibilities—and problems.
Missiles that travel at speeds greater than Mach 5 (five times the speed of sound, or about 1.5km per second), have existed for some time. Intercontinental ballistic missiles (ICBMs) re-enter the atmosphere at up to 8km per second. What is different about the hypersonic weapons in the pipeline is that they are designed to sustain such speeds over long distances, manoeuvre as...