Ballistic missile passes over Hokkaido as US, South Korea hold military exercise.

At 6am local time on August 29, a ballistic missile was launched from near Pyongyang in North Korea. Flying 2,700 kilometers (about 1,700 miles), the missile arced over Japan's northern island of Hokkaido, prompting Japanese officials to issue a civil defense warning to citizens.

Chiho komoriya @Chihokomoriya
Alarm from JP Gov. "A missile was fired from North Korea. Please evacuate to a sturdy building or basement." #northkorea #Japan
6:08 PM - Aug 28, 2017
https://twitter.com/Chihokomoriya/st...76766705664000

Tuesday's launch was the latest in a renewed barrage from North Korea, apparently in response to ongoing military exercises and US plans to station an anti-ballistic missile defense system in South Korea. While the missile passed over Japan, it's not clear that Japan or the US could have done anything to intercept it. And if they had tried, the attempt may have proven to be an embarrassment—potentially reducing confidence in the ability of the US and its allies to defend against an actual attack.

It is not yet clear whether the missile test was successful other than as a provocation. Reports indicate that the missile broke into three pieces and fell into the ocean 1,180 kilometers (about 730 miles) east of Hokkaido. As it passed over Japan, the missile reached an altitude of 550 kilometers (340 miles)—about the same altitude reached by another recently tested intermediate-range ballistic missile. That missile—the Pukguksong-2—had a much shorter flight of 500 kilometers (310 miles). But Tuesday's flight was much shallower and shorter than the intercontinental ballistic missile North Korea tested in July.

The missile launch came as the US and Japan concluded "Northern Viper," an 18-day joint military exercise in Hokkaido. And the launch comes just three days after a trio of short-range ballistic missile launches coinciding with the start of a US-South Korean joint military exercise. Those missiles flew approximately 155 miles from North Korea's eastern coast into the Pacific. The Korean Central News Agency (KCNA) has not yet reported on the launch. But today, the news service ran a commentary stating that the Terminal High Altitude Air Defense (THAAD) system the US is planning to provide to South Korea "can never offer comfort."

This is not the first time North Korea has launched a missile that passed over Japan. In 1998, North Korea launched a three-stage Taepodong-1 prototype ICBM over Japan without permission in an attempt to orbit North Korea's first satellite, Kwangmyŏngsŏng-1. (The satellite failed to reach orbit.) And in April of 2009, North Korea launched its Taepodong-2 three-stage rocket, again flying over Japan before it crashed into the Pacific.

Not throwing away their shot

While the US and Japan have conducted ballistic missile defense exercises and both have Aegis-equipped ships capable of shooting down some ballistic missiles, it would be extremely difficult for the US or Japan to intercept a North Korean intermediate or intercontinental ballistic missile in flight over Japan toward a target such as Guam. The Aegis system is capable of intercepting shorter-range missiles in mid-course with the SM-3 missile, and it also provides "terminal phase" defense with the SM-2 missile closer to the ballistic missile's target. But it's uncertain whether either system would be successful against a "pop up" attack with an ICBM.

The SM-3 Block IIA has an operational range of about 1,350 miles. But range isn't the issue as much as the speed required to intercept. If a North Korean missile were fired to an altitude of over 500 kilometers, success in a shoot-down would depend greatly on how quickly the missile was tracked and the timing of an interceptor launch. Based on the time/distance envelopes for SM-2 and SM-3 missile intercepts calculated from Joan Johnson-Freese (a professor at the Naval War College and a lecturer at Harvard University) and Ralph Savelsberg (an assistant professor at the Netherlands Defence Academy), an Aegis defender would only have a few minutes to get off a shot at an ICBM launch from North Korea. Aegis-equipped destroyers and cruisers would have to be dangerously close to the North Korean coast to get a chance to strike an ICBM in "boost" phase as it rose and could be vulnerable to North Korean submarines if an actual attack were planned.

More local "point defense" missiles would have a better opportunity to intercept, as they'd have greater warning. But while they might have more time to intercept a missile coming in from a high trajectory at Japan (or to intercept an attack on Guam with missile batteries there), the chances of actually making an atmospheric intercept against such missiles could be substantially lower. The main point in their favor currently is that North Korea's re-entry vehicle technology for ballistic missiles is still relatively primitive, and North Korea doesn't have large numbers of missiles available to overwhelm defenses.

If a missile launched by North Korea was deemed not to be a threat to Japan or the US or South Korea, the best choice available to the US and allies would be to simply observe the launch and collect data. Missing a shot at a missile just passing over Japan could have far-reaching political implications, as it would suggest that anti-ballistic missile systems are incapable of protecting people in South Korea, Japan, or Guam. And if a shot hit the missile, it could further provoke North Korea into much more dangerous actions.