Table of Contents

Over the past few years, the United States has restarted its efforts to develop space-based missile defenses. In 2018, the U.S. Congress directed the Department of Defense to identify potential technologies for, and to estimate the costs of, deploying a space-based missile defense layer.1 Research into space-based defenses was funded throughout the remainder of the Trump administration.2 Given their absence from the Biden administration’s Fiscal Year 2021 budget request, their short-term future is less clear—although, over the long term, the United States is likely to remain interested in them, while China and Russia are likely to remain concerned about them.

Space-based missile defenses—involving kinetic interceptors or nonkinetic technologies, such as directed-energy weapons—are capable, at least in theory, of addressing some of the weaknesses of terrestrial missile defense systems. For example, the United States’ Ground-based Midcourse Defense system is vulnerable to decoys deployed in the vacuum of space or to hypersonic gliders that fly below the interceptors’ engagement altitude. Space-based defenses could overcome such countermeasures by engaging a missile in its boost phase, while its engines are still operating and before decoys can be deployed or a glider released. Alternatively, space-based defenses could be configured to intercept ballistic missiles during their midcourse phase when they are unpowered. In particular, directed-energy weapons offer the potential to engage large numbers of incoming warheads (and decoys).

Chinese and Russian officials have repeatedly expressed concerns over existing and possible future U.S. ballistic missile defenses. Russia’s developmental exotic weapons are intended to circumvent them. Because some of these weapons—the Burevestnik cruise missile and Poseidon torpedo, in particular—could evade space-based interceptors, concerns that the United States may develop and deploy space-based defenses could make Russia less amenable to limits on its exotic systems. U.S. officials, meanwhile, have claimed that China is “looking at” nuclear-powered cruise missiles and torpedoes, perhaps because many of its existing programs to combat U.S. missile defenses—including arming missiles with multiple warheads and developing an intercontinental hypersonic glider—could theoretically be vulnerable to space-based defenses.3

Space-based missile defenses present daunting technical challenges and are likely to be extraordinarily expensive.

These concerns may also lead China and Russia to deploy anti-satellite weapons in even larger numbers than currently planned to ensure that they are able to target the satellites on which interceptors would be based or the sensors that would enable them. Such deployments, even if undertaken solely to combat missile defenses, would increase the threat to all U.S. assets in low-Earth orbit.

Space-based missile defenses could also create significant escalation pressures for two reasons. First, to be most effective, attacks on space-based missile defenses would need to occur in advance of a nuclear exchange. Thus, if Russia, say, perceived an impending attack against its nuclear forces, it might attack space-based interceptors or their enabling capabilities preemptively (assuming, that is, that those enabling capabilities, which would include dual-use early-warning sensors, had not already been attacked during the preceding phases of the conflict). Second, space-based interceptors themselves could be used as anti-satellite weapons and thus be attacked, potentially quite early in a conflict, as a way to protect the satellites used to support conventional operations.

In addition to these risks, space-based missile defenses present daunting technical challenges and are likely to be extraordinarily expensive. To engage ballistic missiles during their boost phase, a space-based missile defense system would need to be designed so that interceptors were always located relatively close to all potential missile launch sites. Because satellites in low-Earth orbit are in constant motion relative to the Earth’s surface, hundreds of platforms—close to a thousand, in fact, for global coverage—would be needed.4 Because missiles could be safely fired during any gaps in coverage, which all participants have the space awareness capabilities to identify, space-based defenses would likely have no meaningful capability at all until hundreds of interceptors had been placed in orbit.5 Moreover, to combat missile volleys, each platform would need to be capable of engaging multiple targets in quick succession.

The development of a space-based system designed to intercept missiles during the midcourse phase of their flight would be somewhat less demanding—though still extremely difficult—because only tens of satellites might be required.6 However, such a system would also be less effective since incoming ballistic missiles would have a chance to release multiple warheads, decoys, and other countermeasures, and incoming gliders might have dropped below the minimum engagement altitude. Moreover, the kinds of high-powered, low-weight lasers and power supplies that could make such a system even remotely plausible are still a long way from being developed.7

As a result, the United States is unlikely to ever deploy a meaningful space-based missile defense capability. Nonetheless, its investments in technology development will continue to spur China and Russia to develop countermeasures. This is the central irony of space-based missile defense systems: the United States is likely to pay a significant political and strategic price for a system from which it will never benefit.

