After suffering yet another round of sanctions, as well as a provocative UN speech and further sanctions from President Donald Trump, North Korea's leaders have hinted that more ballistic missile and nuclear weapons tests are to come—including a possible atmospheric nuclear test launched on a ballistic missile.

Given the other antics that North Korea has apparently engaged in—including fake text and social media messages to US Defense Department civilians in South Korea ordering an evacuation of non-combat personnel—making a judgment call on what North Korea may or may not do in the short term is very difficult. But the North Korean military has performed 15 ballistic missile tests this year, including at least two intermediate range ballistic missiles (IRBMs) and one potential intercontinental ballistic missile (ICBM). Combined with North Korea's recent test of what appears to have been a thermonuclear bomb, the new long-range missiles raise the stakes for the US military's ballistic missile defense capabilities.

If North Korea seeks to prove that it can deliver a nuclear weapon effectively, the country's next test could carry an actual warhead. But such a test—or even the suggestion of such a test—could push the US and its allies Japan and South Korea into attempting to shoot down the next launch. That is, if the test fits into the envelope of existing missile defenses—and the risks of a successful (or even failed) test outweigh the risks of trying to shoot it down.

The bigger question is whether the US and its allies are in a position to stop an actual nuclear attack from North Korea. And thanks to some recent events, the answer to that question is uncertain.

Athena’s shieid

Right now, the brunt of the ballistic missile defense mission falls upon the US Navy's Aegis Ballistic Missile Defense (Aegis BMD) system. The Aegis system is being deployed on land as well as aboard ships. Two Aegis Ashore BMD batteries, replicating the entire shipboard system, have been built so far (one in Poland, one in Romania). Japan intends to build an Aegis Ashore system of its own. But, for now, the North Korean intermediate-range missile threat against Japan and Guam is being faced mostly by the Navy's forward-deployed Aegis destroyers and cruisers stationed in Yokosuka, Japan. That force has been diminished by the recent collisions involving the USS Fitzgerald and USS John S. McCain.

The Aegis system began as an air defense system for carrier battle groups, originally intended for a new class of missile cruisers in the 1970s. Since then, it has evolved into a networked sensor and weapons control system with reach far beyond the range of its iconic "stop sign" radar panels. At the core of all the Aegis systems deployed—including the Romanian and Polish Aegis Ashore facilities—is the SPY-1 radar system, a collection of four, 12-foot octagonal panels of S-band radar sensors.

Earlier this year, Ars visited the Navy's USS Rancocas, the "Cruiser in the Cornfield" (also known as the Vice Admiral James H. Doyle Combat Systems Engineering Development Site) and the neighboring Lockheed Martin Aegis assembly operation in Moorestown, New Jersey. Moorestown is where every radar system is given its final "burn in" before delivery. There, we got a walk-through of the guts of an operating Aegis radar room. While Lockheed Martin and the Navy have tested an active phased array version of the SPY-1 (designated the SPY-1E SBARS)—in which each of the more than 4,000 transmitting "radiators" generate its own signal—all the currently deployed SPY-1 systems use passive arrays. The radiators have radio energy generated by eight transmitters pushed through them via a maze of wave guides. (I was not certain if it was the large amount of coffee that I had consumed that day or the several megawatts of radio energy passing inches above my head that made me feel jittery).

The SPY-1D, the current system, is capable of tracking targets the size of a golf ball out to 70 miles, and it can track targets the size of a ballistic missile warhead from more than 180 miles. Soon after the Aegis system's original deployment, it had to be upgraded because the system was triggering alerts from swarms of insects and "ground clutter" caused by stray reflections from terrain. Additional software—Aegis Baseline 9—was added to ships fitted for Aegis BMD, allowing them to track ballistic missiles using input from outside sources. That input includes communication with space-borne and other long-range early-warning sensors, and even shore-based missile defenses like the Patriot system—essential, because its interceptor missile component requires a much farther reach.

Bullet to bullet

The primary interceptor now in use by Aegis BMD ships, the Standard SM-3 IB missile, has been deployed since 2014. It has a range of more than 700 km (380 miles), and it can intercept targets outside the atmosphere—making it the only weapon deployed in the Western Pacific capable of intercepting an IRBM in mid-flight.

