Can we start ground fires with space lasers?

Factual Questions
41

I don’t think anyone has ever seriously suggested the use of lasers in orbit to destroy targets on the ground – the physical problems are too daunting. This is the stuff of bad science fiction and espionage fiction – the movie version of Diamonds are Forever, Robert Moss’ supermarket thriller Death Beam, The aforementioned movie Real Genius (although that caused indirect damage, through popcorn).

The reasons are clear, from the comments above – even the tightest laser beams spread out, thermal blooming in the atmosphere will spread your beam further and shift its direction, the difficulty of generating a beam of suficient power in orbit to enable to to wreak the kind of damage you want (assuming you like that sort of thing).

The Mid-Infra-Red Advanced Chemical Laser (MIRACL*) worked by combining two gases to create a mix that already had a population inversion. It put out a lot of energy, but to do that it had to have enormous tanks filled with the mix gases, so you didn’t get many shots. But at least it avoided the problems of generating a lot of power in orbit and dissipating the waste heat (lasers are inherently inefficient – to get a population inversion they HAVE to generate waste heat, unless they work like chemical lasers).
I remember watching the promotional film of MIRACL blasting a hole in a portion of rocket booster. It looked impressive on TV, but my first question was “how far away was it?” It turns out it was about a kilometer from the target. It melted a hole in the side of a pressurized tank filled with water, which burst like a balloon stuck with a pin. So, for reference,
a.) It didn’t set fire to anything
b.) It was awfully close by – much closer than even LEO

The only laser system that looks as if it really could wreak the kind of havoc you want is the nuclear-pumped laser like Excalibur, Edward Teller’s “Third Generation Nuclear Weapon”, and the catalyst for Reagan’;s March 23 1983 “Star Wars” Speech (that date is frozen in my mind). Set off a nuclear explosion that vaporizes wires strung out along the directions you want the beams to point, then pumps up the vapor to high energy levels by inverse bremsstrahlung and allows an amplified light beam to zap whatever the thing is aimed at. assuming that you’ve aimed properly and that hell explosion hasn’t knocked things out of alignment. (Technically not really a “laser”, since there are no mirrors and no resonant cavity. It’s really Amplified Spontaneous Emission, like a lot of excimer and nitrogen “lasers” are).

So that’s it – you get one shot at multiple targets, until you pop up another satellite with a bomb and laser wires.

Even then, it’s not a sure thing. Most things you want to target require a lot of energy to start burning. Water inside people and plants has to be heated up and boiled off first. A lot of items will reflect or scatter a lot of your light before you can even get started on the burning. It’s really easier, if you’re going to the trouble of sending stuff into orbit, to just drop a rock on them.

*I really do love the acronyms they come up with. I can imagine how they came up with this one:

“Well, sir, it’s a Mid-Infrared laser, so you’ve got “MIR” to start with.”

“Hmm. ‘MIR’ is Soviet for ‘world’, right? How about we add ‘Chemical Laser’ to it. Then it would be MIRCL. That’s close enough to ‘miracle’”

“We could say it’s an Advanced Chemical Laser. That would give us MIRACL. But we still need an ‘E’”

“Hmm. ‘Exoatmospheric’… no. ‘Executive’, ‘Executioner’… too negative. Let’s just leave it at MIRACL. That’s close enough.”

42

All the following is in very round numbers.

@Chronos’ 2 degrees subtends 1 unit at 30 units. So if you could orbit at 30 miles up you’d just need a mirror/magnifier 1 mile across. 30 miles is 150K feet. Well down in the draggy part of the atmosphere. Up at 90 miles you’re starting to get to orbiting, not flying. Now you need a mirror/magnifier 3 miles across.

Ref wiki, 180ish miles is about the bare minimum altitude. So 6 miles across. Something at that altitude will need frequent re-boosting.

To get up to a long-term drag-free orbit is more like double again. So now we’re talking 12 miles across.

Sounds expensive.

43

If a mirror won’t work, what about a giant magnifying glass that will focus, say, a square mile of sunlight onto a spot the size of a hula hoop? That should cook something, no?

44

Well we used to have a technology here at the Dope that was our go to response to these types of scenarios.

45

The kind of damage we’re talking about isn’t “instantly vaporize a main battle tank;” we’re just looking for “barely raise some a small patch of tinder-dry scrub brush to its ignition temperature (~260C) over the course of maybe 30 seconds”.

46

My favorite is the High Energy Laser Systems Test Facility. How would you like to be able to hand out business cards that say “Hellstaff”?

47

Well, I’m not talking about “vaporize a battle tank”:, either.

My point is that raising the temperature high enough to start a fire is surprisingly much more difficult than you think.

