Cheap rockets fired by insurgents are taking a deadly toll in the Middle East. Can a new generation of solid-state lasers blow them out of the sky?

Rockets, mortars, and other forms of artillery have a long and grim history on the battlefield. In a conventional war, an army being bombarded by these from afar can respond by firing back at the attacker’s battery. But you can’t turn the massive firepower of modern armies onto insurgents hiding among civilian populations without courting disaster. Instead of striking the enemy, who run to other hiding spots after firing their weapons, such retaliation would mostly hit civilians.

What the U.S. military dearly wants is a weapon that can defend against such attacks more selectively, shooting down explosive-laden projectiles in the air before they reach their targets. The armament should be easy to field and should strike at the speed of light, but it should not send streams of bullets screaming toward the horizon. In short, the military wants a laser weapon that’s small and rugged but powerful enough to ignite explosive payloads on incoming projectiles while they’re still a safe distance away.

It’s a bold vision for laser defense, bolstered by a dramatic technology demonstration that didn’t make Page One: For five solid minutes in March, an electrically powered solid-state laser pumped out 100 kilowatts of infrared light, the first of its kind to make ”weapons class.”

Each armed service has its own plans for that technology. The U.S. Army and its Israeli allies want truck-mounted lasers to zap short-range rockets on the battlefield or border. The U.S. Air Force wants compact lasers for fighter jets. The Navy wants to defend ships against attacks. And research efforts in China and Russia have been reported as far back as 1995.

And yet, laser weapon R&D is celebrating its 50th birthday this year without much to show for it. In fact, in early April the U.S. Defense Department shelved plans to buy a fleet of 747s to house giant gas-filled antimissile lasers. The old technology was proving too bulky and underpowered to blow North Korean missiles out of the sky without flying within antiaircraft range.

High-energy laser research is at an inflection point. Powered by semiconductors, a new generation of lasers promises new opportunities—and presents a whole new batch of problems.

Laser weapons, like flying cars, have been demonstrated many times, but in the real world their problems have always outweighed their benefits—literally. Weight cripples laser weapons and flying cars alike. Most experimental laser weapons have been so big and heavy that cynical observers have joked that their only conceivable combat use would be to drop them on the enemy.

That’s because the size of a laser weapon is inversely related to its efficiency—and laser efficiencies can be pretty dismal. The red helium-neon gas laser long used for classroom demonstrations turned only 0.01 to 0.1 percent of electrical power input into light. The diode lasers used in today’s inexpensive laser pointer do much better, converting about 10 percent of the electrical energy they draw from their batteries into light. The rest is lost as heat. This is no big deal for a milliwatt-power laser pointer, because the heat generated is negligible. But it’s a thorny problem for a laser weapon. At 10 percent efficiency, it would take 1 megawatt to generate a 100-kilowatt laser beam, leaving 900 kW as heat that must be dissipated somehow.

But that didn’t stop the U.S. Missile Defense Agency from building a megawatt laser. To achieve a 1-MW beam with 10 percent efficiency would require a whopping 10 MW of input energy and produce a hefty 9 MW of waste heat. Nevertheless, later this year a beast with such power, called the Airborne Laser (ABL), will be put to the test of blasting dummy nuclear missiles from the sky.

Here’s how. ABL is the latest example in a class of high-energy lasers called flowing-gas lasers. They are powered by burning chemical fuels like those that drive rocket engines. Hot molecules in the gas emit a cascade of light emissions, producing a powerful laser beam. Rocket-engine lasers have generated infrared beams that can reach a couple of megawatts for a few seconds at a time. The technology used in ABL can turn more than 20 percent of the combustion energy into laser light in the laboratory, but ABL’s efficiency is undisclosed. In such a laser, the exhaust gas carries away the energy left behind as heat.

But so far the US $5 billion ABL can barely squeeze into a Boeing 747. The laser is completely unsuited to the battlefield. It’s being designed to destroy long-range missiles rising through the atmosphere a couple of hundred kilometers away, but it’s vastly overpowered for the comparatively easy job of hitting slow-moving mortar shells only a kilometer or two away. It would be like shooting deer with a cannon. So in 1996 the U.S. Army and the Israeli Ministry of Defense teamed up to test smaller lasers against mortars and rockets. For that task, they tapped Redondo Beach, Calif.–based aerospace contractor TRW (acquired by Northrop Grumman Corp. in 2002) to build a 100-kilowatt-class flowing-gas laser, a compact version of ABL.

The result, called the Tactical High-Energy Laser (THEL), made laser defense look promising. In 2000, it shot down a short-range Katyusha rocket over the White Sands Missile Range in New Mexico. But by 2004 the United States and Israel agreed THEL wasn’t up to the job, ending any further tests.