Re: Fobos-Grunt: analysis of orbit evolution

From: Bill Bard (
Date: Tue Nov 22 2011 - 11:14:28 UTC

  • Next message: Allen Thomson: "RE: Fobos-Grunt: analysis of orbit evolution"

    Looking at the plot, if I read it correctly the actual perigee has remained constant and the apogee has decreased. If this is correct, isn't this normal? I thought that drag at perigee would slow it down decreasing the apogee. What am I missing? Perhaps the models have some generic problem?
    Bill Bard
    On Nov 22, 2011, at 1:53 AM, "Ted Molczan" <> wrote:
    > 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
    > reports?
    > Ted Molczan
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