THE best time to shoot down a hostile missile is straight after take-off. During this initial “boost phase” it moves more slowly, is easier to spot (because its exhaust plumes are so hot) and presents a bigger target (having not yet ditched its first-stage fuel tanks). A bonus is that the debris may come crashing down on the country that launched it—your enemy—rather than you.

But the main advantage of “boost-phase missile defence” is that a missile that has breached the atmosphere and begun its midcourse glide can throw off lots of decoys. In the vacuum of space, tinfoil balloons, or clouds of aluminium strips known as chaff, will keep pace with the missile that released them. Not even the American military can distinguish sophisticated decoys from a warhead (though it might manage with crude ones designed by Iran or North Korea, say).

The downside, though, is that such an approach requires speed. Interceptors (anti-missile missiles) fired from sea or land will probably be too late. Ronald Reagan’s proposed solution was “Star Wars”: armed satellites orbiting above hostile nations’ launchpads. It cost a packet, didn’t work and was scrapped in the 1990s. But some experts say the moment has arrived for a sequel: high-altitude drones.

North Korea’s arsenal of ballistic missiles could probably be countered if as few as three drones were suitably stationed at all times, says Dale Tietz, a former Star Wars analyst. An American Global Hawk drone, which can fly uninterrupted for 30 hours, held 18km above nearby international waters could probably carry several interceptors fast enough to shoot down missiles heading north towards America, he says. It could be alerted to launches by infrared-sensing satellites already in orbit.

Protecting Israel and Europe from Iranian missiles would be harder. Iran is bigger than North Korea, so interceptors would need to be faster (and therefore larger) to reach deep inside its territory. The Pentagon has started to research drone-missile defence, but should be spending more, says David Trachtenberg, a former deputy assistant defence secretary, because the payoff could be “tremendous”.

Such an approach would fail against really big countries like China and Russia (which in any case can launch missiles from undetectable submarines). In one sense this is a plus: what does not work against a country cannot antagonise it. Congress would oppose any system that would spur an arms race, says Kingston Reif of the Centre for Arms Control and Non-Proliferation, a think-tank.

Supporters of drone-missile defence note that America’s existing system, which aims to shoot down hostile intercontinental ballistic missiles with interceptors fired from Alaska and California, has failed every big test since 2008. Sceptics retort that although American drones are stealthy—dozens went undetected over Pakistan during the hunt for Osama bin Laden—better radar and anti-aircraft batteries could render them vulnerable or force them to patrol too far from their intended targets. If North Korea were to develop faster missiles this problem would be compounded, says David Montague, a former head of the missiles division at Lockheed (now Lockheed Martin), a defence firm.

Two years ago a report by the National Research Council advised the Pentagon to give up the attempt to design a boost-phase missile system. The challenge of keeping interceptors close enough to enemy launchpads is “pretty much insurmountable”, says Mr Montague, who was one of the authors. Which camp will prevail is not yet clear. But if the current system fails its next test, probably this summer, the debate will heat up further.