On Earth, it's hard to build due to much higher gravity, higher required orbit, and the atmosphere. Getting much shorter lengths of much less strong fiber to Moon may be more realistic than 20.000 km of super strong carbon fiber to GEO.

I'm not too clued up on the topic, but I was under the impression that a lunar elevator would potentially require much longer cables than on Earth (primarily due to the lack of spin of the moon). From the Wikipedia article [1]

> There are two points in space where an elevator's docking port could maintain a stable, lunar-synchronous position: the Earth-Moon Lagrange points L1 and L2. L1 is 56,000 km away from the Earth-facing side of the Moon, (at the lunar equator) and L2 is 67,000 km from the center of the Moon's far side, in the exact opposite direction <snip />

> Both of these positions are substantially farther up than the 36,000 km from Earth to geostationary orbit. Furthermore, the weight of the limb of the cable system extending down to the Moon would have to be balanced by the cable extending further up, and the Moon's slow rotation means the upper limb would have to be much longer than for an Earth-based system, or be topped by a much more massive counterweight. To suspend a kilogram of cable or payload just above the surface of the Moon would require 1,000 kg of counterweight, 26,000 km beyond L1.

So, we're talking an ~80,000 km one?

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

This is correct, unlike my assumption of a shorter cable above. OTOH due to lower gravity, the tensile strength required is still much lower. To quote the very article you lonked:

> the engineering requirements for constructing a lunar elevator system can be met using currently available materials and technology.

So what you are saying is that a space elevator is theoretically possible. I think that's step 0 in building something. Being theoretically possible is the most basic prerequisite.

Just because something is theoretically possible doesn't mean it's feasible or practical to build in real life. For example, we have known that it's theoretically possible to build vacuume sealed trains, aka the hyperloop, since before the first airplane took flight. And yet whilst you see planes fly everywhere, you don't see any vacuum sealed trains.

A lunar space elevator is feasible with today's materials (just a whole lot of them, way too much for current technology to transport to the moon). Here's a nice talk by Markus Landgraf, a mission planner at ESA, at 33c3: https://media.ccc.de/v/33c3-8407-an_elevator_to_the_moon_and...

It's not just about the tensile strength of the materials. The dynamics of a lunar elevator necessitates and extraordinary amount of weight to be used as a counterweight nevermind the length of the cables themselves.

Just because something ticks the checkbox of being theoretically possible doesn't mean it's practical or feasible to build.

Maybe in a few hundred years we'll have the means to build something on that scale. It still takes $10k to put 1kg in LEO, imagine the cost of transferring to the Lagrange points.

Sure, the amount of materials needed exceeds what current technology can get even into LEO by several orders of magnitude. Nobody's proposing to actually build a lunar space elevator. All I'm saying is that a super-advanced civilisation could theoretically build one from the materials available to us today, which is not true for a space elevator from the Earth's surface.