Trump’s ‘Golden Dome’ plan has a major obstacle: Physics

In sports, the best offense is often a good defense. It’s not clear if the same applies in nuclear war.

In the face of nuclear threats from adversaries like Russia, China and North Korea, some politicians are clamoring for a system to reliably protect the United States from incoming missiles. That’s the aim of President Donald Trump’s plan for a next-generation missile defense system, dubbed the “Golden Dome.” Trump announced on May 20 that an architecture had been selected and that the system would be operational before the end of his term, at a cost of $175 billion. But some scientists suggest that implementing such a system, as called for by a January executive order, would be daunting.

The United States already maintains a nationwide missile defense system aimed at defending against a small-scale attack from intercontinental ballistic missiles, or ICBMs, launched by a rogue nation such as North Korea. But a February report from the American Physical Society concludes that defense against even a small-scale attack is uncertain. And the system’s capabilities are likely to remain relatively limited within the next 15 years, the report argues. The Golden Dome initiative aims to protect the country from more capable adversaries such as Russia and China — a more difficult task.

“Intercepting even a single, nuclear-armed intercontinental-range ballistic missile or its warheads … is extremely challenging,” physicist Frederick Lamb of the University of Illinois Urbana-Champaign, chair of the group that produced the report, said at an APS meeting in Anaheim, Calif. in March. “The ability of any missile defense system to do this reliably has not been demonstrated.”

And as countries come up with new types of weapons that could skirt defenses, the situation is getting even more challenging. Golden Dome aims to defend against not just ICBMs, but also hypersonic weapons, advanced cruise missiles and more. And Golden Dome would take missile defense to space. In addition to ground-based systems, Golden Dome would use potentially thousands of defensive weapons called interceptors orbiting Earth, poised to neutralize attacks.

Golden Dome has drawn praise from missile defense proponents. “The initiative to elevate and prioritize air and missile defense … that’s long overdue and it’s entirely appropriate,” says Tom Karako, director of the Missile Defense Project at the Center for Strategic and International Studies in Washington, D.C. Lower launch costs, proponents argue, make space-based missile defense more realistic than in the past. “I think we’re a lot closer than people recognize,” says nuclear deterrence and missile defense expert Robert Peters at the Heritage Foundation in Washington, D.C.

The U.S. Missile Defense Agency disputed the claims of the APS study, arguing that it relied on older data and unclassified reports that don’t reflect recent improvements and upgrades to the missile defense architecture. “MDA’s Missile Defense System stands ready and fully capable of defending the United States, deployed forces and allies from a rogue nation’s missile attack,” the agency said in a statement.

But critics note that the difficulty of the problem remains. “Technology has advanced tremendously,” says Victoria Samson of the Secure World Foundation in Washington, D.C. “But the laws of physics have not changed, and that’s really what the challenge is.

Hitting a bullet with a bullet

ICBMs are a formidable target. An ICBM launches in a giant arc that sends the weapon it carries careening through space, traversing more than 5,000 kilometers to reach its target. The challenge of intercepting them has been compared to hitting a bullet with a bullet. But this understates the problem: At around 25,000 kilometers per hour, ICBMs speeds are about seven times that of a bullet. What’s more, they’re generally armed with nuclear warheads, each capable of killing a million people, rather than one.

ICBMs have three phases of flight, and there are different possibilities for intercepting the missiles during each phase. In the boost phase, which lasts a few minutes, rocket engines lift the missile to high altitude and high speeds. In the midcourse phase, the engines are jettisoned. The missile enters space, releasing one or more warheads, which continue upward before falling back down again. This part of the trajectory, in which the warheads are moving in an arc under the influence of gravity alone, is what’s known as ballistic motion — hence the missiles’ name. That phase lasts around 20 minutes.

The terminal phase is the shortest: The warhead reenters the atmosphere, descending to its target in under a minute. This period is so short that the only possibility for stopping a weapon is by placing interceptors very close to the point of impact. Such tactics can be used as one layer of missile defense, a back-up protection for sensitive areas like military bases, but it’s not practical for protecting a large country. So concepts for protecting the entire United States typically focus on the boost phase or the midcourse phase.

Midcourse defense

The midcourse phase is the bread-and-butter of the country’s current missile defense system.

Forty-four interceptors in Alaska and California aim to intercept incoming missiles in space. That system — which by some estimates has cost over $60 billion — is known as ground-based midcourse defense. It’s aimed at defending the United States against a small number of unsophisticated missiles from North Korea or another rogue nation.

Critics note that this system has been about 60 percent effective in tests. However, that statistic includes tests going back over 25 years. The tests performed in more recent years have been more successful. “Any time you test a new system, there are going to be failures early on,” Peters says. “That’s how you learn what works.”

