If missiles head to Guam, what could stop them?

North Korea’s HONG KONG — threat to launch four interme- diate-range ballistic missiles into the ocean near Guam could mark the first combat test of the sophisticated missile defense systems of the United States and its Asian allies.

The launches might not happen for any number of reasons. North Korea’s Hwasong-12 missiles might fail, or the United States or its allies could destroy them on the launchpad. Japan and the United States might also decide to do nothing and let the missiles splash harm- lessly into the sea.

But if the four Hwasong-12s do make it off the ground, the options for stopping them mostly rely on hitting them on the way down — in their “terminal” phase.

The Hwasong-12, a domestically developed liquid-fu- eled missile, has a maximum range of 3,000 miles, and hits an altitude of about 470 miles on the way to its destination. The velocities needed for those numbers mean that by the time the missile has been in the air one minute, it is already traveling several times the speed of sound.

At those speeds, a missile trying to chase and hit it from behind would have no chance during this part of the flight, called the “boost phase.” The U.S. Terminal High Altitude Area Defense system, or THAAD, several of which are now stationed in South Korea, could use its radar to track the launches of the North Korean missiles. But it is not designed to hit them as they climb into space.

Once the Hwasong-12’s booster burns out and it reaches the edges of the Earth’s atmosphere, it is no longer accelerating, but is traveling blisteringly fast and higher than some satellites. This part of the missile’s trajectory is called “midcourse,” and it is the most difficult time for an interception, because a fast-moving warhead can also release decoy balloons that are hard to distinguish from the real thing.

But destroying an enemy ballistic missile in space is attractive because it keeps high-speed debris and explosions far from the friendly target. Both Japan and the United States have ships equipped with SM-3 missiles designed for ballistic missile defense. They can hit intermediate-range missiles in midcourse, according to Laura Grego, senior scientist at the Global Secu- rity Program of the Union of Concerned Scientists.

But the trajectory of a Hwasong-12 aimed toward Guam could put the midcourse portion of its flight too far past the Sea of Japan, where the country’s ships carrying SM-3s are usually stationed. Intercepting the North Korean missile at this point would be difficult with- out moving those ships closer to Guam.

As with all ballistic missiles, gravity takes over after the midcourse phase, and the warhead falls toward its target.

The SM-3 can intercept a ballistic missile at this point. The U.S. Navy does not typic ally disclose the exact positions of its warships, but several Arleigh Burke- class destroyers, which are equipped with SM-3s, are permanently based in the western Pacific. If they were stationed near Guam, they could take a shot at the Hwasong-12s.

Missile defense is an attrac- tive but tricky strategy in dealing with missile threats. It has been compared to hitting a bullet with a bullet, and even just testing such systems can be expensive. If Japan or the United States shoots down the missiles, North Korea could see it as an escalation, prompting a military response. If they do nothing, and allow the North Korean missiles to fly unharmed, it’s unclear how Pyongyang would interpret it.

On the other hand, if they try to intercept the missiles but fail, it could undermine the credibility of both coun- tries’ assurances that their anti-missile systems can work.