Newsgroups: sci.astro, sci.physics, sci.mech.fluids, sci.engr.mech,
sci.space.policy
From: "Robert Clark" <rgregorycl...@yahoo.com>
Date: 28 Mar 2005 12:52:00 -0800
Subject: "Rockets not carrying fuel" and the space tower.
http://groups-beta.google.com/group/sci.astro/browse_frm/thread/ab0c8...

 Then this could be used to surround ground based scopes with a shroud
that reaches up at least to the stratosphere, say 100,000 ft. The
density of the air at 100,000 ft. is about 10 mbars and is above
weather variations, which occurs in the lower troposphere. It is also
above most water vapor, important for infrared observations.
 In the "Rockets not carrying fuel and the space tower" thread I noted
that atmospheric pressure decreases exponentially with altitude for a
constant cross-sectional column of air. Therefore to have the pressure
remain constant with altitude you could have the tower around the scope
have a cross-section that decreases exponentially with altitude at this
same rate. Then the tower could be open to the atmosphere at the top
since the pressure equilibrium will prevent air from rushing down to
the bottom of the tower.
 A problem though is the exponential decrease in pressure with altitude
only holds at constant temperature. It may be necessary to have the
tower be heated with a temperature gradient that changes with altitude
to maintain this. Also, since the density of the air at 100,000 ft. is
only about 10 mbars  so it might be possible to have an atomically thin
transparent cover at the top to prevent air flow down the tower. Then
this thin cover would only minimally reduce light collection and cause
only minimal distortion.
 Another possibility is that such an atmospheric envelope would allow
arbitrarily large liquid mirrors to be constructed on Earth. A key
problem with liquid mirrors is the wind they kick up due to the
rotation distorts the surface:

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."
http://www.universetoday.com/am/publish/pristine_view_universe_moon.h...

 Noted telescope maker Roger Angel also suggests magnetic bearings be
used to solve another problem of large liquid mirror scopes, supporting
the rotating structure with minimal friction:

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