The first line of defense in America's next anti-missile system fails or succeeds in a window of 90 seconds. That's all the time there is, designers estimate, for a satellite to...

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The first line of defense in America’s next anti-missile system fails or succeeds in a window of 90 seconds.

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That’s all the time there is, designers estimate, for a satellite to detect the flash of an enemy launch, determine that it is real and send a counter-missile.

It all happens too fast to include a human in the loop.

“Time is of the essence,” said Craig van Schilfgaarde, the Northrop Grumman engineer in charge of the project.

Known as “boost phase” interception, it is designed to be the first “layer” of defense, firing rockets at enemy missiles immediately after launch, when they are most vulnerable.

The military has deployed parts of the two other layers in the missile-defense system — one targeting missiles as they cruise in midflight and the other aimed at descending warheads above their targets.

The three layers are the cornerstone of President Bush’s plan to defend the United States against nations such as North Korea and Iran that gradually are developing the ability to build weapons with global reach. But the system has faced serious problems.

The midcourse missile failed a test Dec. 15 when it shut down before leaving its silo on Kwajalein Atoll in the central Pacific Ocean. It was the second failure in a major test in two years.

On Dec. 17, the Pentagon announced that it was dropping plans to activate the existing pieces of the missile-defense system this year.

The boost-phase reaches into an even more complex realm of design, in part because of the speed with which it must identify and destroy an enemy missile.

The payoff could be big. Terry Little, executive director of the government’s Missile Defense Agency, said the boost-phase interceptors could destroy 80 percent to 90 percent of enemy ICBMs, leaving the other layers to take care of the rest.

But a recent Congressional Budget Office (CBO) technical report suggested that the boost-phase system, scheduled for deployment in 2011, would press the far edge of what is physically possible in such a system.

Philip Coyle, who headed the Pentagon’s testing office during the Clinton administration, said the design of the boost-phase system already is buckling under its complexity.

“The (congressional) analysis confirmed that boost-phase missile defense isn’t practicable,” Coyle said. “You can’t fool Mother Nature.”

Today’s missile-defense programs were inspired by President Reagan’s Strategic Defense Initiative, a plan to shield the nation against an all-out nuclear attack using satellite-fired interceptors.

Dubbed “Star Wars” by opponents in Congress, Reagan’s program fell victim to technical dead-ends, cost overruns and concerns that it would violate the 1972 Anti-Ballistic Missile Treaty, which banned nationwide missile-defense systems.

Missile defense languished until 2002, when Bush withdrew from the treaty.

Instead of trying to defend against all-out nuclear attack by a major power, today’s plan targets the less-advanced arsenals of emerging nuclear states.

The entire system is budgeted at about $50 billion over the next five years and is likely to cost several times that sum to build, deploy and maintain.

In July, the Missile Defense Agency began deploying the midcourse interceptors in Alaska. A second battery is scheduled for deployment next year at California’s Vandenberg Air Force Base.

Mobile Patriot anti-missile systems, a key part of the descent layer (also known as the terminal layer), already have been deployed.

The allure of striking enemy missiles in the boost phase is that they are easily identified by their plumes soon after launch and, because they are ascending, cannot use their full bag of tricks to dodge and deceive.

To destroy a missile in the boost phase requires an unprecedented coordination of space-based sensors, signal-analysis computers, interceptor agility and enough thrust to lift a 10-ton object to about 20 times the speed of sound in less than a minute.

Each interceptor consists of a two-stage booster, followed by a liquid-fuel rocket that steers the kill vehicle on the last leg of its journey to the target. It would travel at about 13,400 mph.

The CBO report said that defending against missiles from large countries might require interceptors that travel up to 22,000 mph — beyond today’s technology.

After infrared sensors on satellites detect the enemy launch, interceptors would be directed to the target by terrestrial command stations that constantly update the target’s flight path. Onboard sensors would take over at close range.

The interceptor’s goal is to strike the enemy missile before the warhead separates from its rocket, usually at an altitude below 300 miles.

The interceptors gain speed and agility because they don’t have to haul a heavy explosive warhead. Instead, they are designed to destroy their target with the force of collision.

This “kinetic” attack — described as hitting a bullet with a bullet — demands uncanny accuracy.

“What is the precision required? I would characterize it as within less than a meter” over hundreds of miles traveled, Little said.

To catch an ICBM streaking across the sky, interceptors would be placed about 600 miles from the target’s launch site on land or sea.

The military also is developing an airborne laser to shoot down ICBMs as they ascend.

“These guys are very, very immature in their development,” said Northrop’s van Schilfgaarde, referring to the missile programs of North Korea and Iran. Even if their technology improves, he said, “we have tremendous flexibility.”

However, even before it has gotten off the drawing boards, the boost-phase system has drawn criticism from a variety of scientists and engineers, who see it as technological hubris.

It’s a needlessly costly, complicated system for a threat that could, for example, be more easily neutralized with pre-emptive strikes, said Ted Postol, a missile expert at the Massachusetts Institute of Technology.

The Missile Defense Agency said 27 nations, including several with unstable governments, have ballistic missiles. No rogue nation can deliver a nuclear or chemical warhead to the United States, but each is striving to improve its technology. And proliferation is accelerating.

One of the most complex parts of the boost-phase interception is its sensing and targeting system. Launch commands would have to be automated because the launch window would close long before a human being could evaluate sensor data, particularly if several ICBMs were fired at once.

Yet, spy satellites that would direct the action are far from foolproof.

“Sensors are subject to huge (signal) noise problems, so you have to be careful not to launch too soon,” said David Mosher, an anti-missile expert with the Rand Corp. in Arlington, Va.

“Even bonfires are a problem,” said Coyle, the Clinton Pentagon official. “If you make them hot enough with chemicals, to our satellites at first glance they look like a rocket going off.”

Bigger doubts involve interceptor accuracy.

Midcourse missiles, which use a similar kinetic attack, have a spotty record. They have hit targets in five of nine tests, succeeding only under what Coyle regards as rigged conditions.

Even against slower-moving short- and medium-range rockets, anti-missile systems have been troubled. Patriot interceptors failed to hit nearly all of their targets during the 1991 Persian Gulf war, according to a congressional investigation and an analysis by outside scientists. In the Iraq war, Patriots mistakenly downed two coalition aircraft.

Some advocates of missile defense in Congress insist that only a space-based system — a new “Star Wars” — could provide sure global coverage.

But an orbital defense would pose even more formidable technical challenges and cost up to $224 billion, according to the CBO report.