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Depends on a lot of things, but most undersea cables have several fibers (typically around 10 fibers), and each fiber runs many wavelengths (typically 40 or 80). Whatever your bandwidth is per wavelength (there are lots of options, at different costs, up to maybe 400gbps) that's a huge amount of bandwidth to replace.

Space lasers probably don't do multiple wavelengths, and aiming a lot of laser transceivers is going to be challenging.

> Alternatively, is there a wavelength of light to which water is completely transparent? Can we shoot lasers of that frequency through open water instead of through cables?

Even if you could, that's only going to work for short hops, long distance fiber includes repeaters every so often (50-100 km) to deal with loss, it would be tricky to build those under the sea ... and you need to power them, which means a cable, might as well include fiber with your power cable.

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Starshield (military arm of SpaceX/Starlink) does this for satellite imaging.

Normally in imaging, you first need to get the satellite over the location you want to image, at the right time - then let it orbit over a downlink location.

There are companies making a lot of money by having downlink capability in useful places (eg: near the poles) - combined with a fat pipe to the Internet.

With Starshield, the satellites talk to each other over laser - so the satellite doesn't have to wait to travel to a downlink location - just relay to of that is already there.

So for certain special cases it can help - but I think you'll find a cable stuffed with multimodal fiber had vastly more bandwidth.

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Someone else with more experience could chime in but I would put it at less practical due to latency (sats are very high up), bandwidth (one laser vs a bundle of 1000s of fiber optic cables), cost (to overcome latency you need a closer orbit which requires more sats), single point of failure, lifespan, intercepted communications, solar flares… and that’s just off the top of my head.
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I don't actually think latency would be that much worse.

Of course, it depends on the altitude of the satellite but speed of light in a fiber optic is only ~2/3rds of speed of light in a vacuum so you can afford some detour

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You probably can’t get under 500 km orbit, so 2000 km round trip is the shortest you can do between you and a ground based server. That’s 150 ms minimum round trip latency.
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> speed of light in a fiber optic is only ~2/3rds of speed of light in a vacuum

That made me wonder: Why can't we use vacuum cables?

(the sort of questions I should probably be bothering an LLM with.. someone should make a LMGPTTFY)

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If you figure out how to make cheap vacuum cables you will probably become very rich.

But otherwise chains of microwave links does get much closer to c in applications where you want absolute lowest latency (or much more commonly, when digging a fiber line is too expensive)

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> Why can't we use vacuum cables?

Hollow-core fiber is basically this!

> LMGPTTFY

"let me go prompt that for you" or "let me GPT that for you"?

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latency lower for satellites, trading firms have looked at this already, no idea if anyone has actually done it

bandwidth would indeed be worse

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starlink sats are some of the lowest orbiting satellites in space - 330-370km for v3. this helps latency as well as easy deorbit at end of life. the v3 sats have multiple 400 gbps lasers for meshing... I can't decide if your comment is sarcasm or not actually because starlink is fully functional and doesn't have any of those issues
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A submarine cable is a bundle of hundreds of strands, so you need quite a few satellites to replace one cable.
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Typical submarine cables at at absolute most 24 strands, more commonly 4, 6, 10 or 12 for modern ones. Terrestrial cables can easily be hundreds and that's quite mundane.

The high capacity in submarine fiber is achieved through DWDM and coherent modulation 100Gbps+ transceivers, not massive parallelism in fiber use.

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