Working with high powered lasers over the years, I or the people I have been working with have blasted holes in plastic and in cinder block walls, but we’ve never set anything on fire. Even that MIRACL test didn’t set fire to anything.

The closest I’ve come is to unexpectedly melt a hole in a black synthetic shirt I was wearing. Even though the visible dye laser beam I was working with was spread out, the black fabric absorbed enough light energy that it caused it to actually melt – something I didn’t realize until I felt the warmth on my stomach and looked down. If I’d been wearing a white shirt I would’ve been fine.

But I’ve never seen anything set afire with a laser beam. It’s tougher than you’d think.

48

A magnifying glass is subject to the same focusing limits as a mirror array. From the perspective of a person on the ground, right in the middle of the target looking up at the sky, seeing an orbital magnifier so big that the area of the lens is twice the apparent size of the sun, the absolute maximum heat the person on the ground would feel is…twice the regular amount.

49

What a letdown. :frowning:

50

51

Stupid people have as many children as smart people, by and large. It takes a lot of stupidity to demonstrate a link between lack of intelligence with a lack of producing children; the Darwin Awards is a collection of people who have demonstrated this link.

52

In fact, one could say that stupid people have more children than intelligent people, because intelligent people tend to use BC.

53

Seems to me that a chemical laser in space would have to be a one shot device. Getting the fuel up there isn’t free. Far more cost effective to hand some evildoer a powerful (about $100) 405nm laser, a ticket to LA, and some cash for his effort. Matches might be cheaper and more effective.

54

I think “laser” is just common colloquialism for “directed-energy weapon.” A “space laser” wouldn’t necessarily have to literally be a light-based weapon. Would a microwave, particle beam, plasma, or other directed-energy weapon be any more effective?

55

I’m confused, what has this and the Darwin Awards to do with energy weapons in orbit?

56

I’ve done a search. You can find videos of people starting matches and suchlike with powerful lasers, especially when focused. Note that they have to have some “dwell” time, and they’re pointing at extremely flammable things intended to burn.

so you CAN start a fire with a laser – but for the most part you have to work at it, and have favorable circumstances. It’s easier to just strike the match.

With high power lasers, especially pulsed ones, when you feed a lot of energy into a small area the result is to blow it apart, rather than to start it burning. Hence the holes punched in plastic and cinder blocks, or the skin of that rocket booster. or (for practical purposes), those industrial lasers used to machine metal or burn wood, or those medical “laser scalpels” used to cut skin or shape corneas.

I looked on the Glowforge site (Glowforge uses a CO2 laser, like a lot of metal-cutting lasers), and they say there’s a poential for starting a fire on small bits of paper if you don’t clean the device out

I’m sure than a really big heat-inducing Zap from an satomic-pumped ASE laser would have a potential for starting a fire, but I still wouldn’t bet on it.

57

If someone wanted to set remote fires and can’t get people there, the easiest way to do it is with a drone.

It is trivially easy today to build a drone that can fly itself to a gps coordinate 50 miles away. No radio required. Then all you’d have to do is start dropping simple fire-starting devices. I could build one of these things in a couple of weeks for $500. That’s oh, maybe 10 million times cheaper than a space laser, much stealthier, and completely untraceable.

There are videos on youtube of amateurs flying remotely controlled planes using FPV goggles, over disrances of tens of miles. The drones fly themselves to GPS waypoints, then the pilot rakes over and can fly like he’s there. Using that level of sophistication you could fly around and drop little fire bombs wherever you wanted.

But with a fully autonomous one there’s not even radio communications to pick up and track. Once you launch it, there’s no way to cnnect you to it. A giant space laser would be instantly detected, and every country would know who launched it.

58

Lets make this simpler. What’s the feasibility of a plane-based laser? How high can it fly and still accommodate the size and weight of the lenses, mirrors, and power source needed to start a wildfire?

59

Distance isn’t that relevant. Blooming is a non-linear phenomenon that only matters at very high power densities, as the laser heats the air, causing refraction, and eventually turning it to plasma.

We already know that the atmosphere doesn’t cause significant distortion at low power densities, because satellites can take clear pictures of the ground at centimeter resolution.

It’s true of course that once you are in the blooming regime, then distance matters. It’s just that 50 kW/m^2 isn’t close to that. Machine_Elf also makes the excellent point that the beam path is moving, so you can’t even count on any heating effect over time.

60

I haven’t used a Glowforge specifically, but I can say that a 60 W Epilog cutter can easily light a 1/4" sheet of MDF on fire. Not just “potentially”; it’s the inevitable result if you aren’t careful. In fact you almost always get some minor flames, though they tend to go out on their own.