Updated, 6:17 p.m.

For years, the Pentagon has tried to find a means to shoot down intercontinental ballistic missiles that could one day be launched from North Korea and Iran. That’s meant developing an expensive slew of interceptors to shoot down missiles mid-course — and, more ambitiously, during the “boost phase,” right after launch when the missile’s rocket engine is still firing. But a new report funded by the U.S. military’s missile-defense bureau says the military’s efforts to develop boost-phase interceptors are doomed to fail.

According to the nonpartisan National Research Council, the U.S. “should not invest any more money or resources in systems for boost-phase missile defense” (.pdf). Better, the council argues, to put money into the far more practical effort to destroy missiles during their ascent into the atmosphere. And the council’s report — released on Tuesday and funded by the military’s Missile Defense Agency — also endorses a scheme to fortify the East Coast of the United States with interceptor batteries that Danger Room has criticized in the past.

Boost-phase missile defense is “not practical or feasible,” the council concludes — the first time it’s waded into the contentious debate over missile defense. The time window for shooting down a missile at that initial launch phase is just too narrow. Were Iran to launch a liquid-fueled intercontinental ballistic missile at the U.S. homeland — which it doesn’t have and can’t do — the council estimates about four minutes of boost time before the missile enters into mid-course flight. For solid-fueled missiles, shave off another minute.

Hitting the missile during mid-course flight is the best bet at bringing it down. That’s the point when the ICBM has blasted into outer space, but hasn’t re-entered the atmosphere, giving the Pentagon enough time and space — in theory — to track and destroy it. Once an ICBM re-enters the atmosphere, however, taking it down is difficult, to say the least. That’s the “terminal phase,” when the missile is only moments away from hitting its target.

Too much has to go right with boost-phase missile interception. Boost-phase missile defense systems have to be situated close to an ICBM launch pad, which is impractical, since we’re talking about a hostile nation that presumably doesn’t want U.S. missile defenses nearby. Iran, for instance, keeps its launch pads buried deep inside its territory, which would mean flying aircraft-mounted missile interceptors within range of Tehran’s anti-aircraft missiles. The only feasible scenario where such a weapon could be useful is in North Korea — though that has thorny political implications — or “late enough enough the war so that an opponent’s air defenses have been thoroughly suppressed.”

And then you have to justify the cost. It’s possible to put boost-phase interceptors in space. But this would require hundreds of new satellites and half a trillion dollars in costs over the next two decades, according to the report. At the low end, Washington would need to build 650 satellites at a cost of $300 billion. But to be effective, the U.S. would have to send up close to 2,000 satellites. The cost for that, the report notes, would be correspondingly greater.

In short, there’s a reason why the Pentagon is littered with the shards of failed boost-phase missile defense systems like the Missile Defense Agency’s unreliable Airborne Laser which sought to zap missiles out of the sky, and the Kinetic Energy Interceptor. The Airborne Laser’s cost ballooned to $4 billion, and was expected to cost $92,000 an hour to fly, even had it worked. But it didn’t. The Kinetic Energy Interceptor also went far over budget, originally estimated at $4.6 billion but growing to cost an estimated $8.9 billion.

But the council isn’t giving up on missile defense. It argues that the military should focus its efforts into building mid-course interceptors like Aegis, the Patriot and the Terminal High-Altitude Area Defense system, or THAAD. It also argues that the U.S. ought to build a third major ICBM interceptor site on the U.S. east coast, to supplement the ones in Alaska and California. It’s an idea that has purchase from House Republicans — and one that this blog has criticized.

The council argues that Iran and North Korea’s efforts to acquire ICBMs could get leave New York, D.C. or other east coast cities exposed. Should either adversary nation acquire them, the council estimates it would take only about 40 minutes to reach the eastern seaboard. It argues building a new eastern interceptor site, using “previously demonstrated technology and implementations long known to be effective,” would “provide much longer and more effective concurrent threat observation during engagements by both X-band radars and the onboard sensors of the [interceptor] while closing on the threat complex.” Translated from the wonk: it would give the U.S. a better shot at tracking and destroying the incoming ICBM. Fort Drum, New York and northeastern Maine are two places the council proposes might host the interceptor sites.

It’s a debatable proposition. This blog has noted that analysts have been predicting Iran would soon develop an ICBM for 20 years now; and North Korea’s rocket technology, crucial for an ICBM, is weak. Beyond that, the Missile Defense Agency has said its existing defenses have the east coast covered, and will be supplemented by forthcoming interceptor placement in Europe and the Mediterranean. An east coast interceptor battery could also cost between $2 and $4 billion to set up and maintain.

The council, however, fears that the hesitations over an east-coast missile site are too optimistic. It criticizes the Missile Defense Agency for assuring that the east coast is protected, and argues that “layered” interceptors at multiple sites around the country would provide a more realistic assurance against future missile threats. And its rationale is consistent with its rejection of boost-phase interception: when defending against ICBMs, you want to give yourself as many chances to prevent the deadly missiles from reaching your territory as possible.

Updated, 6:17 p.m.: This post has been changed to give the National Research Council’s proposals for an east coast-based interceptor site a more thorough consideration.