Solution Concept

A trilateral prohibition on the testing and deployment of any space-based weapons designed to counter ballistic or boost-glide missiles would provide a verifiable means to manage concerns about space-based defenses. It would apply to all such weapons—kinetic or nonkinetic—but would not affect the deployment of space-based sensors to detect missile launches or track missiles during flight. Such a ban would not require any state to forsake a capability that it was remotely likely to deploy but would lessen Chinese and Russian incentives to develop or retain potentially destabilizing countermeasures, such as exotic strategic capabilities and sophisticated anti-satellite capabilities.

The United States is unlikely to ever deploy a meaningful space- based missile defense capability. Nonetheless, its investments in technology development will continue to spur China and Russia to develop countermeasures.

In theory, a state would need to overcome two major hurdles to deploy a useful space-based missile defense system. First, space-based interceptors would need to be tested under realistic conditions—that is, launched from orbit to engage targets in the atmosphere. Without testing, space-based missile defenses would probably be highly ineffective. Indeed, the United States’ experience with its Ground-based Midcourse Defense system highlights the challenges of obtaining reliability even after more than two decades of testing.8 Second, hundreds or thousands of interceptor platforms would need to be built and launched into orbit, requiring many dozens, if not hundreds, of launches—which is well beyond the ability of any of these countries to conduct in a limited time with current launch resources.

Although prohibiting either testing or deployment would be enough to prevent the development of reliable space-based missile defenses, the proposed prohibition covers both, while relying on NTM for verification. The proposed fifteen-year time horizon, with an option to extend the agreement in five-year increments, represents a potential compromise between the United States (which would likely prefer a politically binding agreement) and China and Russia (which would presumably prefer a treaty with no expiration provisions).

A Prohibition on the Testing and Deployment of Space-Based Missile Defense Weapons

China, Russia, and the United States should conclude a treaty prohibiting the testing or deployment of space-based missile defense weapons for fifteen years, with the option of extending the agreement in five-year increments by mutual consent.

Specifically, the parties should agree to prohibit the following:

  • The testing of space-based missile defense weapons
  • The deployment of space-based missile defense weapons in orbit

In implementing these provisions, the following definitions would apply:

  • “Space-based missile defense weapon” means any weapon, based on any physical principle, that is located in Earth orbit and designed to counter ballistic missiles or boost-glide missiles or their elements in flight trajectory.
  • “Ballistic missile” means a weapon-delivery vehicle that has a ballistic trajectory over most of its flight path.
  • “Boost-glide missile” means a weapon-delivery vehicle that sustains unpowered flight through the use of aerodynamic lift over most of its flight path. A reaction control system designed to change a vehicle’s attitude is not considered capable of powering flight.

The parties should hold an annual implementation meeting to discuss compliance or implementation issues. They should also commit to discussing urgent compliance concerns through regular diplomatic channels.


The prohibition would be verified through NTM, with efforts primarily focused on assessing compliance with the ban on testing.

Preparations for the launch of a target missile could be monitored with satellite imagery and potentially other information-collection techniques (including signals and human intelligence). Such preparations would probably not constitute definitive evidence of noncompliance since it would likely be unclear whether the planned test was of a prohibited space-based missile defense weapon, but they could cue enhanced intelligence-gathering efforts.

The test itself would present various opportunities for detection, depending on whether the missile defense weapon was kinetic or nonkinetic and at what point in its trajectory the target missile was engaged. In all cases, the launch of the target missile could be monitored with early-warning satellites. For a successful boost-phase engagement, the target missile’s plume (ejected hot gasses that are the source of intense infrared radiation that can be detected by satellites) would presumably be rapidly and prematurely extinguished.

Space situational awareness capabilities could also be useful for verification. They might be capable of detecting the launch of kinetic interceptors or the debris resulting from an engagement, especially during the midcourse of a target’s trajectory. In theory, ground-based sensors, such as radars, could be used. In practice, space-based sensors, which provide continuous coverage, would likely be more useful. (The benefits and limitations of different space situational awareness technologies are discussed in appendix C.)

Telemetry—the data transmitted from the target missile and the missile defense weapon for diagnostic purposes—could also be useful in detecting a test. Such signals could be intercepted with electronic-intelligence satellites or ground-based radars. Even if they were encrypted, as seems likely, their existence would still be evidence of a prohibited test.

