Missile Defense from Space

The partial deployment ofthe U.S. ground-based missile defense system in recent months—and more specifically, its technical failures—naturally raises the question of basing a ballistic missile defense system in space. Would such a system work?

A ballistic missile is most vulnerable during its boost phase, when it is not maneuvering and the still-burning rocket presents a strong infrared signature. The boost phase for a liquid-fuel intercontinental ballistic missile (ICBM) lasts some 250 seconds, while a solid-fuel ICBM may burn out in 170 seconds.

The U.S. military has understandably shown a great deal of interest in boost-phase missile defense. A recent study by the American Physical Society, in College Park, Md., analyzed two types of space weapons that have been proposed for intercepting incoming missiles during the boost phase: space-based interceptors (SBIs) that would propel a kinetic "kill vehicle" into a collision with the missile (much like the ground-based interceptors currently being deployed) and space-based lasers.

As the study noted, the size of the constellation of SBIs or lasers that would be needed grows in proportion to the number of simultaneous launches that might occur. For example, if a missile-defense constellation can handle at most three simultaneous missiles from a small region, an adversary could surely defeat this defense by launching four.

For use against missiles launched from, say, the small state of North Korea, boost-phase interceptors on nearby ships, or on Russian territory south of Vladivostok, would likely be considerably more capable, not to mention cheaper, than space-based interceptors. What's more, these fragile battleships of space would need to be protected from preemptive attack; we describe in the main text how low-Earth-orbit satellites are relatively easy to destroy.

Another proposal for space-based missile defense involves intercepting ICBMs in the 20 minutes of their midcourse fall through space. Though this has been a mainstay of missile-defense advocates since the Star Wars days of the mid-1980s, it is not part of the current administration's program for national missile defense. In large part, this is because midcourse SBIs have no technical advantage over ground-based interceptors and are more expensive.

Although the purpose of this article is not to analyze in depth the prospects for intercepting ICBMs, it is worth mentioning that systems limited to destroying missiles in the vacuum of space (that is, midcourse systems) will be useless unless they can deal with the countermeasure of cheap and easily deployed balloon decoys.