I’m talking about the kind of weapon that was in “Die Another Day”, it was a satellite orbiting the Earth that focuses sunlight into a tight beam that is highly destructive, easily ripping through buildings and creating a crater where it hits.
Could this kind of weapon be practical in real life? Especially in terms of whether it can be made to be destructive enough to be worth the trouble.
One big problem (as per high energy optical lasers) is thermal bloom - the focused sunlight heats the air it passes through, causing thermal distortion (heat shimmer) that damages the focus, and thus the effectiveness.
Also, clouds. It will take such massive amounts of energy to drill through a heavy cloud layer, and your opponent will figure out what you are aiming at before it gets hit. You would really have to wait for a sunny day. The “Die Another Day” weapon was only really useful against the static minefields and other fixed emplacements of the Korean DMZ.
Finally, size. You need really big space-based mirrors with great orbital and directional coordination. Easy to make (foldable mylar) but not really a surprise weapon.
Si
Interesting. I was wondering the same thing. Except using it as a weapon we could use it to drill trough the top layer of ice on Jupiter’s Europa or Calisto (Which ever one has an ocean underneath).
After that, we could send a submersible probe down there and see if we can find any interesting lifeforms.
Sounds like a good way to destroy whatever ecosystem there is there.
It would, however, be a possible tool to create/steer hurricanes.
It would have to be a big mirror - the solar energy density out at Jupiter is much lower than at earths orbit (~1/27th from the inverse square law). Also, it would be much harder to keep the mirrors in a suitable orbit, as the space is gravitationally complex. Finally, there are more bits of ice and rock moving around Jupiter. You’re gonna break a few mirrors, and that means lots of bad luck.
I prefer the Quantum Weather Butterfly for that. Of course, you either don’t know where the hurricane is going to go, or how fast it is going to get there. 
Si
I’m sure the Monolith would intervene before that happened. Us being the dumb monkeys we are.
The mirror would need to be incredibly massive in order to have any practical effect from orbit. Imagine a truly enormous satellite big enough that it takes up about the same % of the sky that the sun does. So even the *best *a mirror of that size could do is to make that spot on the ground seem like it’s getting double the sun’s normal output. So it’ll certainly get warmer, but that’s about it.
What if there was a massive field of solar reflectors in the Sahara that could send a beam through a network of reflecting satellites? To redirect it back to Earth. That sounds like a more practical way of doing something like this. The only major issue is that those satellites would have to be positioned exactly right.
Seems to me that would be less efficient. You have loss of focus and power anytime your light beam passes through an atmosphere, and if you make the beam pass through the atmosphere twice, losing power and focus on the way up from the Sahara and on the way down, then you’ll find it much more difficult to create a useful weapon.
Not that it’s a bad idea, I’m actually all in favor of useless weapon systems that supervillains create, thinking they’ll be able to hold the world hostage, but that can’t actually hurt anyone. 
If you are just using reflective optics, the smallest spot you’ll be able to make on the earths surface is the distance from mirror to ground divided by 114. Lets say your mirror/mirror system is in low earth orbit. 250 mile divided by 114. Lets call that 2 miles wide.
For focusing lenses and mirrors there is a specification called the F number. That is the focal distance divided the diameter of the mirror/lens.
So lets pick a fixed focal length. The sun’s image at the focus/target area is X wide. The mirror is Y wide, reflecting Z watts of sunlight into that X wide image. Now lets make the mirror 2Y wide. It will be reflecting 4 times as much light (the surface area goes as the square of the diameter), but it is still “pumping it” into the same sized target. So the intensity in that target zone is 4 times higher.
A F2 system has 4 times the intensity of an F4 system. An F1 system has an intensity 16 times that of an F4 system.
From playing around with various lens and mirrors over the year, you can use an F4 or so system and focus it on your hand. About all it does is get warm (of course sending that directly into your eye would cause nearly instantaneous and permanent eye damage). To actually get the intensity high enough to actually cause stuff to catch fire in short order you need something in the F2 to F1 range IIRC. And thats stuff thats rather dry and easy to catch on fire like paper and cardboard.
So, if you want things bursting in flames or melting you are going to need a mirror system in orbit thats about 120 miles across at a minimum. And all that is going to do is set some things on fire. Its not going to be like one of the Star Trek ship phasors blasting the crap out of the target area from orbit. You could probably scale it down by a factor of 2 to 4 if you just wanted to melt roof tops, ruin car’s paint jobs, and kill of some types of vegetation in the area.
Oh, I could be of by perhaps a fator of 2 to 4 (WAG) due so scaling effects (or way the hell off due to doing the math in my head wrong). But it’s pretty obvious the mirror’s goinga have to be pretty damn big.
The reason I’m saying to put it in the Sahara is because there is 1000’s of square km of empty hot (no clouds, less moist air?) space there, perhaps that would offset the loss of energy because of passing through the atmosphere.
Just using mirrors, you’ll never get a mirror with the same apparent size as the Sun to look brighter than the Sun. Adding more mirrors will just increase the size of your spot on Earth, not make the space mirror look brighter at some location on Earth. So using mirrors in the Sahara will never work better than using the Sun directly.
ETA: And no, there’s no way to somehow use the Sahara mirrors and the Sun to overcome this. You could get two spots on Earth, but not one brighter one.
The Giant Space Mirror could easily be defeated by filling the air with particles similar to volcanic ash over any target. This would also stop incoming aircraft.
I recall some discussion that the USA considered such a mirror system during the Vietnam war. Not for random incineration, but to illuminate the jungle at night. It would be the brightness of the full moon or so, making it easier to spot infiltrators. One of the disadvantages, apparently, was that it would also contribute to heavier jungle growth. You win some, you lose some…
On thought occured to me. Even such a system that could not catch things on fire quickly probably would kill anybody not lucky enough to be in or near a building. Just a few minutes of breathing hot assed air is gonna kill you. And if it’s hot enough to be near or past the boiling point of water even faster than that.
Then again, the commuters in Atlanta will probably wonder where this cold front came from.
It’s not relevant how much room is in the Sahara since there’s no limitation size on the lens in space in the OP.
For any given size mirror in the Sahara, your effort would be better spent on a larger lens/mirror in space, since Saharan mirrors would just lose most of their efficiency in the atmosphere on their way up to the satellites anyway.
I don’t see why you can’t create really high temperatures; much smaller solar furnaces can generate temperatures as hot as the surface of the Sun. See example here, where they melt rock and metal in seconds and wood instantly bursts into flames, which certainly sounds like a “solar death ray” as described by the OP.
The difference is that the space mirror would be many miles away from its target, and would be separated from that target by a thick atmosphere. The intensity of the mirror’s radiation would decrease as the square of the distance, so it would have to start out extremely powerful in order to deliver any appreciable amount of heat onto its target–even ignoring the effects of atmospheric diffusion.
A solar furnace is not subject to these limitations.