The biggest verification challenge might be that a state could try to test space-based missile defense weapons against other satellites rather than against target missiles. However, a state would have little incentive to do so. First, this approach would risk creating a significant quantity of orbital debris that could threaten the state’s own satellites. Second, testing missile defense weapons against satellites would not contribute much to the development of boost-phase defenses. It would be more helpful in the development of midcourse defenses—but because reentry vehicles are likely more resilient than satellites and would likely be comingled with decoys and booster debris, the development of a reliable midcourse missile defense capability would probably require additional tests against target missiles under more realistic conditions.9

Ultimately, to gain a meaningful operational capability, a state would need to conduct a lengthy testing campaign, requiring significant resources and personnel. Such a campaign would be difficult to conceal against multiple intelligence-collection techniques (technical and human), even if the state sometimes managed to hide individual tests.

The deployment of space-based ballistic missile defenses could also be monitored with NTM—though here the need to distinguish prohibited space-based interceptors from permitted space-based anti-satellite weapons (or perhaps even permitted space-based ground-attack weapons) could arise. Helpfully, the deployment pattern of a boost-phase space-based missile defense system should be very distinctive. A high degree of coverage and redundancy would be needed to gain any value from such a system. Thus, the deployment of large numbers of weapon-carrying satellites in orbits that provide continuous coverage of potential missile launch sites would provide fairly clear evidence of a state’s intentions—and could be easily detected—even if the exact capabilities of the weapons on any given platform were ambiguous.

Any illicit program to develop space-based missile defenses would have to be large-scale, complex, and prolonged, providing plenty of opportunities for detection.

Distinguishing between a midcourse space-based missile defense system, which could involve tens of satellites, and space-based anti-satellite weapons could be more difficult (which is partly why the prohibition on testing presents the best opportunity for assessing compliance). Yet deploying either system would require a major and prolonged effort, potentially yielding intelligence to help understand that state’s intent. Moreover, as already noted, midcourse space-based missile defense systems are unlikely to be technologically feasible within the lifetime of the proposed agreement and would, in any case, be significantly less effective than boost-phase systems.


Technical feasibility. The proposed treaty should be straightforward to negotiate, largely because it would rely on NTM for verification. Its technical feasibility would therefore hinge on each state’s national capabilities. At least four types of NTM capabilities besides human intelligence could be useful: electronic-intelligence collection, space-based reconnaissance, space situational awareness, and missile launch detection.

Electronic-intelligence collection capabilities and, to a slightly lesser extent, visual reconnaissance satellites are shrouded in secrecy. Publicly available information about each state’s space situational awareness capabilities is discussed in appendix C. This information suggests that the United States’ capabilities are sufficiently sophisticated and persistent to be of considerable use in verifying the proposed prohibition on testing. Chinese and Russian capabilities are likely less sophisticated but could play a meaningful role nonetheless, though there is more uncertainty here.

In terms of missile launch detection, the U.S. space-based early-warning system would allow the United States to reliably monitor the launch of any test target (and possibly even the launch of any space-based kinetic interceptors). Russia is currently rebuilding its space-based monitoring capability; with the launch of a fourth satellite in 2020, its new system reached its “minimum baseline configuration,” suggesting it “can ensure round-the-clock coverage of the most critical areas.”10 China may not yet have a comprehensive space-based early-warning system, though the U.S. Department of Defense assesses that it is now developing one, potentially with Russian assistance.11

Altogether, the various known and unknown potential weaknesses in each state’s NTM capabilities should not prevent effective verification—simply because any illicit program to develop space-based missile defenses would have to be large-scale, complex, and prolonged, providing plenty of opportunities for detection. To be sure, U.S. capabilities are more sophisticated than Russia’s or China’s (though theirs are improving), but the United States would face particularly significant challenges in secretly pursuing a program of the required scale and complexity.

Political feasibility. Russia and China would likely support this proposal. Neither have shown any interest in developing space-based missile defenses, and they view the United States’ potential deployment of such a system as a serious threat. Indeed, Beijing and Moscow have recently invoked the specter of U.S. space-based missile defenses to help argue for a treaty—which they have jointly proposed—that would prohibit the placement of weapons in outer space.12

The primary political impediment to the prohibition on space-based missile defenses proposed here would be resistance within the United States, where there is strong domestic support for missile defenses. Space-based missile defense programs have had a constituency since president Ronald Reagan’s Strategic Defense Initiative of the 1980s.13 While support for such programs today is less broad than for many other missile defense technologies, these programs still have vocal supporters, including in Congress, who invoke them as a potential route to invulnerability from a missile attack.14

For this reason, it could be difficult for the U.S. administration to obtain the Senate’s advice and consent for ratification of the proposed treaty. A politically binding executive agreement would be easier to obtain, of course, but would also be much less valuable to China and Russia, reducing their willingness to grant the United States significant concessions in return. (However, if U.S. policymakers consider a treaty to be totally infeasible, they could explore a joint political commitment or even coordinated unilateral moratoria.)

