Questions to Ask About Proposals for US. Space-Based Missile Interceptors

The Trump administration’s May 20 announcement of a three-year, $175 billion effort to build a constellation of space-based interceptors to attempt to counter any kind of missile from any nation within three years, popularized as the “Golden Dome” missile defense system, has been met with widespread skepticism bordering on incredulity.

Indeed, The Guardian reported May 30 that “two people familiar” with the Golden Dome announcement concede that the space-based interceptor program will only be able to conduct a demonstration under ideal conditions by 2028. In this piece, we will examine some of the many reasons behind the skepticism about the program and identify key questions that remain unanswered.

Before diving into technical questions about space-based interceptors, however, it is worth reiterating that the primary and central concern with strategic missile defense continues to be its potential to aggravate the nascent three-way arms race between the United States and its two main nuclear rivals, Russia and China. This problem was explored in depth in a recent Arms Control Today feature article by Igor Moric, and is worth bearing in mind even while we dive deeper into technical challenges. 

Space-Based Incredulities

The Congressional Budget Office (CBO) released an update of its cost estimates for several space-based interceptor constellation possibilities several weeks before the White House press conference on the Golden Dome. The CBO estimate found that—despite the drastic decline over the last decade in space-launch prices—a space-based interceptor constellation would still cost between roughly $160 and $540 billion dollars over 20 years.

A closer look at the assumptions underlying these figures points the way toward the right questions to ask Golden Dome proponents. Journalists should be pressing the administration for answers on the record, while members of Congress should ask for detailed briefings and seek testimony in public hearings on these and other questions.
 

Notes on Table: APS refers to the American Physical Society Panel on Public Affairs; CBO refers to the Congressional Budget Office; NRC refers to the National Research Council of the National Academies. In its May 2025 report, the CBO reviewed three older scenarios first developed in a 2004 CBO study and a 2012 NRC study.

Sources: American Physical Society Panel on Public Affairs, Strategic Ballistic Missile Defense: Challenges to Defending the United States (College Park, MD: American Physical Society 2025); Congressional Budget Office, “Effects of Lower Launch Costs on Previous Estimates for Space-Based, Boost-Phase Missile Defense,” May 5, 2025; 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: National Academies Press, 2012); Congressional Budget Office, “Alternatives for Boost-Phase Missile Defense,” July 2004. 

The table above summarizes the key assumptions of several cost models for space-based missile defense constellations developed by independent experts over the last two decades, in the period after the George W. Bush administration withdrew the United States from the Anti-Ballistic Missile Treaty in 2002 and kick-started a new generation of interest in missile defense.

Some proponents of Golden Dome have argued that the declining cost of space launch makes the program economical. Given the recent public acrimony between Space X executive Elon Musk and the Trump administration, it is an open question whether Golden Dome will benefit from the vast reductions in space-launch costs made possible through Space X innovations over the last decade. In the table above, the potential implications are reflected in the sharp differences between the American Physical Society (APS) estimates of launch costs, which adopt prices from the 2012 National Research Council study, and the updated Congressional Budget Office study, which assumes current, lower market rates.

But space-launch costs are only one of two key cost drivers. The other, equally important factor is the size of a space-based missile defense constellation, or in other words, the number of satellites involved. The size of the constellation is, in turn, a function of other variables: how performant the interceptor is, how quickly a decision to fire can be made (“decision time”) which in turn depends on overall sensor-shooter system integration, and what kind of threat the system is intended to handle. The logic is simple. The faster and more agile the interceptor missile, the fewer satellites are required but the heavier each will be. The faster the system can decide to fire an interceptor, the more time the interceptor missile will have to reach its target. And the more sophisticated and numerous the threat envisioned, the more satellites will be required.

Leaving aside the cost of space launch and focusing on the size of proposed constellations, there remain three primary categories of questions.

