Flash! Zap! Boom! Lasers come of age

By Sydney J. Freedberg Jr.

April 23, 2001

H.G. Wells dreamed up death rays in 1897. His idea of making weapons out of light has been a pop-culture fantasy for a century, a laboratory curiosity for 40 years, and a political punch line for 20, ever since President Reagan's scheme for anti- missile rays was derided as "Star Wars." Even as recently as 10 years ago, the consensus was that light-beam weapons--lasers, in today's lexicon--were "a waste of everyone's time and money," said Navy War College professor Bill Martel.

But today, at a facility in Wichita, Kan., a Boeing 747 freighter is being converted to carry the prototype of America's first operational energy weapon, the Airborne Laser, built by TRW. With tests against real ballistic missiles planned for 2003, and a three-plane squadron scheduled to debut in 2007, the ambitious $11 billion program is just the most prominent of several military laser weapons now in development.

The joint U.S.-Israeli Tactical High Energy Laser (also built by TRW) has already successfully shot down crude, short- range, terrorist-style "Katyusha" rockets--though, at roughly the size of a small house, this prototype laser is too bulky for the battlefield. The Air Force is about to award a contract to equip its lumbering transport planes with small lasers that can blind incoming heat-seeking anti-aircraft missiles. Both of these weapons are defensive in nature. But by 2005, the Army is hoping to test the "Airborne Tactical Laser," which could shoot out enemy tires, tank tracks, and antenna from aircraft aloft.

These are not the Flash! Zap! Boom! blasters of the movies, though. The lasers being developed today are silent and invisible, tuned to infrared wavelengths beyond human sight, and the way they attack their target is less like the "Death Star," and more like a welding torch. They don't explode things; they melt them.

These lasers also have a drawback--their energy comes from large tanks of industrial chemicals, which have to be mixed until they glow, like an outsize high-school science project. And they are so bulky that one weapon fills a large aircraft, or a small building. Reagan's dream of a space-based laser--compact and hardy enough to launch into orbit--is still at least a dozen years away from a first flight test. But for all their limits, real-life lasers offer the military the intriguing possibility of quietly, quickly disabling a potential threat from a safe distance.

The Pentagon actually has used lasers for years: U.S. troops dropped laser-guided bombs on Vietnam, and laser sights and rangefinders are commonplace in today's military. What's novel, however, is using a laser not just as a tool to aim a weapon, but as a weapon in its own right. Surprisingly, despite the technology revolution of the past 20 years, the most powerful laser weapon on the planet today remains TRW's Mid-Infra-Red Advanced Chemical Laser, built for the Navy in 1980. The catch to this MIRACL weapon is that it takes up most of a five-story building in New Mexico--too big to ever fit on a ship at sea.

The problem in developing a workable laser weapon isn't with the laser itself, but with the bulky, balky supporting systems: the chemical mixing modules to power it, the coolants to keep it from melting itself, and--above all--the computers to control, aim, and focus it. A laser is more than a very bright light. It derives its power not from the intensity of light, but from the precise control of the light's wavelength. An ordinary searchlight beam, for example, is like a crowd of people ambling down the street, all at their own pace, while a laser's "coherent" light is like a phalanx of soldiers marching in lockstep, shoulder to shoulder, ready to bowl over anyone in their way. And keeping light in tight formation is not easy. Clouds, smoke, water vapor, and even empty air all diffuse and distort passing light (that's why the sky looks blue, not clear).

So, to keep the Airborne Laser's beam on target, the system must monitor atmospheric turbulence in its line of fire and adjust the beam several thousand times a second with a "deformable mirror" that can flex its reflective surface in precisely measured millionth-of-a-meter increments. The computing power required to make those adjustments is roughly equal to that of 125 late-model personal computers equipped with Pentium chips. Fitting that computing power onto an airplane became possible only recently. "The key thing, really, is the miniaturization of computers," said Alan Shaffer, the Pentagon's director of science and technology plans and programs. "That enables everything else."

