Explainer: what is ballistic missile defence – and would it stop a missile from North Korea?

North Korea’s test this week of an intercontinental ballistic missile has reignited interest and debate on the feasibility of ballistic missile defence systems, and whether countries such as Australia should seek to acquire them.

But what are these systems, and how do they work? How effective would they be in providing a defence against a potential missile attack?

How do they work?

All ballistic missile defence systems consist of a network of tracking and guidance radars, and the interceptor launchers.

On detecting a ballistic missile launch, the radars track the missile’s trajectory, fire an interceptor to shoot it down, and prepare further interceptors to be launched if the first one misses.

This is referred to as a “shoot-look-shoot” strategy, as opposed to a strategy of saturation – where the defender simply shoots as many interceptors as possible in the hope of achieving a kill.

Modern defence systems use interceptor missiles carrying kinetic kill vehicles. These are warheads that are non-explosive and designed to destroy incoming ballistic missiles by simply crashing into them.

All of the systems mentioned below are intended to work in conjunction with one another. They are integrated to provide the ability to shoot down ballistic missiles throughout their flight path. However, they are also capable of operating independently, although with less effectiveness than if operated in conjunction with other systems.

Missile defence systems in the region

The US and its allies in the Asia-Pacific currently deploy several ballistic missile defence systems. These would be used in the unlikely event that North Korea decided to actually launch a ballistic missile attack.

The first and most prominent is Terminal High Altitude Area Defence, or THAAD, which the US has deployed in South Korea. THAAD is designed to shoot down ballistic missiles in the terminal phase of flight – that is, as the ballistic missile is re-entering the atmosphere to strike its target.

The second relevant system is Patriot PAC-3, which is designed to provide late terminal phase interception – that is, after the missile has re-entered the atmosphere. It is deployed by US forces operating in the region, as well as Japan.

Perhaps the most capable system currently in operation in the region is the Aegis naval system, which is deployed on US and Japanese destroyers. It is designed to intercept ballistic missiles in the mid-course phase of flight – that is, when the missile is outside of earth’s atmosphere and transiting to its target.

What all of these systems have in common is they are theatre ballistic missile defence systems, designed to provide protection against short-, medium- and intermediate-range ballistic missiles. Intercontinental ballistic missiles, such as the one tested by North Korea this week, fly too high and fast for these systems to engage with.

Aegis has demonstrated some limited capability to engage targets similar to intercontinental ballistic missiles. It was used to shoot down a malfunctioning spy satellite in 2008, but has never been tested against an actual intercontinental ballistic missile target.

The only system expressly designed to shoot down intercontinental ballistic missiles is the US Ground-based Midcourse Defence. However, this has a very patchy record in testing. By the end of 2017 it will only have 44 interceptors deployed.

How effective are they?

None of these systems is 100% effective, and most have an iffy record in testing. Aegis has succeeded in 35 out of 42 tests, while Ground-based Midcourse Defence has had only ten successes in 18 tests. However, THAAD has been successful in 18 out of 18 tests.

Tests are conducted in favourable conditions – and it is reasonable to expect the success rates to be lower in actual combat use.

The true difficulty lies with intercontinental ballistic missiles. An intercontinental ballistic missile can attain altitudes well in excess of low earth orbit. Those fired on a typical long-range trajectory can exceed 1,200km in altitude. The high-trajectory, short-range test shot North Korea conducted this week attained an altitude of 2,700km.

By way of comparison, the International Space Station orbits at an altitude of around 400km.

However, the altitude intercontinental ballistic missiles attain is only part of the problem. The other major challenge facing ballistic missile defence is the truly enormous speeds that missiles attain during the terminal phase. They often hit or exceed 20 times the speed of sound.

A common comparison used is that ballistic missile defence is akin to shooting a bullet in flight with another bullet. The reality is even more extreme.

For example, a .300 Winchester Magnum (a high-velocity hunting and sniper round) can achieve a velocity of 2,950 feet per second as it leaves the barrel. This equates to 3,237km/h, or 2.62 times the speed of sound. An intercontinental ballistic missile can achieve speeds almost eight times faster than this. As a result, it is almost impossible to reliably defend against such missiles.

This is not necessarily a problem for countries such as Japan and South Korea. Any ballistic missile used by North Korea against them would be a shorter-range ballistic missile that these systems could engage.

However, countries should be mindful that these systems provide limited-to-no capability to defend against intercontinental ballistic missiles. In Australia’s case, the only missiles capable of reaching this far from North Korea are intercontinental ballistic missiles. Thus, even if Australia decided to invest in ballistic missile defence, it would provide little-to-no protection from a potential North Korean nuclear attack.