Another complaint is that its tests aren’t realistic, but proponents say it’s not possible to fully re-create realistic conditions. “We have not had North Korea try to nuke the continental U.S. yet, so … it’s not an actual battle test,” Peters says. He points to real-world uses of missile defense in conflicts in Ukraine and Israel. “We’ve been really effective with those tests in real-world operational environments.”

However, none of the weapons shot down in those conflicts were ICBMs. “One should not mix apples and oranges,” says physicist and aeronautics engineer Paul Dimotakis of Caltech, who was not involved with the APS report. “Different types of attacking missiles and their number and sophistication will require different tailored defenses.”

And there’s one particularly thorny snag to missile defense in the midcourse phase: countermeasures. An adversary could release debris or decoys along with a real warhead, for example, thwarting attempts to intercept it.

 “One key reason why the midcourse phase is difficult is because you’re in space, and different objects of different mass travel exactly the same,” says physicist James Wells of the University of Michigan in Ann Arbor,  a coauthor of the APS report. The lack of air in space means that a warhead will travel at the same speed as a balloon designed to mimic it, making them hard to distinguish.

That can make the concept of reliable missile defense in the midcourse phase a bigger task than it otherwise seems. “People say, ‘We got to the moon, why can’t we do this?’ Well, the moon didn’t suddenly move out of the way,” Lamb said at the APS meeting. “It’s a huge technical challenge to identify what the target is.… That’s been the bugaboo of midcourse intercept from the very beginning.”

A boost from space

The trickster tactics of the midcourse phase aren’t possible in the boost phase, during which the warheads remain within the missile, and the entire package travels through the atmosphere.

“There’s this perennial dream of intercepting in the boost phase,” Wells says. But because the boost phase lasts only a few minutes, “that time pressure is enormous to get an interceptor there.”

Boost phase is over so quickly that any interceptor would need to be positioned very close to the launch site. And for a large country with an inaccessible interior, like Russia or China, that’s a no-go — on Earth’s surface, at least.

Interceptors in low Earth orbit — an altitude of 2,000 kilometers or less — could do it. But those interceptors would be orbiting, rather than parked over the country of interest. To be certain of taking down a missile, a large constellation of satellites would be needed. And to protect the United States from salvos of multiple missiles at once, the number of satellites would have to increase further.

Ensuring protection from just one North Korean ICBM would require more than 1,000 interceptors in orbit, the APS report finds. Protection from 10 might demand over 30,000 interceptors, depending on missile type and other assumptions. For comparison, there are about 12,000 active satellites in orbit around Earth, most in SpaceX’s Starlink network.

Golden Dome aims to protect not just from North Korea but also from attacks by more capable adversaries, such as Russia and China, who together have hundreds of ICBMs. But Golden Dome is not intended to be impenetrable, Peters says. “I don’t know anyone who is credibly making that argument.” Instead, Peters says, it would prevent a small-scale attack, with a few low-yield nuclear weapons. To thwart Golden Dome, the idea is that an adversary would need to launch a substantial barrage — one certain to provoke a massive nuclear war.

It’s unclear how many ICBMs Golden Dome would aim to neutralize and how many satellites would be needed. But even for small-scale attacks, defending from Russia and China would demand more satellites simply to cover a wider geographic area than needed for North Korea alone.

“It really is an enormously huge difference to be defending against a small region [versus] a large continent,” says astrodynamicist Thomas González Roberts of Georgia Tech in Atlanta.

Depending on the specific objectives, Golden Dome could require an untenable number of satellites, he says. “I would call a lot of these proposals infeasible, but in reality, we don’t know what these proposals are really asking,” Roberts says. Without specific goals for the numbers of ICBMs to be intercepted, from which countries, it’s unclear how plausible the plan is.

Trump shared few specifics in the May 20 news conference, saying “Golden Dome will be capable of intercepting missiles even if they are launched from other sides of the world and even if they are launched from space.”

That, Roberts says, suggests an extensive system. “You would be hard-pressed to find a system that could do that for $175 [billion]. Even the most optimistic assumptions behind boost-phase missile defense would suggest that that is impossible.” What’s more, Roberts says, to do it in three years would require a faster launch cadence than ever before.

The price tag is bound to be a thorny issue. Already, over the past 70 years, the United States has spent more than $400 billion on missile defense, according to the APS report. The budget bill that is currently working its way through Congress would lay out $25 billion for Golden Dome in fiscal year 2025. And a May 5 Congressional Budget Office report suggests that, even with lower launch costs, the space-based effort alone would cost between $161 billion and $542 billion over a period of 20 years.

“It’s really complicated,” Samson says. “It’s like a bumper sticker type thing. It’s easy to say … ‘Do you want to be defended against ballistic missiles?’ And everyone says, ‘Yes, of course, I’d love to.’ Great, but here’s all the things you need to look at.”