If everything else works, the Airborne Laser will pull off a variant of the old magnifying-glass-and-ant trick--only this time, the "ant" is an incoming rocket a hundred miles away and closing fast. Experts predict that lasers will be most effective in airborne, and ultimately space-borne, settings, high above the clouds. Battlefield lasers may be less effective because of the potential for their beams to be baffled by rain, fog, dust, and smoke. Down at sea level, where the air is even thicker and more saturated with water, the situation is worse. That's why the Navy gave up on lasers in the 1980s. Brookings Institution analyst Michael O'Hanlon said: "If I had short-range Katyusha rockets" up against a tactical laser defense, "I'd probably wait for a cloudy day" to fire the rockets.

Weather is not the only obstacle that lasers encounter: Targets with absorbent or reflective coating could foil the light beams, according to O'Hanlon. Tests show, however, that lasers could burn away some reflective coatings in less than a second. More problematic for a laser beam might be heat-absorbing tiles, such as those that protect nuclear warheads when they re-enter the atmosphere. "You can throw laser energy at a re-entry vehicle all day long. It's nothing compared to the heat of re-entry," Martel said. Such tiles are expensive, heavy, and hard to maintain, which might put them out of reach for some U.S. enemies, but more-mundane materials can soak up a laser too, according to Don Slater, Boeing's chief engineer on the Airborne Tactical Laser. "We can't do much to a brick," for example, he said.

This is the surprising secret of real-world lasers: They actually aren't very powerful. At peak power, the Airborne Laser generates one megawatt of energy, enough to run a large factory, or a thousand homes for about five seconds. But after traveling more than a hundred miles, the beam doesn't have enough energy to blow up its target. Fortunately, it doesn't have to. "All we have to do is cause a fracture in the metal," program spokesman Ken Englade said. "If we can cause that missile skin to crack, because it's under pressure and it's full of fuel, it's going to explode."

Likewise, the Army's Tactical High Energy Laser works by overheating the target rocket and setting off the high-explosive warhead it carries. The Airborne Tactical Laser, according to Boeing's Slater, "can melt through metal at something on the order of a millimeter per second." That's pretty effective for penetrating rockets, but it's a different story on the battlefield-the armor on a battle tank is typically one foot thick, or more.

Ironically, if you compare the impact of the cutting-edge Airborne Laser with the Air Force's standard 2,000-pound bomb, "the bomb is undoubtedly producing much, much more energy," said Charles Primmerman, a former laser scientist at the Massachusetts Institute of Technology, who now heads the Pentagon's High Energy Laser Joint Technology Office. The difference is that the laser's energy is focused, whereas the bomb's energy is scattered in all directions. For much of military history, such indiscriminate explosions were the only way to ensure a hit. But today's technology makes pinpoint precision possible, if not guaranteed. Gary Swart, Lockheed Martin's chief fire control designer, said the Airborne Laser can "hit a basketball a hundred miles away."

Aside from accuracy, a laser's other great advantage is speed. By definition, a laser travels to its target at the speed of light. That's 186,000 miles per second. Even the fastest missile travels only a few thousand miles per hour. Trying to catch one missile with another is a tricky game, as the Army discovered with its Patriot missiles during the Gulf War. But a laser beam moves 30,000 times as fast as any missile.

That speed is the laser's killer advantage. A laser cuts the time between seeing the target and hitting it to zero--and if it misses, the time between realizing the miss and firing again is zero as well. As an added bonus, each of those shots is fairly cheap: An Airborne Laser blast should consume no more than $10,000 worth of chemicals, and the rest of the multimillion- dollar weapon can be reused-whereas firing a missile blows at least $250,000 of hardware with every shot, hit or miss. The basic drawback of anti-missile missiles is that they are roughly comparable in cost and speed to the incoming missiles, so enemies can cost-effectively deluge any defense with more offense. "The only way out of that box is some sort of efficient beam weapon," such as an anti-missile laser, which is cheaper and faster than its target, argued Loren Thompson, a consultant and analyst at the pro-defense Lexington Institute.

The demand for missile defense drives most laser programs today. But, in the long run, the speed, precision, and efficiency of lasers may make them uniquely suited to the new American way of war: high-tech low-casualty, and long-range. Their bulk, low power, and problems in penetrating the atmosphere at low altitudes make lasers of little use against tanks, and (perhaps mercifully) inefficient against human beings. But against fast- moving, relatively fragile targets, such as missiles, lasers may be the best option around. "They won't fill every role, but they will fill a very important niche," said the Pentagon's Shaffer. "It's a complement, not a replacement, to existing weapons."

By Sydney J. Freedberg Jr.

April 23, 2001