A US military plane equipped with a powerful laser has moved a step closer to becoming a viable weapon.
Engineers have started flowing chemical fuel through the laser to test its sequencing and control.
This will set up the first test firing of the weapon aboard the aircraft while it is on the ground.
The US Air Force's Airborne Laser (ABL) is designed to shoot down enemy ballistic missiles in the early stages of their flight.
"The Airborne Laser team has done a great job preparing the high-energy laser for these fuel tests, which will lead the way toward achieving 'first light' of the laser aboard the aircraft," said Mike Rinn, vice president of Boeing, which is prime contractor on the project.
"Once again, we made and demonstrated enormous progress toward ushering in a new age of directed-energy weapons."
Airborne intercept
The ABL illuminates the missile with a laser tracking beam, while computers measure its distance and calculate its course and direction.
After acquiring and locking onto the target, a second, high-power laser fires a three-to-five-second burst from a turret located 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 COIL is housed in the back half of the 747. The aircraft's front half contains the beam control/fire control system, developed by Lockheed Martin, and the battle management system, built by Boeing.
Pros and cons
After the laser has undergone a series of test firings on the ground, the entire system will be flown to carry out functional check-outs.
The first airborne intercept of an in-flight ballistic missile is planned for 2009.
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 could be a concern.
However, it may be difficult to position the intercept weapon - in this case the aircraft - to shoot down the missiles. There is also limited time in which to destroy the missile in this early stage of flight.
The ABL's range is limited by the distance its beam can propagate through the atmosphere and remain focused.
This is affected by atmospheric conditions, turbulence (changes in air density as the result of heating and cooling), humidity, clouds and the time of day.
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.