RE: Phobos-Grunt video

From: Ted Molczan (
Date: Thu Jan 05 2012 - 02:34:53 UTC

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    Thierry Legault wrote:
    > hello, and happy new year 2012, to all members of the list!
    > This were a special new year's day morning for me, since I was at the
    > edge of the French Riviera Observatory to take the last passage of
    > Phobos-Grunt over France, after weeks of bad weather.
    > The video with all available shooting details is on this page:
    > There is no sign of tumbling, and a video taken 24 hours before in
    > close conditions shows PG in a similar orientation. The unexpected
    > thing that I realized when I looked carefully at the video is that PG
    > is moving backwards, with its solar panels deployed but at the
    > opposite of the Sun. It's not surprising that it had no energy to communicate!
    Thierry, congratulations on obtaining an excellent result, on a most challenging object.
    As you pointed out, F-G does appear to be oriented with the main propulsion module forward into the direction of travel.
    I suspect this behaviour may be similar to that of a shuttlecock: the nearly full fuel tanks at the main propulsion
    module end of the spacecraft are analogous to the cork of a shuttlecock; the solar arrays at the opposite end are
    analogous to the feathers. 
    Here is a thesis, "Passive Attitude Stabilization For Small Satellites", by Samir Ahmed Rawashdeh, based on using
    precisely this phenomenon to provide attitude control for a cubesat:
    See Fig. 4-1 on pg. 40 (p. 53 of the pdf), and also Fig. 4-2 on the following page. Then look at the results of the
    attitude simulation, in Fig. 7 on pg. 47. On the previous page, the author states:
    "Figure 4-7 shows the time response of the simulation. The angle relative to the velocity
    vector is plotted in blue, and the angle relative to the nadir vector is in green. It appears
    that the satellite begins to track the velocity vector within 3 hours. The plot of the angle
    to nadir also shows the satellite lining up with the velocity vector 90 degrees from the
    nadir vector (for a circular orbit)."
    I believe there is sufficient detail in your imagery, as well as in the documentation of the time and location of the
    observations, to support analysis with the aid of 3D visualization software, to get a better idea of the exact
    orientation of F-G with the respect to the velocity vector.
    Ted Molczan
    Seesat-l mailing list

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