The system will shoot down ballistic missiles in their "boost-phase"
The US military has carried out the first test-firing of a laser weapon system housed aboard a 747 plane.
The Airborne Laser (ABL) was conceived to shoot down enemy ballistic missiles in the early stages of their flight.
Engineers conducted the test on the ground, firing the laser out through a turret mounted on the nose of the plane at a simulated target.
An airborne intercept of an in-flight ballistic missile is planned for 2009.
The multi-billion dollar ABL programme has been running for more than 12 years.
Scientists are reported to be working out other uses for the flying weapon - which could help secure continued funding. These extra missions include shooting down surface-to-air missiles, cruise missiles and even enemy aircraft.
In September, engineers fired the high-energy laser into a calorimeter aboard the aircraft. But this is the first time the beam has been fired along the full length of the 747.
"The team has now completed the two major milestones it hoped to accomplish in 2008, keeping ABL on track to conduct the missile shoot-down demonstration planned for next year," said Scott Fancher, vice president and general manager of Boeing Missile Defense Systems.
Onwards and upwards
The latest ground test was carried out by the US Missile Defence Agency at Edwards Air Force Base in California.
A laser beam travelled the length of the aircraft at one billion km/h (670 million mph).
It raced from the aft (back) section that houses the laser, through the beam control and fire control system, and out through the nose-mounted turret.
When the laser beam emerged from the aircraft, it was captured by a diagnostic system which also provides simulated targets for the laser.
The high energy laser is fired from a turret in the aircraft's nose
The next step is to carry out some long duration firings of the laser.
"Once we complete those tests, we will begin demonstrating the entire weapon system in flight," said Michael Rinn, Boeing vice president and programme director for the ABL.
The ABL is designed to illuminate an enemy missile with a laser tracking beam, while computers measure its distance and calculate its course and direction.
After acquiring and locking on to the target, a second, high-power laser fires a three-to-five-second burst from the turret in the 747's nose.
The beam heats up the pressurised fuel tank of the outbound missile and causes it to rupture, destroying the missile.
The high-energy weapon is a Chemical Oxygen Iodine Laser (COIL) capable of producing megawatts of power.
Built by defence giant Northrop Grumman, it is designed to destroy "all classes" of ballistic missiles, including tactical ballistic missiles (TBMs) and intercontinental ballistic missiles (ICBMs).
Its fuel consists of chemicals found in hair bleach and drain cleaner - hydrogen peroxide and potassium hydroxide - which are then combined with chlorine gas and water.
The beam control system will acquire and track targets
Destroying ballistic missiles during their boost phase - while their rockets are firing - carries several advantages.
The bright, hot rocket exhaust aids detection, discrimination and targeting of the missile. And it is much more difficult to use countermeasures, such as decoys, during this phase of flight.
The wreckage will usually land in enemy territory, although collateral damage in surrounding countries would be a concern.
According to an American Physical Society report in 2004, the Airborne Laser could shoot down a typical liquid-fuel intercontinental ballistic missile (ICBM) from up to 600km away.
However, against solid-fuel ICBMs, which are more resistant to heating, the useful range would be about 300km.
This would be too short to defend against solid-fuel intercontinental ballistic missiles launched from Iran or North Korea, the US report explained.