Its followup, the SM-3 Block IIA, is advertised as having triple that range (2,500 km, or 1,350 miles). Although it could potentially intercept ICBMs, it's still early in its deployment and won't be fully in service until 2018.

SM-3 missiles are "hit-to-kill" interceptors—they have to collide with a ballistic warhead target to destroy it. So far, the IIA is two for three in intercepting live targets. Its last test failed due to operator error—a tactical data link identified the test missile as friendly, and a confused sailor hit the self-destruct button on the shot. But the Aegis system overall has a much longer record of success than the other major ICBM interceptor system in the US' inventory, the Ground-based Midcourse Defense Interceptor—which had its first successful intercept test against an ICBM class target in June (with a handful of other target intercepts).

Both the GMD and Aegis systems have important advantages over the two other ballistic missile defense systems deployed to South Korea, Japan, and Guam: the Terminal High Altitude Air Defense System (THAAD) and the Patriot Advanced Capablility 3 (PAC3) upgrade to the Patriot Missile System. First of all, unlike THAAD and PAC3, GMD and Aegis can intercept their targets outside the Earth's atmosphere. That means they can prevent the high-altitude explosion of a nuclear warhead, and they can prevent the radioactive debris from a destroyed warhead from potentially raining down on everything below.

THAAD and PAC3 are more of a "point-defense" solution aimed at preventing ballistic missile threats from hitting their target areas, and they have much shorter ranges than the SM-3 missile. They're also intended primarily to defend against medium-range and short-range ballistic missiles, not the higher and faster flying IRBMs and ICBMs.

Aegis is the system that will most likely be brought to bear against any near-term North Korean missile threat to Japan or Guam. But an actual ICBM launch headed for the US would likely fall to the GMD system. That's because, when facing an ICBM launch profile, Aegis BMD ships with the current interceptors would have to hit the launch in boost phase—before it got too high and started flying too fast for the interceptors to catch. That would mean being dangerously close to the North Korean coast, in all likelihood. And if North Korea was really lighting things off, that would leave the country vulnerable to coastal missiles and submarine attacks.

Strange game

On the off chance that North Korea does launch an ICBM toward the US mainland any time soon—just one—there is a high likelihood that it could be shot down by the GMD system as it stands now, from sites in Alaska and California. The US and Japan might also be able to defend with Aegis systems against medium- and intermediate-range attacks on Japan and Guam. The probable success rate of such a defense begins to drop depending on how many missiles North Korea could successfully launch in a first wave.

The situation for South Korea (and much of Japan) is much more precarious because of the number of short and medium-range missiles North Korea is believed to have. Though there are probably under 150 medium and intermediate range missile launchers deployed by North Korea, and far fewer nuclear warheads—and there are significant doubts about the accuracy of any current North Korean missile system—launching everything in one wave at South Korea and Japan could potentially overwhelm defenses, and defenders would have to deal with the fact that some could be conventional missiles or simply decoys intended to eat up defensive assets.

If North Korea does launch an IRBM or ICBM with a thermonuclear warhead to do a demonstration explosion, akin to the US "Frigate Bird" test—a May 6, 1962 test using a live sub-launched Polaris missile detonated over the Johnston Atoll, the only known nuclear test using an actual ballistic missile—it's likely there will be little warning that such a test is taking place, though its timing may be tied to some North Korean anniversary observation or an event. But there are significant risks involved for both sides for this sort of test, which would be the first atmospheric nuclear test since China detonated a bomb in 1980.

North Korea has yet to demonstrate any sort of accuracy in targeting longer-range ballistic weapons, and a launch failure could result in the scattering of plutonium near and downwind of the launch site. If "successful," there could be a range of primary and secondary effects, depending on how high it is detonated, ranging from damage to satellites, terrestrial communications, and electrical systems from the electromagnetic pulse generated by a blast to fallout created from lower-altitude detonations touching the ocean.

If the US or Japan shoot down the missile, there are additional risks—if the warhead is not destroyed outside the atmosphere, a wide scattering of plutonium might float down in the atmosphere over great distances. And if the US and Japan tried and failed to shoot the missile down, such an embarrassment would throw doubt on the ability of the nations' governments to defend against an actual attack.

Listing image by DPRK state media, via Getty Images