Secretary Gates and the North Korean Missile Threat

On January 11, U.S. Secretary of Defense Robert Gates made headlines during his trip to China by forecasting an increased threat of nuclear missile attack on the United States by North Korea. Referring to the next five years, Gates said, “I think that North Korea will have developed an intercontinental ballistic missile within that time frame,” but added, “I believe they will have a very limited capability.”[1]

This immediately led to questions about the meaning of what appeared to be a new threat assessment. Some reports said it reflected new U.S. intelligence on North Korean missile and nuclear programs, but others believed this was not the case.[2] Indeed, another report cited “U.S. officials” as saying that the assessment was not new but simply reflected the National Intelligence Estimate (NIE) on North Korea from 2001, which predicted that “before 2015 the United States most likely will face ICBM threats from North Korea.”[3]

A lot has happened since the 2001 NIE, however, raising the question why Secretary Gates would have made such a high-profile statement based on an old assessment. His comments may have been intended as a signal to China, urging it to take steps to make sure this threat does not materialize, or else it would lead to additional U.S. involvement in the region to protect itself.

On the other hand, Gates’ statement that “We consider this a situation of real concern and we think there is some urgency to proceeding down the track of negotiations and engagement,” may indicate that he was highlighting the threat as a prelude to resuming talks with North Korea with the goal of putting limits on its nuclear and missile programs.[4]

Whatever is behind Secretary Gates’ statement, it is worth looking at the current status of North Korea’s missile program, and what further developments would be required to for it to pose a threat to U.S. territory. There are few hard facts about the state of North Korea’s missile and nuclear programs. However, based on what has been seen it is possible to make various assessments.

I argue below that North Korea has shown the ability to make technical progress on both its missile and nuclear programs, and is capable of putting together the components to make a long-range missile. Should it decide to push ahead in a focused way, Pyongyang could eventually develop a limited—and unreliable—ability to strike U.S. territory with a long-range missile carrying a crude nuclear warhead.

Whether that would take five years or longer is hard to say. However, past NIEs have continually overestimated the pace of North Korea’s advancement. To what extent North Korea’s actual pace is determined by technical or resource limitations is not clear. But a significant part of the slow pace of its missile program over the past 15 years was the result of the flight test moratorium that Pyongyang observed from 1998 to 2006, which North Korea saw as a quid pro quo for ongoing discussions with the United States. The most effective and verifiable way of preventing Secretary Gates’ prediction from coming to pass would be to convince the North to reinstate a test moratorium, as he has suggested.

Status of North Korea’s Missile Program

The longest range missile that North Korea currently deploys is the Nodong, which is believed to have a range of up to 1,300 km with a 700 kg payload. While there has been speculation about a longer range theater missile based on the Soviet R-7 missile (called SS-N-6 in the United States), which was first deployed as a sea-based weapon by the Soviet Union in 1968, there have been no flight tests of this weapon.

The Unha-2, which North Korea used in an unsuccessful satellite launch in April 2009, represents a significant advance over its past launchers.[5] This is in large part due to the technology used in the second stage, which is considerably more advanced than that seen in previous tests.

The Unha-2 appears to be constructed from components, such as the second stage, that may not have been manufactured in North Korea. If true, this would mean that North Korea’s indigenous missile capability could be significantly constrained if it can be denied further access to such components. An important factor in understanding the North’s program is therefore to clarify this issue.[6]

While North Korea has twice demonstrated an ability to successfully use staging in a launch—in the Unha-2 and the TaepoDong-1 (TD-1) in 1998—neither test was fully successful since not all of the missile components worked. For example, the third stage of the Unha-2 reportedly separated from the second stage but did not ignite. Had the third stage operated properly it may have been able to place a small satellite (with a mass of up to a couple hundred kilograms) into orbit at an altitude of approximately 500 km.

The Unha-2 launcher represents a significant increase in capability over the TD-1. It is considerably larger than the TaepoDong, with a first-stage diameter of 2.4 meters compared to 1.3 meters. The Unha is also more than three-times larger in mass than the TD-1 (roughly 80 vs. 25 metric tons). This is an important consideration since rockets consist mainly of fuel and the amount of payload they can lift is roughly proportional to their overall mass.

The first stage appears to use a cluster of four Nodong engines housed in a single missile casing and sharing a common fuel tank; the first stage of the TD-1 contained only a single Nodong engine.

