Comment by Razengan

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.

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.

Modern satellites, in particular Starlink, use optical transceivers that are identical to the ones used for the fiber optic links, with an addition of all the required parts to make them work across free space. So a link between a pair of satellites is very comparable to a single pair of optic fibers.

The cables can go the entire distance between the data centers. The satellites probably connect to their closest neighbors, and any long distance transmission would have to make multiple hops, and share the bandwidth with everything else in the system going through the same nodes.

Since there are hundreds of undersea cables, with a dozen fiber pairs or so each, the cable system has several orders of magnitude greater throughput compared to the current constellation of satellites.

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.

A submarine cable is a bundle of hundreds of strands, so you need quite a few satellites to replace one cable.