Can Donald Trump really build an Iron Dome over America?

“We will replenish our military and build an Iron Dome missile defense system to ensure that no enemy can strike our homeland,” Donald Trump promised at the recent Republican Party convention. “Israel has an Iron Dome. They have a missile defense system,” he said. “Why should other countries have this, and we don’t?”

For one thing, it is technically impossible to build a system that can protect the United States from ballistic missile attack.

It is not for lack of trying. Since President Ronald Reagan announced his ambitious Strategic Defense Initiative in 1983, the country has given more than $415 billion to our best military contractors, employed tens of thousands of workers and the best scientists in the effort. Nothing has worked.

All we have to show for the effort is a basic system of 44 ground-based interceptors deployed in Alaska and California. Under ideal test conditions they have been able to hit a target only half of the time. The program is essentially on hold while a new interceptor is designed.

Trump may mean that we can simply deploy a U.S.-built version of Israel’s Iron Dome. That system works fairly well; why not simply build an American version?

Because Iron Dome is designed to intercept short-range rockets, not intercontinental ballistic missiles. Each Iron Dome system can defend an area of roughly 150 square miles. We would need to deploy more than 24,700 Iron Dome batteries to defend the 3.7 million square miles of the continental United States. At $100 million per battery, that would be approximately $2,470,000,000,000.

Still, it might be worth $2.5 trillion if the system could truly defend the country. But it can’t. Iron Dome is designed to intercept relatively primitive rockets and mortars that travel under 44 miles. That is fine if you want to defend San Diego from rockets launched from Tijuana, some 35 miles away. But the system couldn’t even protect Mar-a-Lago from missiles fired from the Bahamas, some 80 miles away.

That is because long-range missiles pose a fundamentally different, more complex threat than short-range missiles.

We now have—after decades of effort—systems that can reliably intercept short- and medium-range missiles that travel tens or even hundreds of miles. These missiles are relatively slow, large, and hot targets. They travel mostly through the atmosphere, preventing them from deploying any kind of decoys against the interceptors.

Reliably intercepting long-range missiles that travel thousands of miles and are fast, small, and cold as they speed through outer space has proved impossible, particularly if the adversary deploys countermeasures like decoys, chaff, and jammers. The interceptors can’t see the target and even when the warhead enters the atmosphere, stripping away the decoys deployed in space, it is traveling so fast (around 4 miles per second) and is so small that it is an extraordinarily difficult target. Add in the ability of the adversary to simply overwhelm defensive systems with more warheads than there are interceptors, and the defense has insurmountable problems.

Ronald Reagan sought to solve this dilemma by deploying laser weapons in space. These, theoretically, could overcome the built-in advantage the offense enjoys. It was a fantasy. The American Physical Society—the country’s premier association of physicists—concluded in 1987 that it would take decades simply to determine whether such technologies were even feasible.

This sent the program away from “Star Wars” laser weapons back to kinetic-kill weapons. After several years of pursuing impractical “Brilliant Pebbles” schemes that would house thousands of interceptor rockets in huge space “garages,” the program was forced to go back to ground-based systems, even with their inherent limitations.

Has the technology improved? Trump thinks so. “Ronald Reagan wanted this many years ago, but we really didn’t have the technology many years ago. Remember, they called it starship, spaceship, anything to mock him,” he said at the convention. “But now we have unbelievable technology. And why should other countries have this, and we don’t? No, no, we’re going to build an Iron Dome over our country, and we’re going to be sure that nothing can come and harm our people.”

While short-range directed energy weapons are now feasible, scientists are nowhere close to achieving the kinds of power, beam control and precision tracking require for space weapons. Nor have engineers overcome the substantial cost, maintenance and operational difficulties of putting dozens or hundreds of weapons in space. House Armed Services Committee Chairman Adam Smith, D-Washington, warned in a comment on Trump’s 2019 Missile Defense Review, “a space-based interceptor layer…has been studied repeatedly and found to be technologically challenging and prohibitively expensive.”

Even with the science and technology against him, Trump believes so strongly in this vision that he has made it the one defense plank in the new Republican Party platform other than “strengthen our military.” There are 20 points in the official GOP agenda. Number eight is: “PREVENT WORLD WAR THREE, RESTORE PEACE IN EUROPE AND IN THE MIDDLE EAST, AND BUILD A GREAT IRON DOME MISSILE DEFENSE SHIELD OVER OUR ENTIRE COUNTRY — ALL MADE IN AMERICA.” (All caps in the original.)

Similarly, Project 2025 calls for making missile defense “a top priority.” It treats the problem as if it were just a lack of political will, arguing that we must “abandon the existing policy of not defending the homeland against Russian and Chinese ballistic missiles.” It returns to the “Star Wars” vision: “Invest in future advanced missile defense technologies like directed energy or space-based missile defense that could defend against more numerous missile threats.”

In 1994, Rep. Newt Gingrich similarly had one and only one defense plank in his 10-point “Contract for America”: to deploy a national missile defense system. Then, too, Republican leaders arrived at their strategy by listening to conservative activists. The Heritage Foundation—the group behind today’s Project 2025—assembled a report that called for ending “the Clinton Administration's policy of intentionally leaving American cities and territory open to missile attack.” The report argued that for a few billion dollars America could develop and deploy “affordable, effective ballistic missile defenses.” All that was lacking, the report stated, was “a proper understanding of missiles defenses and the political will to build them.”

It was complete nonsense. A Republican White House and a Republican Congress spent billions but got nowhere with the scheme. Thirty years later, Donald Trump is trying to pull the same fast one, relying on the same group to sell the snake oil. It may be rhetorically appealing—Gingrich did capture the House—but it is utterly without scientific merit or strategic sense.

Let’s hope the American people have learned from the follies of the past.