The second stage appears identical to the Soviet SS-N-6. There have been reports for years that North Korea had acquired some number of SS-N-6 missiles in the 1990s.[7]

The SS-N-6 utilizes liquid fuels (UDMH and nitrogen tetroxide) that are more advanced than those used in the Scud-B, a 300 km range Soviet missile from the 1960s, and therefore has a high thrust. Since the SS-N-6 was deployed on submarines, the missile has a compact design with a lightweight aluminum casing.[8]

The third stage of the Unha-2 appears to be very similar if not identical to the upper stage of the Iranian Safir-2 launch vehicle, which placed a small satellite in orbit in February 2009. Both appear to use the small steering motors from the SS-N-6 for propulsion.[9] This appears to be a concrete indication of cooperation between the Iranian and North Korean programs.

Capability as a Ballistic Missile

North Korea has conducted two nuclear tests, but is not thought to have designed a nuclear warhead that can be delivered by a missile. A device built for an underground test is not constrained by size and weight limits. Turning it into a deliverable warhead has proved to be difficult for countries in the past, requiring a number of years and multiple nuclear tests.[10] A first generation plutonium warhead could have a mass of 1,000 to 1,500 kg. North Korea is currently thought to have enough separated plutonium for fewer than 10 weapons.

Figure 1 shows the ranges from the North Korean launch site to several locations. North Korea already has a missile capability against Japan with its Nodong missile.

Distances from the North Korean launch site to locations around the world (figure made in Google Earth).

While some experts refer to a possible North Korean long-range missile as the TaepoDong-2 (TD-2), that name does not appear to refer to a specific missile configuration. Any long-range missile North Korea might develop in the near term would probably be a variant of the Unha-2 launcher. Analysis[11] suggests that if the Unha-2 could carry a 1,000 kg warhead, it would have a range of 10,000 to 10,500 km, allowing the missile to reach Alaska, Hawaii, and roughly half of the lower 48 states.

However, since the Unha-2 was designed to launch a relatively lightweight satellite, its structure is unlikely to be strong enough to allow it to carry a warhead of that weight. The upper stage would probably have to be redesigned, and the additional structure would add mass that would reduce the missile’s range to less than 10,000 km. Similarly, the range would be shorter if the warhead mass was greater than 1,000 kg.

If a 1,000-kg payload were instead launched by the first two stages of the Unha-2, its range could be 7,000-7,500 km, allowing the delivery system to reach Alaska and parts of Hawaii, but not the lower 48 U.S. states.

There are a number of hurdles North Korea would have to overcome in order to field such a missile. First, since Pyongyang is not believed to have a nuclear weapon light enough to deliver on such a missile, it would need to conduct further nuclear tests to reduce the warhead’s size and weight.

Second, in order to deliver a warhead on a long-range missile, North Korea would need to develop a reentry heat shield. Reentry heating increases rapidly with the reentry speed of a missile, so a 10,000 km range missile would require a significantly better heat shield than that developed for the much shorter range Nodong missile. Because heat shield techniques and materials have been known for many years, North Korea should be able to develop an adequate shield, but that shield could still be a major source of missile inaccuracy.

Developing a heat shield that gives relatively high accuracy is a very difficult engineering task. For example, making the warhead very blunt rather than pointed would cause it to lose most of its speed at high altitude, which would significantly reduce the total heating. But during the slower descent the warhead would be subject to high-altitude winds for a longer time, reducing the accuracy. Streamlining the warhead so that it passes through the atmosphere quickly, on the other hand, leads to high heating rates, which would require more sophisticated heat shielding, such as an ablative coating. Asymmetric ablation can cause strong lateral forces on the warhead during reentry, which would reduce the accuracy. As a result, a missile based on the Unha-2 would likely have an inaccuracy of 10 km or more. Of course, such low accuracy could still be sufficient for a terror weapon aimed at a large population center.

Third, the Unha-2 was test fired from a known, visible site and requires days to prepare for launch. During that time, the missile would be highly vulnerable to attack. Reducing this vulnerability would require launch sites that were concealed from view. For example, missiles could be stored on transporters in caves, and could be rolled out, erected, and fueled. North Korea might try to develop such a capability in the future, but at this point it is still trying to get its missiles to function properly.

Finally, ballistic missiles are complex systems. Given its limited supply of fissile material, North Korea may well be reluctant to place a warhead on a missile that is likely to fail. Understanding and improving launcher reliability would require a series of additional flight tests.

