A tip sets the plan in motion--a whispered warning of a North Korean nuclear launch, or of a shipment of biotoxins bound for a Hezbollah stronghold in Lebanon. Word races through the American intelligence network until it reaches U.S. Strategic Command headquarters, the Pentagon and, eventually, the White House. In the Pacific, a nuclear-powered Ohio class submarine surfaces, ready for the president's command to launch.

When the order comes, the sub shoots a 65-ton Trident II ballistic missile into the sky. Within 2 minutes, the missile is traveling at more than 20,000 ft. per second. Up and over the oceans and out of the atmosphere it soars for thousands of miles. At the top of its parabola, hanging in space, the Trident's four warheads separate and begin their screaming descent down toward the planet. Traveling as fast as 13,000 mph, the warheads are filled with scored tungsten rods with twice the strength of steel. Just above the target, the warheads detonate, showering the area with thousands of rods-each one up to 12 times as destructive as a .50-caliber bullet. Anything within 3000 sq. ft. of this whirling, metallic storm is obliterated.

If Pentagon strategists get their way, there will be no place on the planet to hide from such an assault. The plan is part of a program—in slow development since the 1990s, and now quickly coalescing in military circles—called Prompt Global Strike. It will begin with modified Tridents. But eventually, Prompt Global Strike could encompass new generations of aircraft and armaments five times faster than anything in the current American arsenal. One candidate: the X-51 hypersonic cruise missile, which is designed to hit Mach 5—roughly 3600 mph. The goal, according to the U.S. Strategic Command's deputy commander Lt. Gen. C. Robert Kehler, is "to strike virtually anywhere on the face of the Earth within 60 minutes."

The question is whether such an attack can be deployed without triggering World War III: Those tungsten-armed Tridents look, and fly, exactly like the deadliest weapons in the American nuclear arsenal.

QUICK HIT

The military is convinced that in the coming years it will need to act with this kind of speed against threats—terrorist leaders, smuggled nuclear or chemical arms—that emerge and disappear in a flash. There may be only hours, or minutes, to respond. "We know how to strike precisely. We know how to strike at long distances," says Kehler, whose office is in charge of the Defense Department's Global Strike mission. "What's different now is this sense of time."

The leading candidates to deliver Prompt Global Strike's swift knockout punch are the sub-launched Trident II missile and the X-51, a cruise missile launched from a B-52 and boosted to supersonic speed by a rocket. A scramjet takes it hypersonic.

Every strategist remembers Aug. 20, 1998, when the USS Abraham Lincoln Battle Group, stationed in the Arabian Sea, launched Tomahawk cruise missiles at an Al Qaeda training camp in eastern Afghanistan, hoping to take out Osama Bin Laden. With a top speed of 550 mph, the Tomahawks made the 1100-mile trip in 2 hours. By then, Bin Laden was gone—missed by less than an hour, according to Richard A. Clarke, former head of U.S. counterterrorism.

The American military already has weapons that can destroy just about anything in a matter of minutes: nuclear missiles. That terrifying capability was designed to contain Soviet adversaries. But as the Cold War recedes into memory, U.S. strategists worry that our nuclear threat is no longer credible—that we are too muscle-bound for our own good. Are we really prepared to wipe out Tehran in retribution for a single terrorist attack? Kill millions of Chinese for invading Taiwan? The answer is no.

Paradoxically, the weaker our enemies have grown, the less ominous our arsenal has become. Military theorists call it self-deterrence. "In today's environment, we've got zeros and ones. You can decide to engage with nuclear weapons—or not," says Capt. Terry J. Benedict, who runs the Navy's conventional Trident program from a nondescript office a few miles from the Pentagon. "The nation's leadership needs an intermediate step-to take the action required, without crossing to the one."

In 2001, Defense Department planners began searching for something that could hit a foe almost instantly without risking a nuclear holocaust. Most of the solutions—unmanned bombers, faster cruise missiles, hypersonic "glide vehicles" coasting in from space—required a decade or more of development. The Navy, however, had been testing conventionally armed Trident II missiles since 1993. With a few hundred million dollars, strategists said, the first Prompt Global Strike submarines could be ready to go in just two years.

The $60 million conventional missile needs to be far more accurate than the nuclear version. But the multiple warheads can lock onto GPS coordinates while streaking through space. Upon entering the atmosphere, the warheads use flaps to steer to a target. With the Trident II's range of 6000 nautical miles, subs armed with the missiles could threaten a whole continent's worth of enemy positions. "Now," says Benedict, who leads the Trident conversion effort, "we've got the capability to hold all of these targets in all these hot spots at risk at one time."

