Shooting down a ballistic missile with a laser sounds like science fiction. For a brief period around 2010, it was an American aircraft parked on a runway in California. The Boeing YAL-1 was a 747 with a megawatt-class laser built into its fuselage and a swivelling turret where the nose radar should be. It worked. It was then cancelled. Both facts deserve equal attention.

The engineering was genuinely remarkable, and so were the reasons it never entered service. The YAL-1 is one of the clearest cases in modern aerospace of a weapon that succeeded technically and failed operationally — and understanding why is more instructive than either the hype or the ridicule that followed.

The concept: hit the missile while it is weak

A ballistic missile is most vulnerable in its boost phase — the first few minutes after launch, while the booster is still burning. It is slow, it is climbing predictably, and it is extremely bright in the infrared. It has not yet released decoys or multiple warheads. Destroy it here and the debris falls back on the launching country, not the target.

The problem is reach. To attack a missile in boost phase you must be close, and you must be there at the moment of launch. The Airborne Laser's answer was to put the weapon on a large, long-endurance aircraft that could orbit near a suspected launch area and fire the instant a booster lit.

How the laser worked

The main weapon was a Chemical Oxygen Iodine Laser, or COIL — not an electric laser but a chemical one. Feeding chlorine gas through a solution of basic hydrogen peroxide produced energised oxygen, which transferred its energy to iodine molecules, which then emitted an intense infrared beam at a wavelength of about 1.315 micrometres. Six laser modules, each the size of a large SUV, occupied the rear of the fuselage. The by-products were hot, toxic chemicals, and the aircraft carried only enough reactants for a limited number of shots before it had to return and reload on the ground.

Firing the beam was only half the challenge. The atmosphere bends and scatters light, so the YAL-1 first fired lower-power tracking and beacon lasers at the target to measure the turbulence between aircraft and missile. A deformable mirror then pre-distorted the main beam to cancel that turbulence out — adaptive optics, the same principle astronomers use, run in real time against a missile climbing at supersonic speed. The corrected megawatt beam left through a 1.5-metre steerable mirror in the nose turret and had to hold on a single point of the booster long enough to heat it to failure.

February 2010: it actually did it

In early February 2010, off the California coast, the YAL-1 proved the concept. It destroyed a boosting solid-fuel target, and roughly a week later a liquid-fuel, sea-launched target — the first time an airborne directed-energy weapon had killed a ballistic missile in flight. For a system this ambitious, that is an extraordinary result.

And yet the U.S. Air Force Chief of Staff, watching the same success, drew the conclusion that would end the programme.

Test footage of the February 2010 boost-phase intercept.

Why it could never work in a real war

The fatal flaw was geometry. The beam's useful range against a hardened, moving booster was measured in a few hundred kilometres at best — and it shrank in cloud, haze or bad weather, which the atmosphere near the ground supplies generously. To be in position, the 747 would have to loiter continuously within that distance of an enemy's launch sites, which for a country like North Korea or Iran means orbiting inside or beside hostile airspace, around the clock, in a large, unstealthy, highly flammable aircraft full of jet fuel and laser chemicals.

That aircraft would itself be an obvious target for fighters and surface-to-air missiles. Keeping even a modest number on station over a crisis would require a fleet, tankers, escorts and bases — and each jet could only fire a handful of times before withdrawing to reload. The physics worked; the operational arithmetic did not.

The boneyard

Defense Secretary Robert Gates had already, in 2009, cut the Airborne Laser down to a research testbed and cancelled the planned second aircraft. The Missile Defense Agency formally terminated the programme in December 2011. On 12 February 2012 the only YAL-1 flew its final flight to the boneyard at Davis-Monthan Air Force Base in Arizona. It sat for two years and was scrapped in 2014.

Its legacy is not nothing. The tracking, beam-control and adaptive-optics work fed directly into the compact electric lasers now appearing on warships and ground vehicles — weapons that solve the YAL-1's fuel problem by drawing power from a generator rather than a chemical reaction. The megawatt 747 was the wrong platform. Much of what it proved was right.

A closer look at Boeing's flying laser and how the system was meant to work.

The engagement sequence from tracking to lethal intercept.

Sources: U.S. Missile Defense Agency; Boeing; U.S. Department of Defense statements (Gates, 2009; Schwartz, 2010); Wikipedia (cross-checked). Superlatives attributed to the MDA/USAF.