[8cvor6j844qw_d6]

> relatively easy to redesign as weapons

There is a fiction I've read years ago that mentioned satellites becoming makeshift weapons by overheating exposed objects (think reactors, gas trucks, oil refineries) by acting as a solar furnace [1] via mirrors.

Not sure/don't recall how it deals with practical issues such as clouds and distance/intensity, but good enough for a story I guess.

[1]: https://en.wikipedia.org/wiki/Solar_furnace

[appplication]

> The temperature at the focal point may reach 3,500 °C

I thought this was interesting because it doesn’t really seem like an applicable top level claim, surely this is referring to a specific furnace, not all solar furnaces?

Then this got me thinking if there is some universal upper bound constraint to these temperatures. E.g. if I recall a telescope can’t make a source object brighter than it actually is, and this just seems like a thermal telescope, so I wonder if that principle applies here or not.

[ben_w]

> I wonder if that principle applies here or not

It applies, but also in practice the maximum temperature is lower than the theoretical upper bound.

https://what-if.xkcd.com/145/

[exe34]

I'm confident that the physics is correct, but I hate when things are translated for the lay public:

> "In general, there's no way to "overlay" light beams on each other, because the whole system has to be reversible".

Behold, my two torches.

[appplication]

Oh funny, my intuition when I was writing that was “there’s probably an xkcd what if about this”, but I imagined it be about surrounding the sun with mirrors. Same idea though in the end

[cobbal]

You may have been thinking of this one instead https://what-if.xkcd.com/141/ . Maybe we should put Randall on the supervillain watchlist.

[cyberax]

You can't focus sunlight at the satellite distances. And this is a fundamental problem, the focal distance varies for each wavelength so your focus point will be smeared. You need monochromatic light for that (a laser).

Edit: and also don't forget that the Sun is not a point source and has an appreciable angular size, further making it impossible to focus it with a reasonably-sized lens or mirror.

[echoangle]

I see your point for lenses but how would that happen with mirrors?

[cyberax]

Yeah, you are correct. The mirrors are not affected by the wavelength, though they are affected by the non-zero angular size of the Sun.

There are some non-linear effects that _are_ dependent on the wavelength, but not important to matter here.

[semi-extrinsic]

Anything that can explode if overheated in a refinery is covered with passive fire protection. This is designed to withstand jet fire exposure, meaning a heat load of at least 1100 °C and at least 250 kW/m2. Ordinary sunlight at peak is about 1 kW/m2, so your mirror needs to cover 250x the visual extent of the sun in the sky.

[kristjank]

007: Die Another Day has it as a main plot point.

[giantg2]

Probably easier to use a MASER

[adrian_b]

Unlike for light, in the microwave frequency domain it is easy to make very powerful microwave sources by other means than by using masers, i.e. with various kinds of vacuum tubes.

Masers are not useful as powerful transmitters, but only as amplifiers with a very low noise or as generators with a very stable frequency.