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Imagine somebody puts a laser cannon in orbit. For my question it probably won't matter if it was a Bond-style villain or invading aliens. This laser cannon is powerful enough to destroy a tank on the surface of Earth, or perhaps even a bunker.

What would an observer on Earth see?

  • Would a short pulse or a continuous beam be more feasible to get through the atmosphere? See How powerful does an orbital laser cannon need to be? A continuous beam could be firing long enough to be seen, a pulse is more problematical.
  • Is the beam itself visible? Or is it obscured by the atmosphere heated by its passage? Would this be a line from the sky to the ground? Does it form immediately? How long does it last? Would it hurt the human eye?
  • Would there be smoke or billowing clouds as the laser hits the target? Or does that depend on the nature of the target?
  • A cannon in LEO would move at high speed relative to the ground, but would that be noticeable? I assuming the laser stays on target, so the movement near the ground is minimal). The laser strike should be over in a few seconds, at most...
  • What is the sound and how far does it carry? Does the answer to Pew Pew Lasers! What would directed energy weapons actually sound like? apply at this scale?
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Humans have successfully built lasers from microwave frequencies to x-ray frequencies. Some of frequency ranges wouldn't penetrate the atmosphere (e.g. ultraviolet-x-ray). Other specific frequencies (e.g. 9-13 $\mu$ wavelength for $CO_2$) are absorbed by the atmosphere's constituent molecules.

Ideal conditions

Under ideal conditions for the laser, the beam won't be absorbed or reflected by the atmosphere. That means the only thing you'd see is a literally blindingly bright spot at the target.

If a human with unprotected vision looks at the target during the shot, there is a good chance they will be permanently blinded. If a human with unprotected vision is in LOS but not looking, there's still a chance they'll be blinded by reflected light. The effects of the atmospheric plasma will be exactly like lightning (sight & sound). The stroke will be perfectly straight and visible even during daylight. It fades from visibility almost instantly even at night. For a <1 sec duration shot the angle change will be <0.5 degrees - so no noticeable fan shape to the shot.

  • Humans doing lab work with even modestly powered lasers (>0.005 W) wear eye protection that is optically opaque to the lasing frequency to prevent loss of eyesight.
  • Humans doing lab work with high powered lasers generally do so from bunkers/control rooms with no optical paths to the experiment.

"The Hard Kill" art from Atomic Rockets
<a href="http://www.projectrho.com/public_html/rocket/spacegunconvent.php#id--Laser_Cannon">The Hard Kill</a> art

Anyone able to see the target (whether looking or not) would be in extreme danger of permanent vision loss.

Realistic conditions

Beam propagation

The reality is that the atmosphere absorbs and reflects every frequency to some degree. This is partially due to the fact that the atmosphere always contains fine particulate matter (aka aerosols). As energy is transferred from the beam to the atmosphere it heats the atmosphere to a plasma.

Due to the difference in emissivity between the plasma and surrounding atmosphere, the plasma acts as a waveguide and channels the beam. Since the plasma is a just super heated gas, that gas will move around due to normal gas dynamic forces such as wind and convection. Which means that even with perfect aim, targeting, and beam director stability, your beam will wander around and hit objects near the target.

Beam duration

The required beam dwell time depends upon how powerful your beam is. It's possible to incapacitate your target through lower powered, long dwell time heating shots. It's also possible for your beam to shatter or drill holes through targets using pulsed lasers. The sudden deposition of large amounts of energy in a very short period of time vaporizes a portion of the target's armor and creates a shockwave in the armor.

In general the long-dwell time lasers require less beam power than the high intensity shots. However, the high intensity shots may incapacitate with lower beam energy.

Effects of orbital motion

Even for the lower powered longer dwell time lasers, we're talking seconds to maybe 2 minutes of dwell time. In 2 minutes a laser satellite in LEO will move approximately 500 miles which equates to about a 55 degree change in incident beam angle (about 1 degree change / 2 seconds).

Beam power

The US military attempted to shoot an orbiting satellite with it's Mid-Infrared Advanced Chemical Laser (MIRACL). This laser is a 1-2 MW Deuterium Fluoride laser. I believe an orbital laser to destroy a tank would need to be at a minimum this powerful and probably something like 10x or more powerful.

Beam sounds

For very high intensity short duration shots, the beam would appear as a perfectly straight stroke of lightning and sound like one too. If the shot managed to explosively destroy a tank or bunker, you would get that detonation sound too.

