The Role of Missile Defense in North-East Asia

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ABSTRACT

This paper discusses specific types of missile attacks the Democratic People’s Republic of Korea (DPRK) might launch in a conflict and identifies the key sources of uncertainty that US and allied political and military leaders must take into account in assessing how effective defense systems might be in stopping these attacks. A key finding is that while missile defenses might be able to blunt some kinds of attacks, the DPRK will have options for retaliatory missile attacks that can reach their targets despite the presence of defenses, and Pyongyang will know which options those are. The existence of this second set of cases is crucial for US and allied leaders to recognize if they are considering taking actions under the assumption that defenses will be effective in protecting US and allied populations.

KEYWORDS:

• Missile defense
• nuclear-weapon use
• DPRK
• United States
• Northeast Asia

Acknowledgments

This work was supported in part by the Program on Science and Global Security, Princeton University.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Notes

¹ For a discussion of the potential roles of missile defense in the strategic context, see Chapter 5 of Grego, Wright, and Lewis (2016a).

² These issues are considered in more detail in Davis et al. (2016).

³ This ineffectiveness was caused by inadvertent countermeasures the Scud’s employed as a result of the spiraling of the warheads during reentry after breaking off from the booster body, and the fact that the Iraqi “stretched Scuds” reentered at higher speeds than the fuse on the Patriots could handle.

⁴ Of course, if the testing conditions do not represent realistic conditions of an attack, then this assessment will overestimate the capability of the defense, as discussed below.

⁵ For an example, see the discussion below of THAAD’s test program.

⁶ Even if a defense system was known with high confidence to have a very high probability of intercepting a missile – say 95% – there would still be a nearly one-in-four chance that at least one missile would penetrate the defense in an attack by five missiles. If the actual intercept probability was 80%, in such an attack at least one missile would penetrate the defense two-thirds of the time. When defending against nuclear-armed missiles, even these high intercept probabilities might be seen as offering inadequate protection to allow freedom of action.

⁷ Note that launching multiple interceptors against each incoming warhead could increase the probability of kill, assuming the kill probability of each interceptor is independent of the others. However, the effective use of countermeasures by the attacker would likely mean that if one interceptor misses its target, all of the interceptors will do so.

⁸ Proposals for space-based defenses require very large systems that are both very expensive and vulnerable to attack.

⁹ For example, a goal for the development of the system that became the US GMD system reportedly was a design requirement of 95% effectiveness with 95% confidence against a small-scale missile attack (Dornheim 1997, 54).

¹⁰ Note that the closing speed of an SM-3 Block II-A interceptor, with a speed of about 4.5 km/s, against a 5,500 km-range ICBM (which has a speed of about 6 km/s) would at most be about 10% larger than the closing speed with a 3,500 km-range target (which has a speed of about 5 km/s) and could be less than that depending on the geometry of the engagement.

¹¹ A 1,000 km range missile would be able to reach speeds of about 3 km/s.

¹² A missile on a depressed trajectory burns the full amount of fuel as on a standard trajectory but reaches a shorter range because it is launched at a lower angle. See Gronlund and Wright (1992).

¹³ The current THAAD deployment is at the Lotte Skyhill Seongju Country Club (36° 3ʹN, 128° 13.5ʹE). This location was picked for political reasons and its rationale is to defend potential targets in the south of the country.

¹⁴ Japan recently decided against deploying Aegis Ashore (Dahlgren 2020), but the basing mode of Aegis would not change these conclusions.

¹⁵ Calculations for this trajectory show that total heating of the reentry vehicle would be essentially the same on these two trajectories, and that the peak heating rate would be several times lower on the depressed trajectory compared to a standard trajectory.

¹⁶ Deploying Aegis Ashore on Guam has been discussed, but there is no current plan to do so.

¹⁷ To date, only one test against an “ICBM-class target” has taken place, but few details are known about this test (US Department of Defense 2020).

¹⁸ These calculations use Nukemap 2.7 (https://nuclearsecrecy.com/nukemap/). The distances are chosen to be somewhat smaller than the 5 psi and 3^rd degree burn radii. The optimal heights of burst for these two cases (chosen to maximize the 5 psi radius) are 0.85 and 1.7 km, respectively, so the warhead would have to go through most of its terminal phase through the atmosphere, which would degrade its accuracy.

¹⁹ Low Earth orbit includes satellites with orbital altitudes below 2,000 km.

²⁰ These distances include regions on Earth within line-of sight of the blast and may overestimate the size of the area where the EMP is significant. For a blast an altitude h, the ground radius is given by r_e*acos(r_e /(h+ r_e)), where r_e is the radius of Earth.

Additional information

Notes on contributors

David Wright

David Wright is a research affiliate in the MIT Department of Nuclear Science and Engineering’s Laboratory for Nuclear Security and Policy. From 1992 to 2020 he was a researcher with the Global Security Program at the Union of Concerned Scientists, serving as co-director of the program from 2002 to 2020. Previously he held research positions in the Defense and Arms Control/Security Studies Program at MIT, the Center for Science and International Affairs at Harvard’s Kennedy School of Government, and the Federation of American Scientists. He received his PhD in theoretical condensed matter physics from Cornell University in 1983 and worked as a research physicist until 1988.