This question is a follow-up to Do bigger telescopes equal better results?

How much bigger does a ground-based mirror have to be to match what a space-based one can do?  I guess I'm asking primarily for visible light, but I'm interested in general too.

I guess on the ground, you're safe from micrometeorites, so it will probably last longer.  At what point does it become cheaper to build a telescope on the moon or something?

It's cheaper.

(1) With adaptive optics you can get 0.1 arc second resolution on the ground (admittedly only on a mountain top with particularly good air flow, but still!). This eliminates one of the major advantages of space until you get above several meters mirror diameter.

(2) Rocket fairings are the shrouds which protect payloads during the supersonic atmospherics speeds reached during launch. A 5 meter fairing is about the largest that can be flown, which limits the size of the one-piece mirrors which can be launched.  (The Dreaded Webb Telescope's mirror is in pieces which will assemble themselves in space -- a very scary and very expensive piece of design.)

(3) Servicing a telescope on the top of Mauna Kea or in the high Chilean Andes is a difficult and expensive process. Servicing a telescope in orbit makes that look like small change.  (Cost comparable to the cost of building a new giant scope on Earth.) And in-orbit servicing can't even be done with current technology except in low earth orbit.

(4) While high resolution is one frontier in astronomy, going deep is another, and going deep requires big mirrors.  A 30 meter mirror on Earth gathers much more light than a 5 meter mirror in space.  The giant terrestrial telescopes simply do a better job of being light buckets for spectroscopy than anything we can yet put into space.

The bottom line is that with the development of adaptive optics, space-based telescopes of currently buildable and launchable size lost their main advantage over ground-based telescopes.  And since they're 10x to 100x the cost, they are simply not worth building for many purposes.

Space based telescopes still hold a significant edge in parts of the spectrum blocked by the atmosphere such as UV and IR (Webb), and for certain tasks involving long-term high accuracy photometry (Kepler) and astrometry (Gaia).  But for general purpose use, the balance seems firmly on the ground's side for large telescopes.

This will change if space flight becomes cheaper -- the SpaceX BFR, for example, with its 9 meter fairing and dramatically lower launch costs, offers great hope for space telescopes.
