The year is 2015, the adversary, North Korea. A missile roars into the sky on a mission to rain destruction on Los Angeles. The United States has about 30 minutes to save the city. Under that scenario, America’s response in the fateful half-hour after the launch will depend not only on what science can achieve, but on decisions made today and in the coming years by President George W. Bush and Congress. Bush last week began promoting an ambitious, but ambiguous, missile defense. He is trying to show that such a system is needed and can work. Skeptics of both claims abound. The president has a number of options. All pose enormous engineering challenges. Some are unspeakably expensive. Several are bones of contention with other world powers, even U.S. allies. None, however, is necessarily impossible. Indeed, they are in various stages of research and development and have been, in some cases, for years. In the North Korean scenario — a commonly studied one in Washington — the rocket is long, shiny and relatively slow at first. It shouts its presence with a terrific tail of fire. Its decoys are not yet deployed. It is as close to a sitting duck as a flying rocket gets. During this vulnerable “boost phase,” the United States has five minutes or less to bore a hole in it with a laser beam. Dreamers love the idea of taking out a missile this early. Bush likes it, too. Two systems have been conceived for this task. The more advanced one — to be deployed, possibly, late this decade — is the airborne laser. Seconds after the North Korean missile launch is detected, a modified 747 flying in nearby friendly or international airspace fires bursts from its nose turret for three to five seconds. A hit would send the rocket debris falling back on North Korea. The fuel for the laser has ingredients found in common hair bleach and drain cleaner. But the system is complex. Targeting the missile is especially hard because laser beams are distorted by air turbulence. Still, the plane gets more than one burst out before the missile climbs out of range. In theory, these lasers can do the job from a few hundred miles away. Ground testing already has shown these lasers to be lethal to missiles. Getting the engineering to work at 40,000 feet (12,000 meters) is the problem. The system still could prove to be a flop. The second system, even more pie-in-the sky, would operate from space. Lasers from as many as 24 scattered satellites would be deployed to shoot down missiles. Unlike the planes, which could not attack launches too deep in hostile territory and must be in constant patrol near the danger zone, space lasers could operate against missiles coming from anywhere, anytime. Even optimists, however, do not expect this system to be tested from space until 2010 or later. Bush, in laying out his hopes for missile defense, did not discuss space-based options. Still, research proceeds. Starting at about 100,000 feet (30,000 meters), the California-bound missile is becoming a tricky target. It is entering the midcourse phase of flight. The rocket stages have fallen away, leaving what is essentially a large artillery shell streaking into space at close to 15,000 mph (24,135 kph). Decoys are deployed to fool America’s next, best and perhaps last shot. A U.S. system of ground-based interceptors — the heart of the planned national missile defense — kicks in when the launch is detected. Advanced radar networks track the trajectory and try to tell the difference between the warhead and decoys. From the continental United States, interceptors rocket out of their silos on a hunt for the attacking missile outside the atmosphere. Ten minutes of frantic calculation and nail-biting ensue until it is known whether an interceptor has scored a hit. “We’d follow the same scenario that police do — keep shooting until the threat is no longer there,” said Air Force Lt. Col. Rick Lehner, speaking for the Pentagon’s Ballistic Missile Defense Organization. Each interceptor has a “kill vehicle” and a booster. Onboard sensors and communication devices allow computers and their human controllers on the ground to make course corrections. No explosives are needed on the interceptor. It is enough to smash into the incoming missile. A nighttime intercept more than 100 miles (160 kilometers) in space could probably be seen from the ground, Lehner said. A fisherman peering from the Pacific into a dark sky might note a “very brief flash of light.” The plan left over from the Clinton administration calls for 100 interceptors at a single site, either in Alaska or North Dakota. Bush must decide whether to pursue a more elaborate ground-based system. Going ahead would violate the Anti-Ballistic Missile Treaty with Moscow, a price he seems willing to pay. A fourth intercept test is planned next month. In tests so far, the interceptor missed twice and hit once. During the successful test, the interceptor was at first tempted by the decoy but veered to make the kill. Bush, like President Bill Clinton, is also pursuing missile defense with the idea of protecting U.S. troops abroad and U.S. allies from short- to long-range ballistic missiles. Those, generally, are ones traveling fewer than 4,025 miles (6,475 kilometers). Bush must decide how much he wants to spend on that battery of regional defenses, which include truck-mounted and ship based interceptors and updated Patriot missiles. Also to be determined is whether any of those defenses can double as protection for the United States from intercontinental ballistic missile attack. In fact, the laser systems were conceived for shorter-range missiles but are being studied now for their utility against ICBMs, which North Korea, Iran, Iraq, and perhaps others might someday be capable of sending across the ocean. Russia and mainland China possess such capabilities now. But the U.S. shield is meant to handle limited strikes from a smaller, unpredictable culprit, or even a mistaken launch. It could not stand against a massive assault from another superpower. For that, the old rule of nuclear deterrence applies: As you destroy us, we will destroy you. Once the North Korean warhead begins its arcing descent back toward the atmosphere, it will take about 10 minutes to reach its destination. Systems to knock out ICBMs in this terminal phase are not under serious development, although Navy interceptors or Patriots might be adapted for that purpose down the road. If nothing has hit it by now, probably nothing will.