First, what type of interceptor will be developed or adapted for Golden Dome? Will it be a derivative of an existing type, such as the midcourse interceptors deployed at sea on Navy Aegis ships or in siloes in Alaska and California? The Department of Defense also has additional interceptor development programs underway already: a Next-Generation Interceptor for the siloed system, and a Glide-Phase Interceptor for Aegis optimized for defeating hypersonic boost-glide vehicles. But the first is not due to enter service until 2030, and the latter until 2035. Given these timelines, how can another new interceptor be designed, built, and tested successfully before the three-year deadline?

Second, what are the overall system’s performance requirements and goals for integrating sensors and shooters? Because these metrics are likely to be classified, a more policy-relevant version of this question is: will the Department of Defense validate its capability to meet its own integration requirements before spending billions on an interceptor constellation which will only work if those metrics are met? If the software and sensor integration effort fails, is found to be impractical, or is delayed, it would be wasteful to acquire and launch an interceptor constellation that cannot perform as designed.

Third, what is the scope of the threat the system is designed to handle, and what degree of responsibility will rest on the space-based interceptors to manage that threat? If, as indicated by the January 2025 “Iron Dome” executive order, the system will be asked to defend against “any foreign aerial attack on the Homeland,” then the number of satellites needed in the constellation will be far higher than in any of the models in the table above. All of those models assumed that the constellation would be designed to defeat only a rogue state threat. Further, as the 2025 American Physical Society study indicates, mandating a requirement that the system be capable of defeating a salvo-launch would increase the satellite requirement by an order of magnitude, to roughly 16,000 satellites.

A further unanswered question in this third category is how the space-based interceptors will be integrated with other existing homeland defense systems such as the siloed midcourse interceptors in Alaska and California. If the space-based component is only designed to destroy 50%, 70%, or 90% of incoming missiles, while other interceptors can effectively mop-up the remainder, then, in theory, the constellation could conceivably be smaller.

These three categories of questions are fundamental to understanding how the Golden Dome system will be designed and whether the plan is feasible. If the administration continues to be unable or unwilling to answer them, even supporters of U.S. missile defense efforts should be hesitant to pour billions of dollars into the project.

Against Obfuscation and Confusion

In conversations about missile defense, it is important to be aware of rhetorical sleights-of-hand made possible by continuing uncertainty about what Golden Dome is and will be.

Notwithstanding the considerable technical challenges confronting any space-based interceptor program, some missile defense experts and proponents argue that Golden Dome is technically feasible. They can make this claim because the administration has remained vague about how central still-to-be-proven space-based interceptors will be in the broader Golden Dome system.

To the extent that, as the president seemed to indicate in his May 20 announcement, the space-based interceptors are the core of Golden Dome, then the technical challenges are enormous – as discussed previously. But the Department of Defense is pursuing a considerable number of pre-existing missile defense efforts that may be supplemented as part of a larger re-labeling of homeland missile defense as “Golden Dome” components.

Those supplements could include additional investments in homeland cruise missile defense, a further expansion of the current array of 44 existing and 20 planned ground-based strategic interceptors that have—after more than 20 years of development and testing—not yet proven effective, or even the deployment of Aegis missile interceptors on land in the continental United States.

Again, such additions would pose problems for strategic stability, prompt adversary countermeasures that could be implemented relatively more quickly and at lower cost, and should be assessed with a critical eye. Each addition also poses substantial cost and technical feasibility challenges, although not quite as insurmountable as those posed by space-based interceptors. For example, it is important to ask whether the effectiveness of new homeland interceptors will hinge on still unreliable warhead discrimination capabilities (i.e. the ability of the defender to distinguish an incoming warhead from decoys). But, in a limited sense, these incremental additions are expansions of proven systems.

To avoid the traps posed by rhetorical maneuvers in the missile defense discourse, it is important to both analyze defense systems separately and to consider the policy implications of the Golden Dome program as a whole. While the technical challenges confronting the proposed space-based interceptor constellation present a vulnerability for that system in particular, the entire homeland missile defense enterprise deserves broader scrutiny as a strategic concept.—XIAODON LIANG, with assistance from LENA KROEPKE