High energy laser anti-missile question

Per this Washington Post article this technology is apparently advancing.


Per this quote from the article why doesn’t the missile launching side simply paint or clad their missle with a highly reflective or polished surface to reflect the laser or do laser energies beyond a certain level destroy even a mirrored surface?

"The modified 747 actually carries three lasers. The first calculates the length of a missile from nose to plume. A second finds the “sweet spot” of the fuel tank. And the third, a high-energy “chemical oxygen iodine” laser with a beam equivalent of 100,000 100-watt light bulbs, burns a hole in the rocket’s hull.

“If we can blow the fuel tank up, we can kill the missile,” Forrest said."

No surface is 100% reflective. Silver is among the best at 97%. But, silver is quite heavy and not durable. You might be able to cover a rocket with a thin layer of silver, but even only working at 3% output a high powered laser could probably melt through that layer fairly quickly. This is assuming that the reflective surface survives the launch at all. One small hole is all it would take for the heat to be transmitted to the rocket.

in terms of reflection… this thread has a few good posts on that:


I believe the most effective countermeasure to lasers is to make the rocket spin (which many do anyway for stability). If the rocket is spinning around the longitudinal axis, the laser must stay on target much longer to get a specific spot to a specific temperature (i.e. it has to heat a ring around the rocket rather than a single spot).

If your laser isn’t Death Star strong, a little spin and a little reflectivity go a long way.

Micco is right. I did not want to imply with my first post that no countermeasures would work, just that going just with a shiney surface probably wouldn’t. In fact, I don’t doubt that if lasers become a major part of a missle defense system, very effective methods of defeating those lasers will also appear.

I asked the same question awhile ago in another thread and I’m too lazy to find the link.
Here were some of the responses I remeber:

1 Megawatt (another way of saying 100,000 x 100 Watt bulbs) of energy (watt is energy, right?) hitting a target in a fraction of a second isn’t like heating the rocket with a bunsun burner or burning an ant with a magnifying glass. Its like setting off an explosive right next to the skin of the rocket. The sudden heat vaporizes that spot which explodes, damaging stuff around it.

Possible countermeasures I’ve heard are reflective armor. abletive armor (which IIRC vaporizes to disrupt the beam) or spinning the rocket. It would probably be more effective to create countermeasures that prevent the rocket from being acquired by the laser in the first place. ie: Stealth, electronic countermeasures, etc.

You folks just remember all these answers about why mirring the surface doesn’t work the next time someone talks about using a mirror to steer your laser beam onto its target.

The other alternative is to swivel around your giant laser, barrel and all.

And how accurately do you think you can point a 747 in flight?

The ABL uses a deformable mirror (similar to the one on the Hubble) to focus the beam. I am guessing that:

  1. Energy is dispersed over a wider area of the mirror and then focused on the target.

  2. The laser device is cooled so that the parts don’t melt or explode.

  3. A highly reflective optical quality mirror is very expensive. It would not be cost effective to coat an entire rocket with one.


Just a tiny nitpick: the Hubble does not use deformable mirrors. Deformable mirrors (aka adaptive optics) is used to correct for atmospheric distortions, so there’s no need for one in space. (Actually, it would still be useful to correct for spacecraft vibrations and manufacturing errors of optics, and might be needed if we aim for much better resolution than the Hubble.)

I actually got to be a part of building/engineering the 747 laser plane. At the time it was just a 747 freighter shell though that got flown out of the Boeing plant to top-super-secret-parts-unknown to get all of the laser modifications done, so I missed the truly interesting bits. But anyway, from my slightly-more-than-layman’s knowledge of the subject…

I read a recent article in a military trade magazine (forget which one, Aerospace Weekly or something) that said that the plane is going to be using three lasers in conjunction. The first is going to be a laser that scans the whole missile target, from nose-cone to the bottom of the rocket plume, and then calculates the best aiming point to hit. The second laser targets that point to illuminate the exact place that the third laser (the big bang) will shoot at. The article implied that it would take a span of time to heat and burn through the target, with the intention of targetting the fuel tank of the rocket to make it explode from the heat on it’s hull. In other words, the laser isn’t meant to destroy the missile or blow it to pieces itself, it is just meant to be enough to heat it to the point where the missile blows itself up.

Interestingly, the article also speculated that a missile could easily be “hardened” with either ablative or reflective surfaces to resist the laser, rendering the 747 laser more or less obsolete at that point.

The article also said that the laser was designed with a maximum range of 200 miles. As to who mentioned ease of targeting from a 747? 200 miles is child’s play to target an object, especially one as large as an ICBM, assuming ideal conditions such as a vacuum. The true range limiter of the laser is apparently due to atmospheric conditions, air variations will disperse and distort the laser, as well as making exact targeting more difficult as range increases, think of the camera footage of space shuttle launches and how diffuse and shimmery the image gets due to air variations. And the shuttle is only being viewed through ten miles of atmosphere.

The plane will likely be flying at its highest altitude to maximize the range of the laser through the thin air (747 ceiling is around 40k feet) and shoot the missiles when they reach higher altitudes in their boost phase.

  1. I don’t know about the 747 device, but in most SDI proposals the laser beam is extremely more likely to actually have a higher power density on the mirror than on the target. Even in the case of the 747 laser, I’ll bet that the power density is comparable – I’ll bet the miror isn’t that damned big. Pull out a book with laser beam propagation equations and figure the relative beam sizes.

  2. and 3. Cooling or high reflectivity won’t help your mirror at all if you get a dust speck on the surface. The laser beam hits that dust speck and heats it up, damaging the surface of the mirror, which as the laser continues to heat it (or in subsequent bursts) damages the mirror surface still more, and so on and so on, in a destructive feedback loop. I’m not just playing devil’s advocate to be difficult here – I speak from personal experience with firing high power lasers. We managed to destroy some high power laser mirrors that were beautifully coated and protected in only a few shots at one of my past jobs. All it takes is a little misplaced dust.

This is whu I thought reflective surfaces on a missile really wouldn’t work as an effective countermeasure.

As the missile leaves its silo it is practically wreathed in smoke and flame. Once flying who’s to say what kind of crud the missile will pick up as it flies throught the atmosphere.

In the end I’d expect the surface of the missile to be fairly dirty. Maybe not horribly so but enough to achieve what CalMeacham is talking about.

Also, my brother (who is a high energy physicist and worked on some SDI stuff), mentioned to me awhile ago that when they were looking to stop ICBM’s they weren’t looking to necessarily blow the missile up. They basically wanted to pop the missile just enough to alter its course. No need to scan the whole thing for the proper spot to hit. Just hit it anywhere and the resulting ‘mini’ explosion would push the missile off course. Not by much but it wouldn’t need to be much. You may be thinking “What good is that?” but from a military standpoint getting the missile aimed for NORAD to hit Reno is a successful stop (or a missile aimed for New York to hit Albany or, if you’re lucky, somewhere in the Atlantic). Still not pretty I’ll grant but better than nothing if you are using the algebra of war to see what works.

Of course, back when my brother was working on this stuff computers were slower and a whole bevy of other technologies weren’t on par with what we can do today. His described method for stopping a missile had the advantage of simplicity (at least over the three laser model being discussed) and less energy needed to be placed on target (again, relatively less). The downside is you still get a missile landing somewhere.

Maybe I’m thinking of something else then. I know I’ve heard of adaptive optics being used somewhere before.
In any event, it sounds like its easier for the laser to damage the mirror than it is for the mirror to reflect the laser.

Maybe the question is “how do you make a mirror that can aim the laser without blowing up?”