Hello "to" the ISS

From: Tom Wagner (sciteach@mchsi.com)
Date: Sat Mar 08 2003 - 15:54:31 EST

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    Please excuse what may be an off topic topic.
    Two things....
    A while back I asked a question about an astronaut saying "hello" from the
    ISS using a signal mirror. Well what about the other way around?
    We can see flares (reflections of sunlight) from silver parts of satellites
    (not true mirrors), some even in a sunlit sky. Now, can anybody estimate how
    large of a rear-surface mirror would be needed by a person on the ground to
    effectively signal an observer on the ISS? Apparently the ISS windows are
    often (if not always) in a position to see such an event.
    I'm sure there are a number of variables to consider. For example:
    (1) The mirror would need to be in daylight which would position the moving
    ISS in a sunlit sky so it could not be seen directly, at least not at this
    (2) The ground itself---like the bright sky---would lower the contrast of
    the light so the bigger the mirror the better.
    This problem of contrast would limit any misalignment on the part of the
    astronaut doing the watching. Foveal vision might be necessary and is not
    large by any means. For example, off the Net: "While the normal field of
    vision for each eye is about 135 degrees vertically and about 160 degrees
    horizontally, only the fovea has the ability to perceive and send clear,
    sharply focused visual images to the brain. This foveal field of vision
    represents a small conical area of only about 1 degree." I know that if a
    daytime iridium flare is not within my foveal vision I often miss it.
    (3) It would be challenging to place a large mirror at an angle that would
    result in an acceptable alignment. One would NOT need to move the mirror
    during the event, just get it positioned accurately. [I can only imagine
    what the neighbors might think while watching me in my yard putting blocks
    of wood and whatever else I can come up with under a large mirror. Then to
    have me tell them, "Oh I'm just setting up to signal a guy on the Space
    Station."]  :~)  Mounting a shutter to make it blink would make it more
    In contrast to my usual way of thinking, the placement of the Station near
    the sun would be at an advantage rather than a disadvantage, correct?
    Now a question that I would like to get answered is, has any astronaut on
    any orbiting vehicle ever seen a "flare" from something manmade on the
    ground? From orbit w/o optical aid isn't it difficult to see evidence of
    man's existence at all? A sharp reflection then would be evidence of
    something artificial.
    The other thing....
    How bright of an incandescent light (or laser) would a person in a dark area
    need to signal an observer on a high pass of the ISS? This would I'm sure
    depend upon the light pollution that the person is embedded in at the time.
    Again, a matter of contrast.
    Thanks and clear skies,
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