Beam appearance

Whether your beam is pulsed or continuous won't have much affect on its appearance. Either the beam will be powerful enough to cause the atmosphere to incandesce or it won't. Even if the beam is pulsed, the pulsing will be far too fast for the human eye to detect and the atmosphere will continue to incandesce (if it is hot enough to do so) between pulses anyway.

Laser test shoots down drone:
Laser shoots down drone

Note that you can't see the beam indicated that this is a "lower powered" beam incapable of shattering a tank.

What you don't know...

...about laser weapons can kill you. Nearly every laser technology has very low efficiencies. Most of the energy you pump into the laser gets turned into heat. Getting rid of waste heat in a laser weapon is going to be a big problem. In space, it's going to be a worse problem.

Note that laser cannon are notoriously inefficient. Free-electron lasers have a theoretical maximum efficiency of 65%, while others are lucky to get a third of that. This means if your beam power is 5,000 megawatts (five gigawatts), and your cannon has an efficiency of 20%, the cannon is producing 25,000 megawatts, of which 5,000 is laser beam and 20,000 is waste heat! Ken Burnside describes weapon lasers as blast furnaces that produce coherent light as a byproduct.

Other information

From the Laser cannon section of the Atomic Rockets website (a must read for every world builder) - from a section on laser damage during a space battle.

A single pulse with a total energy of 100 MJ would have the effect of the detonation of 25 kg of TNT. Everyone in the compartment who is not shredded by the shrapnel will have their lungs pulverized by the blast.

That same 100 MJ delivered as 1,000,000 pulses of 100 J each could very well drill a hole. The crew see a dazzling flash and flying sparks. Some may be blinded by the beam-flash. Anyone in the path of the beam has a hole through them (and the shock from the drilling of that personal hole could scatter the rest of them around the crew compartment). Everyone else would still be alive and would now be worrying about patching the hole.

Although it occurs to me that the jet of supersonic plasma escaping from the hole being drilled could have the combined effect of a blowtorch and grenade on anyone standing too close to the point of incidence, even if they are not directly in the beam. The effect would probably be similar to the arc flash you can get in high power, high voltage electrical systems, where jets of superheated plasma can cause severe burns from contact with the plasma, blast damage from the shock waves, blindness from the intense light produced, and flash burns from the radiated heat.

A continuous beam could have enough scattered and radiant heat to cause flash burns to those near the point of incidence, along with blinding those who are looking at the point of incidence when the beam burns through. If it burns a wide hole, people die quickly when the compartment explosively decompresses, throwing everyone into deep space. If it burns a narrow hole, the survivors who can see can just slap a patch over the hole to prevent the escape of their air.

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    $\begingroup$ Not much more to add to that. IF you are intent on attacking a target on the ground, you are probably better off using kinetic energy weapons (Thor, AKA Rods from God). Or you can nuke them from orbit. It's the only way to be sure.... $\endgroup$ – Thucydides Jan 25 '16 at 1:58
  • $\begingroup$ I like that answer, just waiting a bit to accept it. So what an observer of a short shot (less than a second) would see is a mostly straight line, glowing from the plasma, and possibly a brighter spot directly on the target if there is LOS. A few seconds later depending on the distance there is the bang, and some time after that the plasma line starts to fade. I guess the orbital movement woud not visibly fan the line. Right? $\endgroup$ – o.m. Jan 25 '16 at 17:24
  • $\begingroup$ @Thucydides, time might be an issue. Deorbiting the projectile could take more time than the forces on the ground have. Or there are no ground forces, and a single battleship in orbit would have to fire before the target comes over the horizon. $\endgroup$ – o.m. Jan 25 '16 at 17:27
  • $\begingroup$ To be clear - if a human with unprotected vision looks at the target during the shot, there is a good chance they will be permanently blinded. If a human with unprotected vision is in LOS but not looking, there's still a chance they'll be blinded by reflected light. The effects of the atmospheric plasma will be exactly like lightning (sight & sound). The stroke will be perfectly straight and visible even during daylight. It fades from visibility almost instantly even at night. For a <1 sec duration shot the angle change will be <0.5 degrees - so no noticeable fan shape to the shot. $\endgroup$ – Jim2B Jan 25 '16 at 18:22
  • $\begingroup$ The decision between using an orbital laser and KE weapons might come down to cost. Given the massive size of the laser installation's radiators and heat rejection system, bundles of THOR projectiles might be more cost effective. As well, you could orbit dozens to hundreds of THOR systems for the cost of a megawatt or Gigawatt laser, so any point on Earth will have coverage in a natter of minutes. Ideally you have both for different types of targets. $\endgroup$ – Thucydides Jan 26 '16 at 2:33

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