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It may sound like a good idea. It really isn’t.
Reagan’s plan didn’t materialize, but the idea never went away. In 2018—against the Department of Defense’s wishes—Congress passed a bill calling for the Pentagon to start building space-based interceptors.
Unfortunately, it’s a terrible plan. Space-based missile defense is an ineffectual defense at best, and a very dangerous provocation at worst.
Understanding why space-based missile defense is a bad idea requires basic knowledge of how it works—and what it can and can’t do.
In a space-based system, “interceptors” are placed in orbit over the Earth. Design proposals vary, but the interceptors would look more or less like ordinary satellites, with solar panels to power them while they wait in orbit.
If an adversary’s missile launch were detected within range, the closest orbiting interceptor would use rocket engines to accelerate out of orbit and collide with the missile. This occurs during the attacking missile’s “boost phase,” when its engines are still burning, shortly after launch.
The ability to reach ICBMs during their boost phase is a key motivation for putting interceptors in space. If you can destroy the missile very early in flight, the adversary can’t use decoy warheads to confuse the defense—a tactic that limits the effectiveness of current missile defense systems. Basing the interceptors in space is one of the only available ways to get them close enough to the missile during launch.
For the system to work, at least one interceptor must always be in range of the launch site. But objects in low-altitude orbit don’t “hover” over a location—they circle the Earth. This means a single orbit must be filled with 40 or 50 interceptors. And because the Earth rotates, multiple orbits would be needed to ensure a given region is always covered.
To cover North Korea, for example, the United States would need 300 to 400 interceptors, spread among seven or eight orbits. A defense that works more globally—something that covers a larger country like Iran, for example—would require even more.
Such a system would easily become one of the most expensive military projects of all time. The National Academies of Science and Engineering estimated that even an “austere” system of 600 interceptors would cost taxpayers some $300 billion—roughly half of the entire US military’s annual budget.
But the biggest problem with space-based missile defense isn’t the cost—it’s the fragility. Launching several missiles at the same time could overwhelm the system; interceptors might knock down one or two, depending on the system’s design, but the rest would pass through. Doubling the number of missiles that the system could deal with would require doubling the size of the system.
Since the interceptors orbit at an altitude of a few hundred kilometers, they are also extremely vulnerable to anti-satellite weapons launched from the ground on short- or medium-range missiles. Adversaries could use these weapons to punch holes in the defense, rendering it useless.
From a security perspective, a defensive system that’s so easily bypassed doesn’t simply offer less protection; it provides no protection. More money or technology won’t change the underlying physics.
Sometimes military technologies cause conflict all by themselves—even if they’re never used.
In the case of space-based missile defense, even a small number of interceptors would escalate tensions with Russia and China. One reason is because the interceptors could be designed to attack other satellites that serve vital military and economic roles. The interceptors could target not only all low-Earth-orbiting satellites, but also satellites at higher orbits where GPS and communications satellites operate.
Russia and China would likely view any number of interceptors as a threat to their high-orbit satellites and respond strongly, perhaps building their own interceptors or shooting ours down.
Of course, we don’t know exactly what other actors would do, or what counter-reaction the United States would take. But all the likely outcomes—heightened tensions, an arms race, military conflict—would directly threaten US national security.
Therein lies the crux of space-based missile defense: it’s enormously expensive, easily defeated, and fundamentally destabilizing. It creates new problems without solving existing ones. It’s a bad idea.