A series of recent missile tests has added new urgency to Japan’s debate about how it can defend itself from Pyongyang’s increasingly sophisticated arsenal.

Even before Tuesday’s claimed test of a missile with intercontinental range, North Korea had fired a rocket in May that reached an altitude of more than 2,000km before plunging into the Sea of Japan at a speed of more than 5km a second.

Meanwhile in late June, Japan and the US tested their new, jointly developed Aegis SM-3 Block 2A interceptor, designed to protect against exactly such a threat. The interceptor failed; the missile got through.

The outcome of these respective experiments is amplifying discussions in Japan about the future of missile defence – and whether a more viable option is not defence but attack. This would involve pre-emptive strikes against Pyongyang’s missiles on the ground in the moments before they are launched, stretching the country’s pacifist constitution to the limit.

“It requires formidable technology and cost to intercept a missile in flight,” says Itsunori Onodera, former defence minister and a member of parliament for the ruling Liberal Democratic party. “To destroy a missile before it is launched is much easier and cheaper.”

Japan’s existing missile defence system has two layers. The first consists of Maritime Self-Defence Force destroyers equipped with the Aegis system, designed to intercept rockets mid-flight.

The second is the short-range Patriot missile, designed to intercept a ballistic missile in its final moments of descent. Defence analysts in Tokyo say this set-up offers a good chance of success against small numbers of unsophisticated missiles.

Multiple missiles

Pyongyang’s recent tests, however, suggest other scenarios. “If North Korea fired many missiles at once – 10 or more – it would be hard to defend against,” says Naruhiko Ueda, a retired lieutenant-general and now president of the Japan Defence Research Centre.

The Block 2A interceptors are part of Japan’s efforts to upgrade its defences. Tokyo is also considering whether to buy the Aegis Ashore system – similar to the sea-based system but installed on land. With just a few sites, it would be possible to protect most of Japan, improving coverage and freeing up destroyers for other missions.

“The problem with Aegis Ashore is simply cost and time to deploy the system,” says Ben Goodlad, principal analyst at IHS Jane’s in London.

An alternative is adding a third layer of interceptors via the Terminal High-Altitude Area Defence (Thaad) system – whose deployment in South Korea sparked a dispute between Seoul and Beijing because its powerful radar can reach into China.

Made by Lockheed Martin, Thaad is similar to Patriot but intercepts at a greater height. It could be installed relatively quickly compared with Aegis Ashore, but covering the whole of Japan would be costly. Nor does it protect against future advances in Pyongyang’s arsenal.

This concern is prompting a search for more radical options. One idea is to develop a “boost-phase” defence system designed to intercept missiles shortly after they leave the ground, when they are moving slowly and are much easier to hit.

Warhead debris

“If it were possible to shoot down a missile in the boost phase reliably, I think it would have a large deterrent effect, because its chemical or nuclear warhead would fall on the aggressor’s country and they would suffer the damage,” says Mr Onodera.

A boost-phase system appeals to those who want to build up Japan’s defence industry, but development could take decades and cost trillions of yen. “It’s technically very difficult,” says Mr Goodlad. “You have to have weapons in close proximity to the launch.” One possibility would be drones carrying interceptors constantly circling off the North Korean coast – but that would also be problematic under Japan’s constitution.

Mr Onodera argues that acquiring the ability to destroy North Korean missiles on the ground before launch would be a constitutional act of self-defence. Unlike the technological arms race of missile defence, he says, the cost of a strike does not increase as Pyongyang’s rockets become more sophisticated.

When combined with a strike capacity, says Mr Onodera, Japan’s existing missile shield makes sense as a way to tackle any rockets that do manage to launch. “I don’t think it would be possible to destroy 100 per cent of North Korean missiles on the ground,” he says. “Both are needed.” – (Copyright The Financial Times Limited 2017)