In 1988, Lockheed Martin's Trident II D5 nuclear ballistic missile entered service on Ohio class submarines. In the Prompt Global Strike program, each sub would be armed with 22 nuclear Tridents, along with two retrofitted Tridents, each with four independently targetable warheads. here's how a conventional Trident II would work.

NUCLEAR AMBIGUITY

Almost immediately, congressional critics and outside analysts attacked the missile plan. Everyone seemed satisfied that, technically, modified Tridents could meet Global Strike's requirements. But the Pentagon can't explain how the weapon will be deployed and who will be its intended target. "I just don't think they've got a plan for using these things," says a frustrated senior congressional aide.

First, there's the matter of intelligence. If a president is going to launch the first intercontinental ballistic missile attack in history, he'll need overwhelming evidence. Our ability to nail down that kind of quality information is patchy, at best. On March 19, 2003, the United States launched 40 cruise missiles at three locations outside Baghdad in hopes of killing Saddam Hussein and other senior military officials. It turned out the former Iraqi leader wasn't in any of the locations; the strikes killed at least a dozen people, although it's not clear if they were civilians or leadership targets.

The mission failed even though friendly forces controlled the area. At the heart of Prompt Global Strike is a much darker scenario: American troops are far from their intended target—or the enemy's air defenses are too tough to penetrate. "So let me get this straight," says Jeffrey Lewis, a Harvard University nuclear energy and weapons analyst. "We've got exquisite, fleeting intelligence in an area of immediate concern, but no forces nearby and, miraculously, a sub in just the right spot to attack. I suppose there's some chance of that. But it's pretty small."

Video still shows X-51 predecessor being test fired.

Click here for the full video.

More difficult to explain is how a conventional Trident could be launched without provoking a crisis even bigger than the one that it was meant to solve. The Navy's plan calls for arming Ohio class subs with two conventional and 22 nuclear Trident II missiles. (The Navy intends to cut its Ohio class fleet from 18 to 14 subs, with 12 in the water at any one time.) To outside observers, the subs' conventional and nuclear weapons would appear identical—the same size, the same speed, shooting from the same location.

Traditionally, the U.S. strategy is to shoot missiles over the North Pole. But the current, most likely Prompt Global Strike targets, North Korea and Iran, lie south of China and Russia—which would put those countries right under a pole-launched flight path. "For many minutes during their flight patterns, these missiles might appear to be headed towards targets in these nations," a congressional study notes. That could have world-changing consequences. "The launch of such a missile," Russian president Vladimir Putin said in his 2006 state of the nation address, "could provoke an inappropriate response from one of the nuclear powers, could provoke a full-scale counterattack using strategic nuclear forces.

The Navy and Strategic Command have proposed all kinds of fixes to address what a Senate Armed Services committee described as Prompt Global Strike's "nuclear ambiguity issues." The subs could be positioned in different locations for a conventional attack than for a nuclear one, military leaders argue. (But that could put the boats out of position for an instant strike.) Hotlines to Moscow and Beijing could warn leaders in those capitals of conventional missile attacks. That is, if those leaders take us at our word—and don't warn their allies in Pyongyang or Tehran to get out of the missile's way.

Former Secretary of Defense Donald Rumsfeld, in a press conference, didn't seem that concerned. "Everyone in the world would know that [the missile] was conventional," he said, "after it hit within 30 minutes."

Congress is decidedly less blasé. The House and Senate have ordered the Pentagon to come up with something more certain before they'll provide the $127 million requested in this year's budget for conventional Trident modification.

While Trident II missiles with conventional warheads could be deployed in a few years, it may take a decade or more to develop the X-51 WaveRider. The WaveRider destroys targets by simply crashing into them at hypersonic speeds. But the technology in this remarkable missile may have wider applications, including ultrafast planes and new space vehicles. Designed by Boeing and Pratt & Whitney for the Air Force Research Laboratory, the X-51 uses just one moving part—the fuel pump—to hit Mach 5, or 3600 mph.

