Re: What factors determine the brightness of a satellite flare?

From: Robert Clark (bobbygc2001@yahoo.com)
Date: Sun Jun 28 2009 - 11:44:57 UTC

  • Next message: Ted Molczan: "RE: What factors determine the brightness of a satellite flare?"

    --- On Sun, 6/28/09, Robert Clark <bobbygc2001@yahoo.com> wrote:
    > From: Robert Clark <bobbygc2001@yahoo.com>
    > Subject: What factors determine the brightness of a satellite flare?
    > To: seesat-l@satobs.org
    > Date: Sunday, June 28, 2009, 5:17 AM
    > 
    >  I copied below a post to sci.astro about the possibility
    > of sending naked-eye-visible light communications from a
    > Google Lunar X Prize entrant. It's based on the brightness
    > seen with some satellite flares. 
    >  What I need to know is what are the factors that
    > contribute to the brightest satellite flares since then we
    > could incorporate these facts into the design of the
    > reflective surfaces at the Moon to produce the brightest
    > images.
    > 
    >
    
    
      I forgot to attach that sci.astro post. Here it is:
    
    
    ===============================================================
    Newsgroups: sci.astro, rec.radio.amateur.space, rec.radio.amateur.antenna, sci.astro.seti, sci.physics
    From: Robert Clark <rgregorycl...@yahoo.com>
    Date: Sat, 27 Jun 2009 23:54:14 -0700 (PDT)
    Subject: High data rate space transmissions through visible light communication.
    
     I had been thinking about methods of high data rate transmission in
    regards to getting *video* transmissions from Mars orbiter missions. I
    was irritated by the spotty coverage of the Mars surface at the best
    resolutions so I wanted to send real-time *continuous* imaging back to
    Earth receiving stations at the highest imaging resolutions. This
    would require very high transmission rates, much higher than what is
    currently used.
    The idea would be to use light transmissions but only of the on-off
    variety. You would use a large surface, many meters across, capable of
    being alternatively lit up and darkened. There are computer chips of
    course capable of operating at Ghz rates. This would determine if the
    large surface was lit up or not electrically, possibly by using a
    material whose reflective properties can be changed electrically.
    I was worried though about the twinkling seen in point sources, which
    this would appear to be, such as with stars due to atmospheric
    effects. So this might require the telescope(s) to be in Earth orbit.
    The question I had though was whether the atmospheric distortion would
    cause an "on" signal to appear "off" and vice versa? My understanding
    of atmospheric distortion is that it causes the point source to be
    constantly apparently undergoing small shifts in position. But this
    wouldn't be a problem if what you want to determine is whether it is
    on or off. If that is the case then ground based telescopes would
    work.
    In the large reflecting surface, I actually wanted to use separate,
    say, squares on the reflecting surface that could be put separately in
    the on-off position to increase the information transmission rate. But
    that would require being able to distinguish the squares from Earth
    millions of kilometers away. This is why I wanted to use light rather
    than radio for this since the larger wavelengths in radio would make
    the reflecting surface impractically large for diffraction limited
    resolution.
    Even with light you couldn't do this with a single telescope. They
    would have to be widely separated. Combining the signals from widely
    separated scopes is common in radio astronomy but is not nearly as
    successful in optical astronomy. That is because the light wavelengths
    are so much smaller and you would have to have nanoscale accuracy in
    positioning the widely separate mirrors in relationship to each other.
    However, in the case of just detecting an on-off signal this shouldn't
    be as big of a problem as you're not trying to form a usable image,
    but only trying to see if a particular location is on or off. You
    would need though highly accurate timing synchrony between the
    separate scopes, within nanoseconds, to be sure they are detecting the
    same on-off square. Note also here that the shifting in the image due
    to atmospheric distortion very definitely would be bad for using
    ground based scopes.
    
