To clarify:
Oops, sorry. Just saw this.
Wouldn’t the satellite be traveling way too fast over the surface if it was in LEO making sustained energy delivery to one spot impossible? Unless the satellite can start a fire in a few seconds while it is overhead (since on approach and departure it will be firing at an angle through a much thicker blanket of atmosphere) you’d need to be in geostationary orbit or close to it so your satellite moves much more slowly over the surface. But the further you are the more energy you lose and the wider your beam is spread.
Please see my note above.
Oops… Sorry about that.
You would presumably need some kind of phase conjugate tracking system to keep the laser focused on a specific spot until it heats to ignition temperature… Of course, it is very easy to keep the heat source on target if it originates at the point of application, e.g. a match or pyrotechnic device on the surface rather than shooting killer death rays from space, but that isn’t nearly as cool of a conspiracy theory.
Stranger
Damn, I’m sorry. I really got this off-track.
For sure. I was being a little too telegraphic there. I was not suggesting that space power systems are plausible weapons or even plausible improvised weapons.
What I was trying to say is that if somebody wanted a layman’s intro to the obstacles a space laser weapon had to overcome, the article would be a decent primer. Much of which you hit in your post; waste heat rejection and gathering that much power in the first place being the biggees.
Space Based Laser Weapons Since the OP is about burning up things with laser weapons located in orbit, this article is very pertinent and interesting.
Enough Mangetout enough!
How many time do we have to tell you to stop with the logical arguments!
Are we too hung up on the term “lasers”? Lasers mean something specific to science-types, but a laymen probably thinks that a magnifying glass they used to burn up ants as a kid was a type of laser.
So what about using a mirror (or mirrors) as a “sun gun”? That wiki article says a 9 sq km mirror at a distance of 8200 km could “boil oceans or burn cities.” How do they come up with that calculation, and is that a cheaper and easier approach to your reign of destruction?
There is no conceivable way that a 9 square kilometer mirror could have any noticeable effect at all, at that kind of distance. Using the OP’s figure of 10-30 times normal insolation to start a fire, you’d need a mirror with an apparent size 10-30 times that of the the Sun, call it a couple of degrees across.
Well, what if we get a “little” closer than LEO? At some point a dollar store magnifying glass will do the trick.
I think people are overestimating the energy density required here. Lasers for missile defense are in a whole different category since they need to punch through defensive measures and also work in a matter of seconds. There’s several orders of magnitude between a laser that will ignite dry vegetation vs. one that will destroy a missile.
I have a small handheld laser that will light a match. The beam is only barely visible, and probably only then because of dust. There’s no significant blooming. Scaled up to a larger diameter, it would easily ignite dry vegetation.
My small laser has poor divergence, but a high power space laser, spread over a large diameter, could still have sufficient energy density (while not being so high to cause blooming) and low divergence. Power isn’t a problem; you could power the laser continuously with a solar array 1/3 the size of the ISS solar array, and in reality you wouldn’t run it continuously. Some modest battery packs would provide more than enough pulse power capacity.
Sounds like Command & Conquer type stuff.
I think my OP estimate of 50 kw/m^2 was reasonable. The challenge I’m envisioning is primarily related to the distance involved, with beam divergence and tracking accuracy making it difficult to illuminate any given target area continuously with adequate intensity. In my OP, I noted that a 1-MW laser distributed over a 4.5-meter square target would get the job done. The MIRACL beam is just a few inches in diameter when it leaves the source. Call it zero width, and then at a range of 120 miles, a spread of 4.5 meters works out to a divergence angle of 0.0014 degrees. I know that physics sets a lower bound for divergence based on wavelength, and practical constraints move that bound even higher, but I don’t have a feel for what those bounds might be. Is 0.0014 degrees completely impossible?
Assuming that’s impossible, we’d have to go with an XKCD-style approach, i.e. wretched excess: pointing more and more lasers at the target until we achieve our illumination goal. The challenge now is one of tracking accuracy. Suppose for the sake of discussion we need to have a couple dozen lasers pointed at the target. Due to divergence, each beam has substantial width; we don’t need them to overlap with 100% perfection, we just need some tiny patch of target to be lit continuously by a portion of each of those beams, like a smoldering Venn diagram. If we’re using a couple dozen lasers, that must mean each beam is diverged to a couple dozen times the 4.5m square I mentioned, i.e. each beam is about 22 meters square. So I need to target each laser for 30 seconds with an accuracy of +/- 11 meters at a range of 120 miles, i.e. +/- 0.003 degrees. Is this achievable with existing technology? does atmospheric refraction make this impossible to achieve? Is an orbiting satellite stable enough to enable such accurate tracking?
I thought so too. I was referring to those bringing up space weapons as a comparison. 50 kW/m^2 is not very much compared to those systems. Again bringing up my cheap laser pointer, it had 100 mW in a ~1 mm beam. That’s about 100 kW/m^2. And nowhere close to causing blooming.
You’d be better off using a mirror or lens to start the beam out at roughly the diameter you want. That keeps the power density roughly constant as it passes through the atmosphere, and keeps divergence low. Easy enough to accomplish.
The minimum divergence is based on the wavelength and aperture. I don’t have time now to do the math but my intuition is that divergence really isn’t a concern here.
I can’t imagine tracking will be a problem, either. SpaceX is launching satellites now with laser comms systems that can target another satellite hundreds of miles away. And they are by no means the first.
Your laser pointer is nowhere close to blooming over what range? The more air you’re going through, the more significant blooming is.
It’s a long path alright, but two things work in our favor:
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The beam is going to be wide, so the power flux will be low. It’s going to be hard to quickly heat up the air along the beam path; gentle breezes will move slightly warmed air out of the beam path before any significant thermal lensing effects develop.
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Assuming the satellite is in LEO, the path will be constantly moving through the air as the satellite moves overhead, likewise making it difficult to put a lot of heat into any one parcel of air.
Would it count if space lasers were used as projectiles? They would have to be large enough so they wouldn’t entirely burn up in the atmosphere. Properly constructed the lasers would be a glob of molten incendiary material when they hit the ground that could spread over a pretty wide area and likely start a fire in dried out forests.