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Solidifying space mirrors in space?

  1. Sep 16, 2007 #1
    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

    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
    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.

    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.
    H.D. Wanzenboeck, H. Langfischer, S. Harasek, B. Basnar, H. Hutter,
    and E. Bertagnolli
    Vienna University of Technology

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

    Bob Clark
    Edit/Delete Message
  2. jcsd
  3. Sep 17, 2007 #2


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    Science Advisor
    Homework Helper

    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.
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