The Air Force aims to shoot down a theater ballistic missile with the jet by 2002, and if all goes as planned, a fleet of seven ABLs should be flying operational missions by 2008.

The contracting team will use a Boeing 747-400 airliner as the platform for the multi-megawatt laser, which will be designed to track and destroy enemy theater ballistic missiles hundreds of miles away in the early stages of flight. One type of theater ballistic missile is the SCUD - the weapon Saddam Hussein used in the Persian Gulf war to kill 28 American servicemembers and terrorize Israelis.

Secretary of the Air Force Sheila E. Widnall likened the airborne laser to the discovery of gunpowder.

"It isn't very often an innovation comes along that revolutionizes our operational concepts, tactics and strategies," Widnall said. "You can probably name them on one hand - the atomic bomb, the satellite, the jet engine, stealth, and the microchip. It's possible the airborne laser is in this league."

A crew of four, including pilot and copilot, will operate the airborne laser, which will patrol in pairs at high altitude, about 40,000 feet. The jets will fly in orbits over friendly territory, scanning the horizon for the telltale plumes of rising missiles. A tracking laser beam will illuminate the missile, and computers will then measure the distance and calculate its course and direction.

"The system has to be highly automated. The window of opportunity to kill a boosting missile is very short," said Col. Dick Tebay, ABL program director. "A typical missile in powered flight travels at about Mach 4, around two-thirds of a mile per second. The airborne laser's 'bullet,' however, will travel at the speed of light - 186,000 mph."

After acquiring and locking onto the target, a second laser - with weapons-class strength - will fire a three- to five-second burst from a turret located in the 747's nose. The laser's blast literally burns a hole through the missile's metal body, destroying it and raining debris down upon those launching it.

If an enemy fires a salvo of SCUDs, the ABL will automatically slue over to intercept the next missile in the tracking queue, and sequentially engage all targets until the ABL's magazine is empty, that is, until the laser's fuel runs out.

"The airborne laser will greatly change the complexion of ballistic missile warfare," Tebay said. "It offers a high deterrence for an enemy to fire their missiles. There's a high probability that their missiles will never reach the target, and a good chance the missiles will fall back in their own territory."

For the past 20 years, scientists at Phillips Laboratory, Kirtland Air Force Base, N.M., have researched and developed much of the technology the airborne laser will incorporate. In 1981, the Air Force proved the feasibility of such a system with the Airborne Laser Laboratory. Researchers fitted a gas-dynamic laser on a KC-135 tanker, which vaporized five AIM-9 Sidewinder missiles and a low-flying drone simulating a cruise missile.

The airborne laser will fire a Chemical Oxygen Iodine Laser, or COIL, which was invented at Phillips Lab in 1977. The laser's fuel consists of the same chemicals found in hair bleach and Drano - hydrogen peroxide and potassium hydroxide - which are then combined with chlorine gas and water. Each laser shot will expend about $1,000 worth of chemicals compared to the $1 million it costs to fire a defensive missile.

The laser operates at an infrared wavelength of 1.315 microns, which is invisible to the eye, travels easily through the atmosphere and delivers a wallop. The COIL's beam will leave the airplane through a nose turret.

"The COIL basically vaporizes the metal," said Dr. Keith Truesdell, Phillips Lab chief of the applied laser technology branch. "It deposits enough heat to laze a hole through it. It's like taking a magnifying glass and burning a hole through a piece of paper, but we do it through metal."

Although the ABL consumes chemicals instead of using a large power plant, weight is still a big concern.

"We had to make the COIL more efficient - stronger, smaller and lighter - so we could get it on the airplane. Increasing the efficiency and decreasing the weight were our two biggest hurdles," Truesdell said. "Up until 1992 we were studying an anti-satellite ground-based laser. If it's on the ground, you don't care how heavy it is."

By recycling chemicals, building with plastics and using a unique cooling process, the COIL team was able to make the laser lighter and more efficient while - at the same time - increasing its power by 400 percent in five years.

Another hurdle Phillips Lab had to overcome was atmospheric turbulence, which weakens and scatters the laser's beam, rendering it useless. Atmospheric turbulence is produced by fluctuations in air temperature, and it's the same phenomenon that causes stars to twinkle. If the distortion isn't corrected, the laser becomes much less effective.

Scientists at the Phillips Lab's Starfire Optical Range, home to the world's largest satellite-tracking telescope, pioneered a process called "laser beacon adaptive optics," which solved this problem. Adaptive optics relies on a deformable mirror, sometimes called a rubber mirror, to compensate for tilt and phase distortions in the atmosphere.

"We shine a laser into the night and look at the return for backscatter, then adjust a deformable mirror to correct for that," said Lt. Col. John Anderson, chief of Starfire Optical Range. "The mirror has 341 actuators that change at a rate of about a 1,000 per second. The end result is that it increases the beam's intensity on a target. It's a key enabling technology. Without it, you'd have too much beam spreading to do enough damage. If this doesn't work, the whole program doesn't work."

When the Air Force declassified its research into laser beacon adaptive optics in 1991, it yielded a breakthrough in the world of astronomy. Stargazers peeping through a telescope equipped with adaptive optics now can get a clearer, brighter picture of the universe.

"Since 1991, every major astronomical telescope on the planet has adopted an adaptive optics system," Anderson said. "It's revolutionized astronomy."

On clear nights, residents in nearby Albuquerque can see eerie green shafts of light stretch into the heavens as range researchers conduct experiments. Anderson said he's heard that some locals believe the Starfire Range is some sort of guiding light for extraterrestrial travelers.

"Let me assure you that if it's attracting aliens, it's not intentional. We're just trying to measure the atmosphere," Anderson said with tongue in cheek "Inviting ET is only a secondary benefit."

While some people readily accept the existence of aliens as fact, others find the notion of lasers hard to swallow. They think it's still just sci-fi.

"Submarines and airplanes were science fiction in Jules Verne's day - now both are realities," Truesdell said. "The same can be said of Buck Rogers' laser gun. Now the technology exists to make it possible. Lasers have come a long way since the days of pulp fiction, and we're still making advances. It has so many civilian applications - medicine, computers, entertainment, industry. ...

"Lasers have a bright future," the doctor said, seriously. "It's an exploding industry."