One feature of the proposed treaty that would help lessen domestic resistance in the United States is that it would only last fifteen years (with an extension option). Although Beijing and Moscow would presumably prefer an indefinite agreement, a time-limited treaty could help address U.S. concerns about the possibility that technological developments might increase the need for, or enhance the feasibility of, space-based missile defenses.

Another reason for considering a treaty is that limitations on space-based interceptors may be at least somewhat more palatable for the United States than limitations on ground-based missile defenses (which attract particularly intense domestic opposition). Support for developmental capabilities is never as strong as support for existing capabilities. Moreover, the costs associated with deploying space-based defenses are likely to be prohibitive for the foreseeable future. The United States has barely scratched the surface of the needed research and development investments, and the large number of interceptor platforms that would need to be deployed for a system to be at all useful would be exorbitantly expensive.

Finally, if China and Russia want a prohibition of space-based missile defenses, they will have to make significant concrete concessions to the United States in return. One approach would be to package the proposed prohibition with separate U.S.-Russian and U.S.-Chinese measures that were similarly time-bound. For example, Russia and the United States could agree to a strategic offensive arms control treaty that included significant limits on Russia’s exotic delivery systems (which are intended to defeat U.S. missile defenses). Meanwhile, China and the United States could agree to a bilateral fissile material cutoff because, with more clarity about the future trajectory of U.S. missile defenses, Beijing might be willing to commit to refraining from producing any new fissile material for military purposes (see chapter 4). Of course, this kind of triangular diplomatic dance could be difficult to orchestrate and, in practice, negotiators would need to capitalize on the trade-offs that seem attainable at the time.


1 National Defense Authorization Act for Fiscal Year 2018, Public Law 115-91 (2017): Sec. 1688,

2 Kingston Reif, “Congress Calls for Interceptors in Space,” Arms Control Today, September 2018,; and Kingston Reif, “U.S. Seeks New Space-Based Capabilities,” Arms Control Today, April 2019,

3 Keaten, “U.S. Envoy Warns China ‘Looking at’ New Nuclear Technologies.”

4 David Wright, “How to Think About Space-Based Missile Defense,” All Things Nuclear (blog), Union of Concerned Scientists, August 22, 2018,

5 National Research Council of the National Academies, 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: National Academies Press, 2012), 58–61, available from

6 Thomas G. Roberts, “What Can 24 Satellites Do for U.S. Missile Defense?,” Center for Strategic and International Studies, October 18, 2018,

7 Jeff Hecht, “A ‘Star Wars’ Sequel? The Allure of Directed Energy for Space Weapons,” Bulletin of the Atomic Scientists 75, no. 4 (2019): 162–170.

8 U.S. Government Accountability Office, “Missile Defense: Assessment of Testing Approach Needed as Delays and Changes Persist,” GAO-20-423, July 2020, 57–65,

9 In theory, it would be desirable to bypass this problem entirely by simply prohibiting the testing and deployment of space-based anti-weapons; in practice, however, defining what constitutes an anti-satellite weapon presents an insurmountable challenge. For example, between 1993 and 2009, a U.S. Space Shuttle captured the Hubble Space Telescope on five separate occasions for repair and servicing. Did the shuttle constitute a space-based anti-satellite weapon? For other definitional issues, see Michael Krepon, “What Is a Space Weapon,” Arms Control Wonk (blog), March 18, 2010,

10 Bart Hendrickx, “EKS: Russia’s Space-Based Missile Early Warning System,” Space Review, February 8, 2021,

11 Office of the Secretary of Defense, “Military and Security Developments Involving the People’s Republic of China,” 89.

12 For example, Sergey Lavrov, statement, Conference on Disarmament, Geneva, March 20, 2019,; and “Statement of the Chinese Delegation at the Thematic Discussion on Outer Space,” 75th Session of the United Nations General Assembly First Committee, New York, October 28, 2020,

13 Jon Harper, “Special Report: The Legacy of the Strategic Defense Initiative,” National Defense, April 23, 2019,

14 Ted Cruz, “Exclusive Commentary From Sen. Ted Cruz: America Needs Space-Based Interceptors,” National Defense, April 10, 2019,