III. BASEBALLS OVER SHEA STADIUM
"You'll need to empty all your pockets of any cell phones or the like," says the radar engineer, handing me a small box to put my stuff in. Visiting the SBX's radar unit -- the most powerful defensive radar in the world -- requires a whole new set of engineers, who lead me on a separate subtour. After surrendering our personal gear, we turn to a panel behind us that holds twenty-four big brasslike keys. For each of us who plans to enter the radar room, a single key is turned and withdrawn from a large lock. We put the key in our pockets. As long as even one of these keys is missing, the radar will not activate and zap us with a potentially lethal blast of radiation. In the Bruce Willis version of this story, this small room will be crucial in the "final scene of mayhem.

Inside the SBX's big white dome, the radar unit is the size of a small house, its face a giant circular billboard fronted by 45,056 radiation-receiver trumpets. When it is operating, the engineers tell me, the SBX could sit in Los Angeles Harbor and track a high-flying baseball over Shea Stadium in New York. The radar crew swears that if you were in here and the unit was accidentally turned on, it wouldn't really hurt you. But it's impossible not to be thankful that the key is in your pocket. The image in my head is of Marvin the Martian getting fried by a cartoon laser gun into a hovering thread of carbon.

"We do our part," one engineer tells me, referring to the mission of missile defense. And that part, while crucial, is only one small element of the entire shield. That's not to belittle the otherwise gargantuan SBX, but only to indicate just how massive and complex the full system of missile defense is. Actually, the preferred term of art among missile defense savants is "layered." By that, officials mean that there are many, many moving parts to the shield that provide different technological responses depending on the range of the ballistic missile that is headed our way -- short, medium, long, intermediate or intercontinental. Such choices, in turn, depend upon which phase of the missile's parabolic flight path you hope to intercept it in -- at the beginning (boost phase), in the middle (midcourse) or as it descends, hurtling into its target (terminal phase).

If the entire layered shield were up and running, and if an enemy were to fire a missile at America, here's how the system would hypothetically work:

In the first few minutes of the boost phase, a lot of technology would be brought to bear on the incoming missile. First, the rocket's signature plume would be spotted by our satellite systems -- either at takeoff or, possibly, when it broke through cloud cover. The missile would then be tracked by land-based radar systems that are already in position in Alaska, California, Greenland and England. The information would be relayed to officials at command and control -- probably in Colorado -- who would confirm it as a missile launch and order a response.

The boost phase of an ICBM lasts roughly four minutes, a period when the missile is rumbling at its most sluggish. It is the optimal time to blow it up. Unfortunately, the designers of our missile defense shield have yet to figure out a system that is capable of reliably intercepting a boost-phase rocket. The idea that is furthest along -- but still years away from working -- is to mount an airborne laser on a Boeing 747. The plane would attempt to focus a laser beam on the climbing rocket until its metal housing heated up and caved in. Or there's the Kinetic Energy Interceptor, also still in the idea phase, which would swoop down from space, or be fired from land, and crash into a boost-phase missile.

Should the missile get past the boost phase, it enters the midcourse phase. If the rocket is short to medium range, it could be shot down from sea by one of the Navy's three Aegis ships -- a key part of the shield -- which use high-powered radar to track and destroy incoming missiles. But if the rocket is a longer-range ICBM, it would spend twenty minutes or so in flight -- giving us more time to hit it. This is where the crew on board the SBX really goes to work. Using the unit's massive radar, the SBX would relay extremely precise information about the rocket's location back to central command. A course would get plotted and one of the U.S. interceptor missiles currently on standby in the ground in Alaska and California would fly toward the target.

But an interceptor missile won't have much of a chance of hitting its target if, as most experts assume, the enemy's nuclear warhead is surrounded by numerous decoys and chaff to baffle the radar. An interceptor can currently launch a single EKV, or Exoatmospheric Kill Vehicle, a device armed with its own infrared sensors designed to guide it on a collision course with the incoming missile. But with decoys and chaff, engineers now envision the interceptor launching MKVs, or Multiple Kill Vehicles. After receiving coordinates from the SBX, the onboard computers of this hypothetical interceptor would track the decoys, discriminating between the fakes and the real thing, and then launch kill devices on independent trajectories to destroy them all.

After the midcourse phase comes the terminal stage -- the final thirty seconds or so of a missile's descent. Short-range to medium missiles, which move more slowly than ICBMs, would be taken out by the PAC-3, the latest version of the Patriot interceptor made famous in the Gulf War. Or they could be brought down by Terminal High Altitude Area Defense, a device that looks like a big truck carrying sewer pipes -- except the pipes can tilt back off the rear bumper and shoot down a Scud from 125 miles off.

But if the enemy missile is a fast-moving ICBM, then the terminal-phase plan is just that: Everyone within its blast radius dies in a supernova of light that's 3,000 times brighter than the sun. There is no technology exclusively dedicated to stopping a nuke in its terminal stage.