Rocket booster The X-51 is carried to 45,000 ft. by a B-52 bomber or a fighter jet, then released. A rear-mounted Army Tactical Missile Systems rocket kicks in to propel the 1600-pound missile to Mach 4.5 and 100,000 ft. The rocket then drops away and the X-51's engine takes over. Internal inlet The missile's sharp nose funnels shock waves produced at hypersonic speeds into a rectangular opening on the craft's belly. The shock waves compress the air, eliminating mechanical parts that normally do this. Isolator This component adjusts airflow—which can reach 2500 pounds per square foot—to a stable pressure for the combustor. Slowing airflow increases drag on the vehicle, but allows for more complete combustion. Combustor Thrust is created when the compressed air mixes with a mist of JP-7 jet fuel and is ignited. Because hypersonic speeds generate sustained temperatures of up to 4500 degrees, the propellant also acts as a coolant—and prevents the X-51's engine walls from melting. Airflow PM consulted NASA to estimate the fluid dynamics for external airflow around the nose, engine, stabilizers and tail of an X-51 traveling at Mach 5. The rear contour illustrates the engine exhaust plume shape.

The USS Tennessee and other Ohio class subs carry 24 Trident II ballistic missiles in midship tubes. The 65-ton weapons are about 44 ft. long and 7 ft. wide. (Photograph by Yogi Inc / Corbis)

WAVE-RIDING WEAPON

Some officials in the Defense Department want to answer concerns about the Tridents with more radical solutions: exotic, high-tech devices capable of outracing any machine in their class to catch fleeting foes. If these weapons work as planned—and that's a big if—they could let the Pentagon launch lightning-quick attacks without risking a worldwide nuclear storm.

On the coffee table in his cavernous office in the Pentagon's E Ring, Air Force chief scientist Mark J. Lewis has a model of such a machine, a 14-ft.-long missile called the X-51 WaveRider. With an angled nose, flaps in the middle and an inlet on the underbelly, the device looks like a cross between a spaceship and a futuristic cruise missile. It's designed to go nearly seven times faster than a Tomahawk—a flight from the Arabian Sea to eastern Afghanistan would take 20 minutes—and destroy targets with its own kinetic energy. Test flights are scheduled for 2008.

The pressure, drag and high temperatures associated with hypersonic speeds (typically, greater than Mach 5, or 3600 mph) used to be considered too extreme for an aircraft to handle in a controlled way. Only ballistic missiles and spacecraft burning rocket fuel, shooting into space and roaring back to Earth, could go that fast.

What the X-51 does is to turn some of the most brutal effects of hypersonic flight to its advantage. Take shock waves, for example. Bursting through the air at a hypersonic rate produces a train of waves, one after the other, which can drag down an aircraft. But the X-51 is a "wave rider," with a sharp nose shaped to make the waves break at precisely the right angle. All of the pressure is directed beneath the missile, lifting it up. The shock waves also compress the air to help fuel the X-51's combustion process.

The craft is the same size and shape as a Joint Air-to-Surface Standoff Missile, so it can be attached to a B-52 or fighter jet. It runs on standard JP-7 jet fuel, not on rocket fuel, so it fits in neatly with the military's existing logistical chain. The X-51 is made from a fairly standard nickel alloy, not from exotic materials. And the advanced engine technology is very real. In 2004, NASA broke speed records while testing its X-43A, a precursor to the X-51 (see "Breakthrough Awards 2005," Nov. 2005). In a final test flight, the 12-ft.-long aircraft hit 7000 mph—nearly Mach 10. In other words, the X-51 is not just some lab experiment; it's being designed from the start to deploy. "I've got tremendous confidence in it working," the Air Force's Mark J. Lewis says.

That doesn't mean the X-51 will be in competition with a conventional Trident. It will have a range of only 600 nautical miles. And it first needs to be lifted into the air by a plane, then accelerated by a rocket-fueled booster before its hypersonic engine kicks in. But if the 2008 test flight is a success, the X-51 will be the first weapon other than a ballistic missile to fly at hypersonic speeds.

NO CONFUSION

The Trident II iteration of Prompt Global Strike foresaw a pushbutton war, fought from the White House. It assumed that the United States would have few allies or bases abroad from which to attack. Local commanders would be largely circumvented.

But alternate scenarios being drawn up let U.S. forces act much as they do today, only faster. Hypersonic weapons could make that happen. Put an X-51-equipped plane in the air, and it could enable commanders to hit targets for hundreds of miles around in minutes. Tips could be acted on instantly; subs wouldn't have to be in a perfect position in order to strike. Intelligence wouldn't have to race all the way to the Oval Office. Wrong information would produce local damage. And because the X-51 wouldn't be confused with a nuke—or have to fly threateningly over nuclear-armed countries—"you don't worry about starting World War III" when you score a direct hit, Lewis notes.

Hypersonic technology will take longer to develop than a conventional Trident. But the X-51, and weapons like it, might make the most sense for the Global Strike arsenal. After all, they reduce potential fallout from the riskiest part of the program: the human element.

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