     It occurred to me this might be a means of acquiring advertising
    support for a Google Lunar X Prize entrant. I had also been trying to
    come up with a method of having an illuminated image either on the
    Moon or in lunar orbit that would be visible to the naked eye on
    Earth. Such an idea was discussed here:
    
    moon advertising.
    put a billboard on the moon.
    http://www.halfbakery.com/idea/moon_20advertising
    
     I wouldn't be in favor of doing this in a way that would actually
    advertise a product. But I was thinking about it as a way of sending a
    message in favor of, for example, world peace. In this case you could
    still have advertisers who could say in TV commercials for example
    they supplied funding to support the mission and the message.
     BTW, I would be in favor of advertisers who could pay to have
    advertising signs set up at the rover landing site so that if anyone
    who wanted to log on to the the rover transmissions or who watched a
    TV program on the rover transmissions would see the ads. This to me is
    something different than an ad that someone would be forced to see
    just by looking up at the Moon.
     In any case you would need something large enough so that with naked
    eye resolution at the lunar distance it would still be
    distinguishable. This page gives the naked eye resolution at the lunar
    distance:
    
    Purpose of Building Telescopes.
    http://www.astronomy.org/astronomy-survival/telepur.htm
    
     According to this page the resolution of the human eye at the lunar
    distance would be about 22 miles. One single object clearly couldn't
    do this. However, if you had separate illuminated landers or orbiters
    at this large distance apart they could be used to send a message
    visible to the naked eye on Earth.
     It could work with orbiters by the example set of satellite formation
    flying by the Cluster mission:
    
    Cluster mission.
    http://en.wikipedia.org/wiki/Cluster_mission
    
     I also needed to find how large a brightly illuminated surface needed
    to be at the lunar surface to be visible by the naked eye on Earth. I
    thought of the example of the "Iridium flares":
    
    Satellite flare.
    http://en.wikipedia.org/wiki/Satellite_flare
    
     The Iridium satellites have 3 antennas that happen to be also
    reflective in visible light, totaling 4.8 m^2 in area. According to
    the Wikipedia page, the flares can be up to -8 in apparent magnitude,
    though typically at +6 magnitude, and are produced by an individual
    antenna, so by one of area 1.6 m^2.
     I'll assume the brightest flares are produced just by the orientation
    the antennas happen to be in so we could make our reflective surfaces
    be oriented with respect to the Sun to get the greatest brightness.
    For the same size surface, the brightness would be lessened by the
    greater distance to the Moon. The Iridium satellites are at about 780
    km altitude so the Moon is about 500 times further. This would lower
    the brightness by a factor of 500^2 = 250,000.
     This page gives the apparent brightness commonly visible by the naked
    eye in urban areas as +3:
    
    Apparent magnitude.
    http://en.wikipedia.org/wiki/Apparent_magnitude
    
    The 250,000 times lesser brightness at the lunar distance for an
    Iridium sized reflective surface would give it a +13.5 higher apparent
    magnitude so up to +5.5 in apparent magnitude. To make our reflective
    surface be at +3 apparent magnitude we could make the area be 10 times
    larger, so at 16 m^2 area, or a square 4 meters across.
     We would need a method for a flat reflecting surface of unfolding it
    to this size. It might be easier instead to have the reflecting
    surface be a balloon inflated by stored gas. Since this would be in a
    vacuum, you wouldn't need much gas pressure or mass to accomplish
    this.
     Another consideration is that because of the brightness of the Moon
    it could swamp out our illuminated surface. For the orbiter, this
    could probably be alleviated by having the orbiter have a highly
    elliptical orbit, (this also would be beneficial in minimizing the
    required delta-v and fuel load) then it would be visible at the higher
    distances from the Moon in its orbit. For the landers it might work
    for them to land in the dark lunar maria.
    
     To communicate the message though we would need a method to turn on
    and off the reflecting surface. One possibility would be to have the
    reflecting surface consist of very many small squares that could be
    rotated to reflect toward the Earth or away. Another possibility might
    be to have it covered with LCD's. Whichever method it would have to be
    both lightweight and low power.
     For our first attempts we probably would not want to send so many
    orbiter or landers at once to form a naked-eye visible image. We would
    first send just a single one to test it out. Note that this method
    with a single vehicle could still be used to send high definition
    video by having our single reflective surface be turned on and off at
    the required rate, about 256,000 times per sec with compression.
    
    
         Bob Clark 
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