Solidifying space mirrors in space?

[RGClark]

Tell me if this is feasible. Liquid mirrors may cost only 1/100th the
cost of a comparable solid mirror:

Friday, June 22, 2007
Molten Mirrors.
Liquid mirrors could enable more-powerful space telescopes.
By Katherine Bourzac
http://www.technologyreview.com/Nanotech/18961/page1/

A disadvantage is that they must be pointed up because they need
gravity pointing downward along their vertical axis to operate. Still
their simplicity and low cost is what led their being proposed to be
put on the Moon. The above article is actually about putting such a
mirror on the Moon because the liquid mirrors need gravity to
operate.
But couldn't we just form the parabolic shape of the mirror in space
by rotating a molten substrate while at the same time creating the
gravity by accelerating the mirror by a propulsion method? We would
then let the mirror cool so that we would wind up with a solid mirror
that no longer needed to be rotated or accelerated to hold its shape.
The advantage of this is that after the acceleration is cut off the
mirror would be in zero gravity and therefore would not have to have
the thickness required to hold its shape as for mirrors on Earth. Then
we might be able to get mirrors of much greater size then for current
Earth bound mirrors. We could also then point it in any direction
because it would be a solid mirror.
I was thinking about this first for glass mirrors since rotating
molten blanks is how large mirrors on Earth are currently formed:

Making a Giant Mirror to Scour the Skies.
by Ted Robbins
All Things Considered, July 27, 2005.
http://www.npr.org/templates/story/story.php?storyId=4773461

As described in this article, the building holding the mirrors is two
stories tall and the glass weighs 20 tons. However, it may be this can
be shrunk in the zero gravity environment of space. The glass has to
be heavier for an Earth mirror because it has to hold its shape after
the rotation and after it is allowed to solidify. This wouldn't be the
case for a space mirror so its mass would be much less. Therefore the
structure holding it probably also could be much smaller.
However, as indicated in this article you need three months for the
glass to solidify so you would need to provide the acceleration for
this length of time. However, it probably is the case you could make
the acceleration much smaller than 1 g for this to work. On the Moon
for instance it's only 1/6 g. Still though you would need a great deal
of power for the heating elements at the temperature required to keep
the glass melted.
Instead could we just use mercury for the substrate? The temperature
could be even less than 0 C for the mercury to become liquid. Then
when we cut off the heat the mercury would rapidly solidify in the
cold of space, presumable maintaining it's parabolic shape in zero g.
So you wouldn't have to provide the acceleration for a great length of
time, perhaps only hours or days.
A couple of problems. If the mercury were exposed directly to space
at near zero pressure it might boil or evaporate off despite the cold
temperature. So you might have to provide some background air pressure
for it. You could have a very thin transparent cover to maintain the
air pressure. Likely the pressure required would not have to be very
high so we could make the cover very thin. Also, if you pointed the
mirror too close to the Sun the mercury would rise in temperature
again to melt. You would avoid this but avoiding looking in the Suns
direction during observations. This is not that severe a limitation.
Hubble has to do the same thing because of its sensitive optics.
A potentially severe problem though is whether or not the mirror
would need polishing after it solidified. The glass mirrors for
example require a year of polishing after they solidify. It's not
clear if the mercury mirrors would require polishing after they
solidify. They obviously don't require it as liquid mirrors on Earth.
It may be possible to do the polishing using some type of automated
nanometer-scale deposition method. For instance, this method allows
deposition at 100 nm accuracy:

Versatile Nanodeposition of Dielectrics and Metals by Non-Contact
Direct-Write Technology.
Author(s):
H.D. Wanzenboeck, H. Langfischer, S. Harasek, B. Basnar, H. Hutter,
and E. Bertagnolli
Vienna University of Technology
Austria
http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=2587&DID=118545&action

Using the recently developed "superlenses" it might be possible to do
better than this since they allow microscopy at subwavelength
resolution.

Bob Clark
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[mgb_phys]

It is difficult to generate accelaration - sustaining 1g would need an awful lot of fuel, most attitude adjustment rockets work in milli-g.
With a liquid mirror you still need to hold the liquid in a tank while it forms and heat it to keep it liquid which in space would need a lot of insulation.

There was a plan to have an inflatable mirror - a plastic balloon which would be expanded to the correct shape by compressed gas/exposive and then the inside coated with a reflective material and it cut in half to create a mirror.

 

[tacman]

Thank you for your detailed response and for providing additional resources on the topic of using liquid mirrors in space. It seems that there are some potential advantages to using liquid mirrors, such as cost and size, but there are also challenges that need to be addressed, such as maintaining the mirror's shape and potential polishing requirements. It is certainly an interesting idea and one that could potentially be explored further. However, more research and testing would be needed to determine its feasibility and practicality for use in space.