RE: Fobos-Grunt: analysis of orbit evolution

From: Ted Molczan (
Date: Tue Nov 22 2011 - 06:53:22 UTC

  • Next message: Greg Roberts: "Optical 21 Nov 2011"

    The unusual evolution of the orbit of Fobos-Grunt (11065A / 37872) continued through 2011 Nov 21 UTC. Below is my
    updated plot, using the methodology described earlier:
    The plots now include the SPG4-propagated orbit, in addition to the STOAG, to make clear that both are in substantial
    agreement on the expected evolution, and in disagreement with the actual, as revealed by USSTRATCOM's TLEs. Also, I have
    added a plot of the mean eccentricity, which clearly shows that the orbit has circularized much more rapidly than
    expected due to drag alone. The mean eccentricity is as defined in SGP4-based TLEs, i.e. for argument of perigee = zero.
    In case anyone is wondering whether USSTRATCOM's TLEs are somehow in error, I can state that the trajectory data reduced
    from Paul Maley's recent time-stamped video is in close agreement with near-contemporaneous TLEs, in terms of time and
    track, but relative earlier and later TLEs, the track residuals are very large, even after allowing for Earth's rotation
    over the interval of the time residuals. This is mainly due to the ~35 percent faster than normal precession of the
    argument of perigee, as well as the aforementioned circularization, which I believe to be manifestations of the same
    phenomenon, probably due to a propulsive release of matter from the spacecraft, which may also explain the unusually low
    rate of decay - barely half the expected value, based on the object's dimensions and mass.
    If anyone is aware of a natural phenomenon not modeled by SGP4 or STOAG, that could account for the evolution of the
    orbit, I would be interested to see the results of supporting calculations.
    Finally, the list received word overnight of an observation of F-G from San Francisco, apparently the pass of Nov 22 at
    01:44 UTC, during which it was flashing with a period of variation of about 20 s. Have there been any other similar
    Ted Molczan
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