With new threats coming daily from North Korea, the question naturally arises whether the United States or any country could reliably defend against a missile attack. The head of U.S. Pacific Command told Congress yesterday that “we have a credible ability to defend the homeland,” but systems deployed by the U.S. and its allies in the past have consistently fallen short.

Fortunately, so has the threat. North Korea’s medium-range missile, thought to be a Musudan, is designed to deliver a warhead up to 2,500 miles, but there are no reports that it’s ever been tested, and its technology is based on the Cold War-era Russian R-27 Scud, also with an unverified record. Experts doubt that North Korea is capable of fitting the Musudan with a nuclear payload, so the threat would be from a conventional bomb.

That’s why this weekend the U.S. began moving a THAAD, or terminal high-altitude area defense, missile battery to the island of Guam, 2,100 miles to the southeast of Pyongyang. The THAAD system is a kinetic or “hit to kill” weapon, meant to stop an incoming threat by ramming it head-on. THAAD’s maker, Lockheed Martin, says in promotional literature that it has “a track record of 100 percent mission success in flight testing.” That’s true, at least for the production article. When the prototypes were fired in 1999, THAAD failed in its first six attempts, but then two consecutive target strikes convinced the U.S. ballistic missile defense organization (BMDO) to order batteries costing at least $818 million apiece. Still, THAAD has never been used in real-world conditions.

Along with the ability to shoot down threats, missile defense systems are judged on what the Pentagon calls their cost-to-kill ratio. North Korea’s missiles are far cheaper than any system designed to intercept them. The U.S. Patriot system, built for the Army in 1976, has been compared unfavorably in this regard to an Israeli system called Iron Dome, which is far less costly. But the comparison bears closer scrutiny. Each firing of an Iron Dome Tamir missile, developed in part with $200 million in U.S. aid, still costs at least 50 times more than the crude rockets lobbed by Israel’s enemies.

When the Israel Defense Force (IDF) announced last year that Iron Dome was up to 95 percent effective against rockets fired at Israel’s border cities by the Islamist group Hamas, U.S. media were impressed. The U.S. Army had initially claimed that Patriots stopped only 80 percent of the Soviet-designed Scud missiles Iraq launched at Saudi Arabia during the 1991 Gulf War, and a government audit later estimated the success against all Scuds at no more than 25 percent. Some analysts have urged that funds for the Patriot be cut back and that the Pentagon imitate the Israeli technology. Yet Patriots have been fired some 600 times in tests and exercises since 1991 and have benefitted from decades of advances in computing, radar, guidance, and software. Today’s Patriot uses phased-array radar that sees better through urban “clutter.” Its missiles launch faster and, with improved fins and attitude control, are more nimble than those of the 1991 system.

And, even though precise numbers for any defensive system are classified, it now appears that the Israeli estimate of Iron Dome success is also too high. In a single week in November, the IDF reported that it shot down 400 rockets, but that number was only a third of the total number of rockets fired at Israel. A decision to fire Iron Dome missiles depends on whether the trajectory of the incoming rocket threatens a populated area; most of the 1,200 rockets were left to fall. Of the rockets engaged, Iron Dome did destroy up to 95 percent.

In after-action studies of the Patriot system’s performance in the 1991 Gulf War, both the U.S. Army and General Accounting Office estimated its effectiveness against all missiles fired, not just against missiles engaged. In the 2003 Iraq war, the Army changed its tactic. It reported that Patriots “scored a perfect nine for nine.” The Arms Control Association posed questions about 14 Iraqi missiles, beyond the nine destroyed, that were not engaged; the Army did not address the inaction. 

In 2003, Patriots killed a U.S. Navy F/A-18 pilot and two crewmen of a British RAF Tornado; an F-16 might have suffered the same fate had it not fired on the Patriot battery first. Today, with input from radar, operators can distinguish aircraft from missiles, but the improvement has been evaluated only on test ranges.