North Korea is on track to conduct a record number of missile tests this year, with the ultimate goal of developing an intercontinental ballistic missile (ICBM) capable of delivering a nuclear warhead to the continental United States. During his 2017 New Year’s Day speech, North Korean Supreme Leader Kim Jong Un said that his country had “entered the final stage of preparation for the test launch of an ICBM.” North Korean state media outlets have repeatedly asserted that despite international protests, the country has the right to conduct such a test at the time of its choosing. And during an April 15 parade in Pyongyang, the regime showed off two different kinds of vehicles, six apiece, each carrying large missile canisters apparently designed to carry ICBMs.

No one doubts that Kim Jong Un wants an ICBM; many, however, wonder whether his missileers are close to delivering one that works. North Korea today has at least four paths to a working ICBM, and although each has its drawbacks, taken together, they suggest a country that will likely succeed before too long.

WHY BUILD A MISSILE?

There are obvious reasons for North Korea to seek the capability to strike the United States with a nuclear weapon. Washington is Pyongyang’s primary adversary and the one power in the world that threatens Kim Jong Un with the fate that befell Libyan dictator Muammar al-Qaddafi and Iraqi President Saddam Hussein: forcible regime change. The North Koreans believe that Qaddafi, who was toppled by a NATO intervention in 2011, made a fatal mistake by abandoning his nuclear weapons program in 2003 and that Hussein, who was deposed in 2003, doomed himself by allowing the United States to build up its forces in neighboring countries before the Iraq war. Pyongyang wants nuclear weapons both as a deterrent against invasion and as a part of its strategy for repelling one. If war was to break out, North Korea would likely use large numbers of nuclear weapons against U.S. forces in Japan and South Korea, hoping to shock the United States and blunt an invasion.

A threat to use nuclear weapons against U.S. forces throughout South Korea and Japan would be credible only if North Korea is also capable of striking the United States—U.S. officials have suggested that Kim Jong Un would be committing suicide by using nuclear weapons in this way, but they do so safe in the knowledge that Washington and other U.S. cities are out of North Korea’s reach. That is why the development of an ICBM is an essential component of Pyongyang’s nuclear strategy.

THE SCUDS ARE ALRIGHT

The basis of North Korea’s missile force was a pair of Soviet-manufactured Scud-B missiles provided by Egypt in the late 1970s. Pyongyang is believed to have reverse engineered the Egyptian Scuds in order to create missiles of its own. Today, short-range Scud missiles form the bulk of North Korea’s arsenal and have been sold to countries around the world, including Egypt, Iran, Pakistan, Syria, Vietnam, Yemen, and the United Arab Emirates. North Korea has also created a series of longer-range missiles using Scud-based technology, including the Nodong medium-range ballistic missile as well as space-launch vehicles—carrier rockets used to reach outer space—that Western analysts call the Taepodong and Taepodong-2.

The Scud-B missiles that North Korea imported could carry a 2,200-pound payload just under 200 miles. Today, the country’s extended-range Scuds can carry a half-ton payload more than 600 miles, while the Nodong, a larger version of the basic Scud missile, can deliver a similar-sized payload just under 750 miles. North Korea’s most recent missile test, on May 29, was of a Scud-C missile with improved guidance that is more accurate than the regime’s other short-range Scud missiles.

Kim Jong Un inspects a Hwasong-12 rocket in an undated photo, released May 2017. KCNA / Reuters

North Korea has used this basic technology to develop a launcher, which it calls the Kwangmyongsong and the United States calls the Taepodong-2. In theory, Pyongyang could use the Taepodong-2 as an ICBM, and a three-stage version might travel over 9,000 miles—capable of hitting the East Coast of the United States. But the process of assembling and fueling North Korea’s space launchers is lengthy and requires the use of a massive gantry. Such a missile would be vulnerable during its long preparation to launch—an obvious drawback that limits its strategic value. These problems have led North Korea to develop alternative missile systems. One of these is an ICBM called the KN-08.

MUSUDAN-CE THE NIGHT AWAY

After the collapse of the Soviet Union in 1991, North Korea sought more advanced technologies for its missile program—technologies that would scale better than those in the Scud missiles supplied by Egypt. During the 1990s, reports emerged that engineers from Russia’s Makeyev Missile Design Bureau, which had designed the Soviet Union’s submarine-launched ballistic missiles (SLBMs), were helping Pyongyang to copy a Soviet-era SLBM called the SS-N-6. The United States calls the North Korean version of this missile, which appeared in the middle of the following decade, the Musudan.

The Musudan uses more powerful propellants than the Scud, which allows North Korea to build longer-range missiles that are compact enough to be carried by vehicles. But it is also a far more complicated missile. Soviet designers needed to keep the SS-N-6 short enough to fit inside a submarine launch tube, so they submerged the engine inside the fuel tank. This complex design has proved hard for North Korea to replicate. All but one of Pyongyang’s six or eight attempts to launch a Musudan have failed. (It is not always easy to tell what has been launched if it explodes.)

Just as North Korea has attempted to create an ICBM using Scud-based technologies, it has also tried to make one based on the Musudan. Despite the challenges associated with this design, North Korea seems to have used a pair of the submerged engines as the basis for its KN-08, an ICBM that would be able to deliver a nuclear weapon–sized payload to Washington.

No state that has developed an ICBM has been unable to develop a reentry vehicle capable of delivering a warhead.

