Space-based interceptors make even less sense now

The rationale behind Golden Dome’s mandate for space-based boost-phase defense made some sense. If orbiting interceptors could hit an enemy missile very early in flight—before it could deploy countermeasures—they would avoid the Achilles’ heel of defense systems that target missiles in midcourse. But now the Pentagon and contractors are proposing to also use space-based interceptors for midcourse defense, which would jack up the cost while defeating the purpose of going to space in the first place.

Today’s Ground-based Midcourse Defense, or GMD, system and its 44 ground-based interceptors in Alaska and California are designed to intercept warheads during their 30-minute travel through space. But in outer space, lightweight decoys follow the same trajectory as the heavier warheads—preventing the defense from identifying and destroying the true warhead. Any country that can build a long-range missile and nuclear warhead can also build decoys and other countermeasures, creating a defensive problem that remains unsolved despite decades of work. As a result, the GMD system would almost certainly be ineffective against an actual attack. 

One response to this problem is to focus on a different portion of a missile’s flight: its boost phase, when its rocket motors are burning and before it deploys countermeasures. But the boost phase lasts only about three minutes. The only way to station interceptors close enough to every possible launch point in “peer, near-peer, and rogue” countries is to put them in space. 

And “to station” is a misnomer that conceals the true difficulty and massive resource consumption of creating an effective boost-phase defense. Each interceptor satellite has a very brief time to accelerate to reach a boosting missile, requiring the satellites to be closely spaced to provide a defense without gaps. And because satellites move in orbit, those in position for an intercept will quickly move away and must be replaced by others moving in. (This simulation helps show why.)

To ensure that at least two interceptors are in position to reach a single attacking missile during its short boost time, the system would need several thousand interceptors in orbit. Defending against a salvo of 10 missiles would require a constellation 10 times that size—that is, tens of thousands of satellites. Defending against a full Russian or Chinese attack would require hundreds of thousands of satellites.

And that would be for a missile burn time of three minutes. During the Cold War, the U.S. and Soviet Union demonstrated that they could reduce the burn times of their solid-fueled missiles. If the U.S. deploys a space-based boost-phase system, one should expect Russia, China, and North Korea to work to reduce their burn times enough to effectively eliminate the possibility of boost phase intercepts.

The practical difficulties of boost-phase intercept have led the Pentagon and some advocates to talk about using space-based systems for midcourse defense instead. This would give the interceptors more time to reach their targets—perhaps 15 minutes, rather than less than three. That would reduce the total number of interceptors needed to have one or two in place to reach a single target in space at any time.

But this switch would simply return to the first show-stopping problem: countermeasures. These systems cannot distinguish warheads from decoys, regardless of whether the interceptors are based on the ground or in space. Attempting to intercept all the warheads and decoys each missile releases would vastly increase the number of interceptors required in orbit.  

And it is far more expensive to field a midcourse defense in space. In addition to the costs of building and launching thousands of interceptors, satellites have a limited lifetime, which depends on their altitude. Midcourse interceptors would all need to be replaced roughly every ten years. The cost of building and launching the satellites would come due again and again. 

The Pentagon appears poised to spend enormous amounts of money only to end up where it started: with an ineffective mid-course defense. Policymakers must beware of the promise of high-priced fixes that solve nothing.

The authors are physicists and researchers in the Laboratory for Nuclear Security and Policy at MIT.