In this post I suggested a low-cost means to have permanently erected space towers:
Newsgroups: sci.astro, sci.physics, sci.mech.fluids, sci.engr.mech, sci.space.policy From: "Robert Clark" Date: 28 Mar 2005 12:52:00 -0800 Subject: "Rockets not carrying fuel" and the space tower.
A Pristine View of the Universe... from the Moon. "On Earth, LMTs are limited in size to about 6 meters in diameter because the self-generated wind that comes from spinning the telescope disturbs the surface. Additionally, like other Earth-based telescopes, LMTs are subject to atmospheric absorption and distortion, greatly reducing the range and sensitivity of infrared observing. But the atmosphere-free moon, Angel says, provides the perfect location for this type of telescope while supplying the gravity necessary for the parabolic mirror to form."
A Pristine View of the Universe... from the Moon. "One of the challenges in developing an LMT on the moon is to create the bearings to spin the platform smoothly and at a constant speed. Air bearings are used for LMTs on Earth, but with no air on the moon, that is impossible. Angel and his team are looking at cryogenic levitation bearings, similar to what's used for magnetic levitation trains to get a frictionless motion by using a magnetic field. Angel added, "As a bonus, with the low temperatures on the moon you can do that without expending any energy because you can make a superconducting magnet that allows you to make a levitation bearing that doesn't require a continuous input of electrical power." "Angel called the bearings a critical component of the telescope. "With no air on the moon to create wind, there's no limit to size or reaching the accuracy that you require as long as the bearing is alright," Angel said."
Then such scopes could be arbitrarily large, 100's of meters or even kilometers across, with a cost only 100th that of a solid mirror telescope of comparable size. Liquid mirror telescopes have the disadvantage that they must be zenith-pointing so can only view a few degrees of sky around the zenith. But since they are so low cost and simple to construct we could build many at differenr latitudes to be able to view most of the sky.
Bob Clark