Possible Future Missile Developments

There are a number of steps North Korea might take to develop a launcher with longer range and/or payload capability. For example, it could replace the third stage of the Unha-2 by a stage with higher thrust. It might also improve the thrust of the first stage by using more advanced propellants, and/or decreasing its structural weight by making the body out of lightweight materials, such as aluminum alloys. However, North Korea’s technical ability to make changes of this kind remains unclear.

Gaining substantially more capability would require North Korea to build a significantly larger missile. For example, China launched its first satellite on the Long March 1, which was similar in size and capability to the Unha-2 but had a more advanced first stage. However, for its first intercontinental weapon (the Dong Feng-5), China developed a much larger missile, with an overall mass of 183 metric tons—more than twice the mass of the Unha-2. Developing a delivery vehicle of this size would be a major, multi-year undertaking.

The general assumption for many years has been that in the early 1990s, North Korea successfully reverse-engineered the Soviet Scud missile and began producing its own version. Afterwards, the North was thought to have scaled up the Scud engine to produce the larger Nodong engine. However, there is evidence that North Korea received significant technical assistance from Russian missile designers and that its program may rely heavily on the acquisition of Russian production equipment and possibly key rocket components.

If North Korea is not able to build some key rocket components indigenously, then its missile program may have to rely on combining existing components in clever ways. That could significantly limit which of the steps listed above that North Korea could take in the near term, unless it is able to adapt existing components. For example, it is not clear that North Korea has a rocket engine that can use more advanced propellants. Similarly, Pyongyang may not currently possess or be able to build a third stage of the right mass and thrust to significantly improve the range/payload of the Unha-2.


Despite its resource limitations, North Korea has shown the ability to use what technology it has to build increasingly capable launchers. One must assume that process will continue unless steps are taken to stop it.

However, the pace of North Korea’s progress has consistently been slower than official U.S. estimates. Moreover, a credible threat would not appear out of the blue. Developing a working missile and a deliverable warhead would require North Korea to conduct new missile flight tests and additional nuclear tests which can be detected with high confidence.

If such a threat starts to develop, it is worth emphasizing that the missile defenses the United States is developing cannot be assumed to be an effective response. Any country able to develop or acquire long-range missiles with nuclear warheads would be able to develop or acquire decoys and other countermeasures that could defeat missile defenses, as the 1999 National Intelligence Estimate[12] and other studies[13] have pointed out.

The development of a North Korean threat is not preordained. The example of Pyongyang’s missile flight test moratorium from 1998 to 2006, combined with what appears to be an increased openness to return to the negotiating table, suggests that there may be options for preventing the emergence of the threat that Secretary Gates has highlighted.

[1]L. Shaughnessy, “Gates: North Korea could have long-range missile within 5 years,” CNN, January 11, 2010,

[2] E. Bumiller and D. Sanger, “Gates Warns of North Korea Missile Threat to U.S.,” New York Times, January 11, 2011,

[3] Shaughnessy, “Gates: North Korea could have long-range missile within 5 years.”

[4] Anne Gearan, “Gates: North Korea will pose direct threat to U.S.,” AP, Jan. 12, 2010,

[5] D. Wright and T. Postol, “A Post-Launch Examination of the Unha-2,” Bulletin of the Atomic Scientists, 29 June 2009,

[6] D. Wright, “North Korea’s Missile Program,” September 2010,

[7] Andrew Feickert, “North Korean Ballistic Missile Threat to the United States,” Congressional Research Service, The Library of Congress, 1 October 2003, Reports have also stated that in 2005 Iran bought 18 SS-N-6 missiles from North Korea.

[8] For information about the R-27 missile, see Pavel Podvig, ed, Russian Strategic Nuclear Forces (Cambridge, MA: MIT Press, 2001), pp.319-322.

[9] Geoff Forden, “Safir—Iran Hops Off the SCUD Bandwagon,” 25 August 2008,–Iran%20Hops%20Off%20the%20SCUD%20Bandwagon1.pdf; Theodore Postol, “A Technical Assessment of Iran’s Ballistic Missile Program,” 6 May 2009,

[10] Li Bin, “Nuclear Missile Delivery Capabilities in Emerging Nuclear States,” Science and Global Security, Vol. 6, 1997, pp. 311-331,

[11] Wright and Postol, “A Post-Launch Examination of the Unha-2.”

[12] National Intelligence Council, “Foreign Missile Developments and the Ballistic Missile Threat to the United States Through 2015,”September 1999,

[13] Sessler et al., Countermeasures, April 2000,

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