Donald Trump says he's settled on a design for his planned "Golden Dome" missile defence system, aimed at fending off hypersonic, ballistic, and space-based weapons. However, technical and political concerns mean that the project may die a quiet death.
Based loosely on Israel's Iron Dome - but of necessity having much broader geographical coverage - the scheme involves sensors on the ground, at sea, in the air, and in space.
It would include an extensive constellation of new satellites that could identify launch sites and missile types - including the latest generation of hypersonic missiles and Fractional Orbital Bombardment Systems (FOBS), which can evade traditional radar detection - and track their trajectory.
In the longer term, there are plans for space-based interceptors that would target missiles early in their flight, when the missile’s bright exhaust makes it easier to detect from space.
"Within the last four decades, our adversaries have developed more advanced and lethal long-range weapons than ever before, including ballistic, hypersonic, and cruise missiles capable of striking the homeland with either conventional or nuclear warheads," says Secretary of Defense Pete Hegseth.
"Golden Dome is designed to leverage some past investments, but will also use next-generation technology to defend against the evolving and complex threat landscape."
Hegseth said that the Department of Defense was working with the Office of Management and Budget to develop a plan to fund all this, and was hoping to get $25 billion allocated through the One Big Beautiful Bill.
How much will it cost?
But there are big questions over the cost. Trump has suggested that the system will cost $175 billion in total and could be completed within three years - but the Congressional Budget Office has come up with rather more significant figures.
Even given falling launch costs, it says, the total cost of deploying and operating the constellation for 20 years could hit $161 billion for a bare-bones system, and up to $542 billion for the space-based parts of the system alone.
Meanwhile, says Julia Cournoyer, International Security Programme research associate at think tank Chatham House, the system is dependent on the development of new technologies that don't yet exist.
"Investing the necessary resources to develop such an advanced system would require significant trade-offs that could come at the expense of other defence priorities," she says.
"Pursuing the Golden Dome risks prioritising an expensive and unproven system over more immediate and achievable capabilities, such as improving regional missile defences and cyber resilience to countering emerging threats like drones."
The Center for International & Security Studies at Maryland has estimated that the US would require at least 1,600 orbiting interceptors for the system to work, simply to defend the US from a single launch of the North Korean Hwasong-18 intercontinental ballistic missile (ICBM).
Interceptors would need to be frequently replaced thanks to orbital decay, while attackers could also target the satellites themselves.
"Therefore, to ensure effectiveness of the shield, the United States would have to deploy multiple layers of interceptors in different orbits and a fleet of maneuverable satellites to defend them, multiplying the overall cost of the system and rendering the whole concept absurd," warns associate research scholar at the Princeton University Program on Science and Global Security Igor Moric.
And earlier this year, the American Physical Society concluded in a report that, within a 15-year time horizon, such a system couldn't be expected to provide a robust or reliable defense against anything more than the simplest attacks by a small number of relatively unsophisticated missiles.
Political and legal concerns
There are also huge political implications. North Korea's Institute for American Studies of the Ministry of Foreign Affairs has described the plans as a 'threatening initiative' that sets the scene for nuclear war in space.
Meanwhile, Russia and China last month issued a joint statement in response to early discussions of the Golden Dome plans, describing the system as “deeply destabilizing in nature.”
"This means a complete and ultimate rejection to recognize the existence of the inseparable interrelationship between strategic offensive arms and strategic defensive arms, which is one of the central and fundamental principles of maintaining global strategic stability," the statement warns.
There's also the rather important question of international law. The 1967 Outer Space Treaty — considered to be the foundation of international space law — bans the installation of nuclear weapons or other weapons of mass destruction in orbit or on celestial bodies.
"Golden Dome... plans to expand the US arsenal of means for combat operations in outer space, including R&D and deployment of orbital interception systems. That gives the project a strong offensive nature and violates the principle of peaceful use in the Outer Space Treaty," warned Chinese Foreign Ministry spokesperson Mao Ning last week.
"The project will heighten the risk of turning the space into a war zone and creating a space arms race, and shake the international security and arms control system. This is yet another 'America First' initiative that puts the US’s absolute security above all else."
It's possible that, just like Trump's tariffs, the plans for the Golden Dome are more a negotiating tactic than a firm commitment. It is, after all, a long-term project that would most likely see little actually done during the three years Trump's presidency has left to run.
"Framing the proposal as a starting point for dialogue, rather than a signal of unilateral ambition, could help to stabilize a dangerously volatile moment," says Cournoyer. "Otherwise, the project risks pushing the world one step further towards a more contested, militarized, and insecure future."
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