North Korea conducted a ground test of the KN-08 engine in 2016, demonstrating that the engine works in a controlled environment. But it has yet to conduct a flight test, which is necessary to demonstrate that all the components of the missile would function properly in a realistic launch scenario. Given the Musudan’s awful track record, there are many reasons to think that the first KN-08 flights will fail. Some experts, such as Markus Schiller of ST Analytics, doubt that it can ever be made to work given the engineering challenges posed by the engine. Nevertheless, when Kim Jong Un discussed North Korea’s readiness to conduct an ICBM test this year, the KN-08 was probably the missile he had in mind.

The KN-08 represents a more plausible path to an ICBM than the Taepodong-2. But both missiles emerged from a process of taking existing rocket engines designed by someone else and attempting to use them to cobble together an ICBM. On March 18, North Korea showed something new—a ground test of a new engine that the country’s officials said was completely indigenous (although assistance from countries such as Iran cannot be ruled out). The government’s statement called the event “a historic day which can be called the March 18 revolution.” The new engine appears to use the same propellants as the Musudan, but without the complicated submerged-engine design that has bedeviled North Korea’s engineers.

The statement also warned that “the whole world will soon witness what eventful significance the great victory won today carries.” This was apparently a reference to the Hwasong-12 intermediate-range ballistic missile, which North Korea tested on May 14. Based on images of the missile, my colleagues and I at the Center for Nonproliferation Studies believe the Hwasong-12 uses the new “March 18” engine and is capable of carrying a nuclear weapon about 3,000 miles—just short of the technical definition of an ICBM (3,400 miles) and capable of striking Guam and the Aleutian Islands. Although the Hwasong-12 is not an ICBM, it represents a domestic capacity to design engines that should ultimately manifest itself in far more capable missiles.

Kim Jong Un watches a ground test of the "March 18" engine in an undated photo, released March 2017. KCNA / Reuters

ON SOLID GROUND

Finally, there is North Korea’s rapidly developing solid-fueled missile program. The Taepodong-2, KN-08, and Hwasong-12 all use liquid-fueled engines, which must be fueled prior to launch. Liquid-fueled rockets are typically less mobile and require a larger number of support vehicles, whereas solid-propellant rockets, although they are less powerful, arrive from the factory fully fueled and offer more mobility. Before 2015, North Korea had tested only a few, relatively small, solid-fueled ballistic missiles. But in the past two years, it has successfully tested a two-stage medium-range missile that can be launched from a submarine and, beginning this year, a variant that is launched from land. North Korea’s new generation of solid-fueled missiles represents a far more survivable missile force than its existing Scuds and Nodongs.

It is hard to say how quickly North Korea might develop a solid-propellant ICBM. The major challenge of building such missiles lies in casting motors in the large diameters necessary for them to travel long distances. The diameter of North Korea’s missile is 1.5 meters—an important threshold that, although it represents progress, is well short of the larger diameters needed for an ICBM.

North Korea almost certainly has a compact fission warhead capable of fitting on a future ICBM. Pyongyang claimed that its most recent nuclear test, in September 2016, was for the purpose of standardizing a warhead small enough to arm its ballistic missile force. This was the same language North Korea used earlier in the year when Kim Jong Un posed in front of a mockup of a compact nuclear warhead next to a KN-08—the message clearly being that North Korea had tested the warhead that would arm its ICBM. With five nuclear tests under its belt, North Korea’s claim is line with the progress that other nuclear powers had made at similar points in the development of their programs. For instance, China, by the time of its fifth nuclear test, had both built a nuclear weapon small enough to be delivered by a missile and developed the basic principles for the massive thermonuclear weapon it would test next.

The major question now is not whether the warhead is small enough to mount on an ICBM—it is—but whether it is rugged enough to survive the shock, vibration, and extreme temperatures that a nuclear warhead would experience on an intercontinental trajectory, in which it would be shot into space and then reenter the earth’s atmosphere. A successful intercontinental launch would involve not only the durability of the warhead itself but that of the reentry vehicle—the part of the ICBM that protects the warhead from the incredible heat generated by reentering the atmosphere.

Some analysts, including within the U.S. intelligence community, have expressed doubt about whether North Korea’s current reentry vehicle would survive an ICBM’s journey. But no state that has developed an ICBM has been unable to develop a reentry vehicle capable of delivering a warhead. The warhead fitted to the Hwasong-12 experienced heat loads similar to those of an ICBM (although for a shorter period of time) and survived. Separately, North Korea has published images of an apparently successful ground test of a reentry vehicle last year. Similar doubts were expressed about China’s nuclear warheads in the 1960s, prompting China’s leaders to arm a missile with a live nuclear weapon and launch it across the country to dispel any lingering doubts. Fortunately, North Korea has not chosen to take such a step.

WAITING ON KIM

Any and all of these programs could lead to a functioning ICBM. North Korea could test a KN-08 or a converted Taepodong-2 at any time, although a first KN-08 flight test is likely to fail. Or North Korea could wait to test a far more capable ICBM, based on either the engine displayed on March 18 or the solid-fueled missile program.

North Korea’s state media has stated that the country could test an ICBM at “any time and anywhere determined by the supreme headquarters of the DPRK”—a reference to Kim Jong Un. Kim’s decision, however, will be as much political as technical. Does Kim value a quick demonstration of a crude capability? Is he willing to wait for a more credible ICBM? Or will he wait, hoping to explore diplomatic options to reduce tension on the peninsula? Only time will tell.