Citation needed, for Facebook: A) spent billions of dollars on this specific problem (free space optical links in space), and B) couldn't solve it.

Given that there's systems that have successfully flown doing links from GEO to LEO (e.g. high angular rates again), using several year old conservative technology, it's not so bad.

There's a million little details, of course. Just conduction cooling for fast optical transceivers is going to be annoying in space, for instance.

SpaceX started a LEO system as ambitious as SpaceX for its time back in 2013. again, GEO to LEO is not LEO to LEO, and is also not moon to earth. there are no successful examples of 20+Gbps between LEO satellites. The million details is why SpaceX has yet to turn them on for production.

> SpaceX started a LEO system as ambitious as SpaceX for its time back in 2013.

I don't understand what this means. If you mean Facebook-- Facebook didn't drop billions of dollars in to free space optical comms.

> GEO to LEO is not LEO to LEO

Yes, GEO-to-LEO is worse in every way (assuming the LEO satellites are in the same inclination and have the approximate same orbital period):

* Longer link distances. (More path loss, worse link budgets)

* Higher peak angular rates for pointing.

LEO-to-Earth is mostly worse:

* Shorter link path, but atmospheric dispersion (More path loss, worse link budgets, plus things like multipath).

* Higher peak angular rates for pointing.

* Less demanding pointing precision due to shorter path, though.

The hard part isn't the optical comms in space. The hard part is fitting multiple precision-pointed transceivers into a tiny volume and mass budget.