How Much Would a Space-Based Missile Interceptor System Cost and Does It Make Sense?

In a recently issued executive order, President Trump called on the Department of Defense (DoD) to submit a plan for the “development and deployment of proliferated space-based interceptors capable of boost-phase intercept.” This idea was known as “brilliant pebbles” in the George H. W. Bush administration, and the Trump administration explored starting a program like this in its first term. But previous efforts to field a space-based missile interceptor system did not move forward in large part because the technology was immature, and the costs were prohibitive. But times have changed. This article provides an updated rough order of magnitude estimate for what such a system might cost. The analysis is based on the methodology outlined in a 2004 report by the American Physical Society (APS) and is updated with current assumptions.

Intercepting a missile during the boost phase is ideal because the missile cannot deploy decoy warheads and is much easier to track. It is challenging, however, because of the tight timeline involved. An intercontinental ballistic missile (ICBM) typically has a boost phase that lasts three to four minutes, and it may take 30 seconds or more before a launch is detected and a track is established. Even with the improved coverage of the new missile tracking layer being deployed by the Space Force in the coming years, it is reasonable to assume that a space-based interceptor would have roughly 150 seconds to strike its target during the boost phase.

To be effective, interceptors would need to be based in low Earth orbit (LEO) to intercept the missile inflight. Using the aforementioned APS model, if the interceptors are kept in orbit at an altitude of 500 km, approximately 1,900 interceptors would be needed to provide continuous coverage of all points on Earth with an average of two interceptors. Each interceptor, including propellant, kill vehicle, and support systems, would weigh around 900 kg. Using an 85 percent learning curve, the average procurement unit cost (APUC) of each interceptor in a constellation like this is estimated to be between $4.4 and $8.9 million, for a total procurement cost of $8.6 to $17.2 billion (all costs are in 2025 dollars). An additional $2 to $4 billion would likely be needed for non-recurring development costs, and the constellation would need to be replenished about every 5 years as satellites age and their orbits decay.

Launch costs are perhaps the area where updated assumptions matter the most because launch costs have fallen significantly in the past decade and are expected to fall by another factor of ten in the coming years. At the low end (using the most generous assumptions) launching a constellation of 1,900 interceptors with a mass of 900 kg each would require at least 12 of SpaceX’s Starship launch vehicle with a payload capacity of 150,000 kg each and an estimated cost of $70 million per launch (an aggressive assumption). At the high end, it would require 39 of Blue Origin’s New Glenn rocket with a payload capacity of 45,000 kg and a cost of up to $150 million per launch. Given these assumptions, the overall launch cost would be somewhere between $0.8 to $5.9 billion for the constellation. As in the case of the interceptors, launch costs would be incurred each time the constellation needs to be replenished every five years or so.

The total cost to develop, build, and launch an initial constellation of 1,900 space-based interceptors would likely be on the order of $11 to $27 billion. If this seems like a no-brainer to protect the United States from ballistic missile attack, there’s a catch. The system described above is only sized to intercept a maximum of two missiles launched in a salvo. That means that if an adversary launches a salvo of three missiles, only two could be intercepted and at least one would get through because all of the other interceptors in the constellation would be out of range—what is known as the absenteeism problem.

The grim reality is that the cost of a space-based interceptor system scales linearly with the number of missiles it can intercept in a salvo, excluding development costs. Designing the system to have an average of four interceptors in range (and thus able to intercept a salvo of four missiles at once) requires twice as many interceptors (some 3,800 in total) and twice as many launches. This is true even if multiple interceptors are housed together. A space-based interceptor system for missile defense does not scale well when compared to adversary missile forces. While the costs have come down and the technology has matured, the physics of space-based interceptors has not changed.