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Laura Grego, Senior Scientist, Global Security Program, Union of Concerned Scientists.
Congress, in the fiscal year 2019 National Defense Authorization Act,  has directed the Pentagon to develop a space-based boost phase missile defense system, whether or not the administration’s as-yet-unreleased Missile Defense Review endorses the concept. This defense would be regionally-focused, with a proposed live-fire intercept test during fiscal year 2022. No money has yet been appropriated to carry out these plans.
The aim is to build a constellation of defensive weapons in space to intercept long-range missiles in their “boost phase,” the first three to five minutes of launch, while their engines are burning. Destroying the missile in boost phase provides an advantage—it catches the missile before it can release decoys and other countermeasures that greatly complicate intercepting during the subsequent midcourse phase, when the missile’s warhead is coasting through the vacuum of space.
Boost-phase defense is also an enormous technical challenge. Because launch is short, the defense must be close enough to the launch site to reach the missile quickly. Because North Korea’s geography—a relatively compact peninsula—allows for the possibility of hosting defenses on its periphery, proposals for boost phase defenses have included putting interceptors or lasers on ships,  drones, or airplanes.  This is the motivation for putting missile defense satellites in low Earth orbits—with altitudes of a few hundred kilometers—that periodically pass over the missile’s launch site.
The concept of a space-based missile defense has a long history. While the Reagan-era concepts were abandoned as technically unworkable and too expensive, proponents have continued to advocate for space-based missile defense, most recently a constellation of orbiting kinetic interceptors or lasers.
The Pentagon itself has not asked for money for such a program since the late 2000s and Pentagon officials have repeatedly voiced doubt that it would be useful or cost-effective. The Pentagon Vice Admiral James Syring, then-director of the Missile Defense Agency, said as much when he testified before the House Armed Services Committee in 2016.  “I have serious concerns about the technical feasibility of interceptors in space,” he said, “and I have serious concerns about the long-term affordability of a program like that.”
This judgement is in line with the best publicly available technical advice. In 2005, the American Physical Society conducted an in-depth study of boost phase missile defense and concluded that space based missile defense would be extremely costly.  In its 2012 study, the National Academy of Sciences and Engineering drew on this work and agreed, concluding that a space-based boost-phase missile defense would cost 10 times more than any terrestrial alternative, and a system providing an austere capability to defend against a few North Korean missiles, a constellation of 600 interceptors costing on order of $300 billion would be required. 
While such a system would rank among the most expensive military projects ever attempted, the most serious issue isn’t the cost—it’s the fragility. The system would be vulnerable to being overwhelmed by the salvo launch of several missiles. Doubling the number of missiles that the system could deal with would require doubling the size of the system. 
Since the interceptors orbit at an altitude of a few hundred kilometers, they are also vulnerable to anti-satellite weapons launched from the ground on short- or medium-range missiles as well as to space-based anti-satellite weapons. Adversaries could use these weapons to create gaps in the defense, rendering it ineffective.
That a space-based missile defense system would be unwise from a military and economic point of view is clearly the case for a fully deployed defense, but it holds true even for a small number of orbiting “testbed” interceptors, which would still have significant security costs. While a small number of boost-phase interceptors would not provide any useful defense against missiles, they could have significant inherent anti-satellite capabilities.
The space-based missile defense interceptors could not only reach satellites in low-earth orbit, they could reach valuable military and commercial satellites in mid-earth and in geosynchronous orbits. Most schemes for space-based missile defense require a burnout-velocity for the interceptors of 4-6 km/s. The interceptors will already be in low-earth orbit, possessing a speed on order of 7 km/s. Using a combined speed of 11-13 km/s, space-based boost phase interceptors could carry a kill vehicle to geosynchronous orbits in around an hour.
Because of the expense and the operational challenges, deployment of a fully-realized space-based missile defense constellation is unlikely. However, it is entirely more plausible that the test bed called for in the defense bill could proceed, and a few interceptors would be developed and place in orbit under the guise of research and development. Although this would be small in scope, it would be big in effect.
Putting prototype interceptors in space would surely be viewed by adversaries and allies alike as putting the first dedicated space weapons in orbit. It would likely encourage development by others of anti-satellite weapons to challenge these systems or of similar space-based technologies. Of course, one cannot know for certain what the actions and counter actions would be. But the likely outcomes—heightened tensions, an arms race, risk of miscalculation and misperception leading to a conflict—all decrease security with little in return. Pursuing space-based missile defense continues to be costly and deeply unwise.
 John S. McCain National Defense Authorization Act for Fiscal Year 2019, H.R. 5515. 115th Cong., 2nd Sess. (2018), Retrieved from https://www.gpo.gov/fdsys/pkg/BILLS-115hr5515enr/pdf/BILLS-115hr5515enr.pdf
 Jan Stupl, Götz Neuneck, "Assessment of Long Range Laser Weapon Engagements: The Case of the Airborne Laser," Science & Global Security, 18, no. 1, (2010): 1-60. Retrieved from http://scienceandglobalsecurity.org/archive/2010/02/assessment_of_long_range_laser.html
 Axe, D. (2017, December 7). The Pentagon’s Plan to Send Stealth Fighters Into North Korea Is ‘Cray Cray’. Retrieved from https://motherboard.vice.com/en_us/article/nedned/pentagon-plan-to-send-stealth-fighters-into-north-korea
 The Missile Defeat Posture and Strategy of the United States—The FY17 President's Budget Request, House, 114th Cong. 2 (2016) ), available online at https://armedservices.house.gov/legislation/hearings/missile-defeat-posture-and-strategy-united-states-fy17-presidents-budget-0
 D. K. Barton et al., “Report of the APS study group on boost-phase intercept systems for national missile defense,” Rev. Mod. Phys. 76, S1 (2004). Retrieved from https://www.aps.org/newsroom/pressreleases/upload/BPI_Report.pdf
 National Research Council. 2012. Making Sense of Ballistic Missile Defense: An Assessment of Concepts and Systems for U.S. Boost-Phase Missile Defense in Comparison to Other Alternatives. Washington, DC: The National Academies Press. https://doi.org/10.17226/13189.
 [Union of Concerned Scientist] (2018, September 12). Space-based Missile Defense: Not a Good Idea [Video File]. Retrieved from https://www.youtube.com/watch?v=fEkXTV69yo4&feature=youtu.be
These contributions have not been peer-refereed. They represent solely the view(s) of the author(s) and not necessarily the view of APS.