In science fiction, it's one zap of a laser gun, and you're dead. But real-life energy weapons likely won't work that way.

Take the Advanced Tactical Laser (ATL) that U.S. Special Forces have begun to test-fire. Intended for "covert strikes," the ATL has been sold on its ability to blast away with pinpoint accuracy. A very rough estimate shows, however, that the effects when you target an individual are not quite what you might expect.

The ATL's laser beam is widely quoted as being ten centimeters wide at the target. It's exact power has never been stated, but it's somewhere in the hundred-kilowatt class. (The ATL has a single 12,000 lb laser module while the "megawatt class" Airborne Laser fourteen moduleseach of which is slightly larger, so a hundred kilowatts looks like a reasonable estimate. In addition a hundred kilowatts was the power of the original flying laser, the Airborne Laser Laboratory, and it's the target which new solid state lasers are aiming for, so it seems to be a sort of benchmark for weapons-grade lasers.) It may be somewhat higher (or lower). But by applying a little basic physics we can get a ballpark estimate of what this might do to flesh. For simplicity, I'll assume flesh has similar properties to water. The heat capacity of water is about 4.2 joules per gram per degree centigrade. The heat of vaporization (the energy needed to turn water at boiling point to steam) is 2261 joules per gram.

So if the beam stays on the same spot of the target for a full two seconds –- which is a very long time under the circumstances –- it would in theory boil off a disc around one centimeter deep. In real life, the laser would be much less effective, as smoke and steam would rapidly degrade the effectiveness of the beam. Also in real life, the energy is likely to be focused at the center of the beam. And flesh is not water. And nobody is going to hand around being lasered that long… But we're just trying to get a general idea of orders of magnitude here.

Bullets are lethal when they damage a vital organ (like the heart or the brain) or when they cause rapid blood loss. Most likely, a laser of this type would not easily be able to go deep enough to affect a vital organ. Plus, the laser would will be self-cauterizing, with the heat sealing off blood vessels. It's not going to kill you quickly.

While research in this area tends to be classified. But from what we know, the Air Force considers laser effects on eyes and skin, for the most part. Skin damage is very much easier to achieve than penetration; simply raising skin temperature to (say) 80C/ 180 f to a depth of a couple of millimeters will cause serious blistering (second-third degree burns). If 40% of the body is burned in this way, then the target will be disabled and may die.

A rough calculation suggests that exposed skin would be blistered/burned in under a twentieth of a second, so the beam could play over the target at quite a high rate. It's unclear whether clothing would have much protective effect or whether it would simply ignite and cause secondary burns.

So instead of "zap-and-you're-dead" in normal science fiction style, with a hundred kilowatt laser, it's more a matter of spraying the target all over to ensure they're done. The description of the ATL as a "long range blow torch" is probably quite accurate.

With this type of weapon, the effects are more like napalm than bullets. Humanitarian protests are likely. And one accidental lasering of a civilian could be enough to prevent the ATL being used as an anti-personnel weapon.

Higher power lasers and smaller beam diameters might be able to get a cleaner kill. The Airborne Laser is several times more powerful than the ATL, but is a huge device mounted in a 747. Very short pulse lasers can produce explosive effects on flesh, but they are a different matter to to continuous beams like the ATL, and I suspect will prove to be much more useful.

Incidentally, you can do the same sort of calculations for the Active Denial System (aka the pain beam) which is also rated at around a hundred kilowatts but which has a beam diameter of two meters. Some have alleged that the non-lethal ADS can be set to be deadly. In reality, it could only become a "death ray" with a much higher power or much smaller beam diameter, neither of which is feasible with the current design. If you really wanted to kill people using microwaves, you would use something with a much longer wavelength/lower frequency than the 95 GHz ADS, but that's another story…

Footnote: Tank Zapping At the Speed Of Light

How about busting tanks with the ATL? Well, if we conveniently ignore reflection, heat conduction and other complications and just look at the heat requirement, then the best you can hope for a hundred-kilowatt laser with a ten-centimeter spot diameter is that it can melt through a centimeter of steel in around eight seconds. If you have to vaporize the metal it will take longer.

Since the ATL can only fire for a maximum of around 30 seconds each time, it may not be very useful against armored vehicles. However, it should work fairly well against pressurized vessels used for gas storage or munitions (rockets, missiles etc) which can be set off by heating their outer skin.

[Photo: USAF]

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