Laser weapon fairy tales

Children delight in stories of magical devices that dispose trivially of a threatening presence: Aladdin's lamp or the Good Fairy's magic wand in early tales, the phaser of ''Star Trek'' and the ray gun of ''Buck Rogers'' in contemporary science-fiction myths are all fantasy devices that relieve feelings of childlike helplessness. Very large laser weapons in orbit around the Earth that would protect the United States from a Soviet missile attack have similar psychological appeal but are equally fictitious.

A laser is a device that produces a very intense stream of light waves that arrive in step at a target, so their destructive effect is the maximum possible. Each laser weapon would consist of a powerful laser, a large, movable, precisely controlled mirror to point the laser light beam at the target, sensors to detect the target, and energy stores and power-generating facilities.

Missile-defense lasers would be deployed on satellites in orbits some 1,000 kilometers above the Earth. From this altitude a satellite would be within striking distance of launching sites in the USSR for only a short period during each orbit. To ensure that at least one satellite would be within range at all times the total force would have to include about 50 satellites. A single satellite would have to be capable of destroying an entire flight or perhaps 1, 000 missiles during their boost stage, which lasts for about eight minutes. Therefore the satellite could devote about half a second to each missile.

A laser weapon would damage its target by overheating, melting, or cracking it. Damage is caused only by that fraction of the laser beam energy that is actually absorbed by the target. In general much less than 10 percent of the energy carried by the laser beam to the target would be absorbed by it and cause damage. The rest is reflected and gets lost. So the laser must generate ten times more energy than what would destroy the target.

Laser light has no trouble propagating in the vacuum of space, but a laser beam would spread out due to diffraction, an unavoidable consequence of the wave nature of light. So a beam that starts out one meter in diameter could spread to a 10-meter circle at the target 1,000 kilometers away. That spreading thins out the light, so the beam at the target is a hundred times less intense than it was at the laser.

In order, then, to tear the metal skin of an an ascending ICBM with laser light (something that has been shown to be possible in the laboratory), a laser weapon would have to generate a series of rapid pulses of light some thousandths of a second long, each equivalent to a million megawatts of power. If, instead, the chosen destruction mechanism would be burning a hole in the side of the missile, a 100 megawatt laser would be needed with a continuous beam that would take a few seconds to accomplish its destructive task.

One such powerful laser would not be adequate, then, because it might have to shoot down up to a thousand enemy missiles in something under eight minutes if it were confronted by an all-out ICBM attack, since it could devote less than a half second per missile (which is not enough time even to locate and track a missile). How much fuel would a perfect laser require for such a task? Five tons of fuel and coolant per pulse would be required to crack the skin of a missile and about one ton of consumables would be required to burn a hole in it. So each laser weapon system in orbit would have to be provided with more than 1,000 tons of fuel to be able to attack all 1,000 enemy missiles that it would have to defeat in case of an all-out Soviet attack against the United States.

In all, then, 50,000 tons of fuel would have to be carried in orbit. If the US had four space shuttles and each made four trips a year to outer space loaded just with fuel for the lasers, it would take a hundred years and $100 billion in transport costs alone to move the needed 50,000 tons.

One way to lessen the amount of fuel needed by each laser weapon in space is to make its mirror much bigger and devise lasers that produce light of shorter wavelengths than what is available now. Neither of these developments is forbidden by any physical law and so they are in principle possible. It is conceivable then that at some distant future time a laser weapon suitable for deployment in space could be constructed. Such a feat, however, is technologically extremely difficult and therefore improbable for the foreseeable future. The lasers that we have now are at least a thousand times less powerful than what would be needed for such a weapon, but, even if we ever were able to build lasers with the necessary power, the fuel requirement would obviate any practical antimissile system in space.

Neither the US nor any other country can build the mirrors several meters in diameter with perfect surfaces, yet rugged and steerable, that a laser antimissile system would require. Finally, the sensors needed to detect and track a missile speeding at five kilometers per second a thousand kilometers away would have to be a thousand times more stable and speedy than what we have now. But even if by some miracle we could overcome these technical and economic hurdles, antimissile laser weapons would still be hopelessly susceptible to enemy countermeasures: their sensors could be blinded, jammed, or fooled and enemy ballistic missiles could cheaply be made very resistant to laser light. Under these circumstances, proposals for the erection of a laser antiballistic missile defense in space sound like little more than childlike, wishful fantasies of omnipotence.

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