But the Pentagon's former top weapons tester cast doubt on project, calling it far from militarily effective and perhaps easily defeated by a simple countermeasure.
The so-called Airborne Laser has been developed at a cost so far of about $3.5 billion with the aim of destroying, at the speed of light, all classes of ballistic missiles shortly after their launch. If successful in flight testing and deployed, it would become part of an emerging US anti-missile shield that also includes land- and sea-based interceptor missiles.
"You've demonstrated capability on the ground," Air Force Lt. Gen. Henry Obering said at a ceremony at which the aircraft was rolled out of a Wichita, Kansas, hangar where it has been undergoing modifications.
"Not since that time nearly twenty-two hundred years ago, when Archimedes reflected the sun's rays to set the Roman fleet on fire off Syracuse, has the world seen a weapon that puts fresh meaning into the phrase 'in real time'."
"Let's do it now in flight," Obering told employees of Boeing, the prime contractor, and chief subcontractors Lockheed Martin Corp. and Northrop Grumman Corp. at the event.
Philip Coyle, the Pentagon's chief weapons tester under former President Bill Clinton and now at the private Center for Defence Information, said in an e-mail reply to Reuters that its real effectiveness appeared doubtful.
"If a laser can be developed with enough power to penetrate the atmosphere and still be lethal once it reaches a target, an enemy would only need to put a reflective coating on the outside of its missiles to bounce off the laser beam, making it harmless," he said.
"The Romans could have done the same thing in the myth about Archimedes. Any grade schooler knows that you can set a dry leaf on fire with a magnifying glass. The challenge is to achieve militarily effective damage," he added.
Neither Boeing nor the Missile Defence Agency responded immediately to an offer to rebut Coyle's comments.
PENTAGON SEES BIG POTENTIAL
Engineers are to start installing a high-energy chemical oxygen iodine laser on the modified jumbo jet next year, with the first missile intercept test to take place in late 2008.
Pat Shanahan, vice president and general manager of Boeing Missile Defence Systems, said engineers had demonstrated "enormous progress toward ushering in a new age of technology, namely directed energy weapons."
Obering said the technology had the potential to change the nature of warfare.
"The news from North Korea and Iran has been consistently bleak," he said, referring to programs to "arm ballistic missiles of increasingly long range with lethal payloads."
In Wichita, engineers fully integrated the Lockheed-designed systems that control the beam and firing mechanisms in the aircraft, a modified 747-400F, Boeing said.
"The program achieved most of the objectives of the ground tests and expects to satisfy the remaining ones in the coming months," the company said in a statement.
Coyle said Boeing had omitted the "basic scientific and technical limitations that stand in the way of achieving an effective system."
Northrop Grumman supplies both the high-energy laser and a beacon illuminator laser used to measure atmospheric turbulence that it would encounter on its path to the target.

Copyright Reuters, 2006