Fobos-Grunt: decay estimate

From: Ted Molczan (
Date: Wed Dec 28 2011 - 16:15:22 UTC

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    The mean rate of decay of the semi-major axis of the twenty-four (24) USSTRATCOM TLEs from epoch 11357.54351775 to
    11362.60588000 (2011 Dec 23 13:023 - Dec 28 14:32 UTC) was 1.7179 km/d. 
    Using the STOAG propagator and actual space weather during that period, I found that the mean rate of decay could be
    accounted for using A/m = 0.0011873 mē/kg and Cd = 2.2.
    Using those values with predicted space weather, STOAG propagates the orbit to decay on 2012 Jan 15. Estimated
    uncertainty is +/- 4 days, based on the rule of thumb of 20 percent of the time remaining to decay.
    Below is the plot of estimated A/m since the orbit manoeuvres ceased, through Dec 25. Most of the points are at ~2 day
    intervals, and typically span the preceding ~2 day period. For Cd = 2.2, the mean A/m was 0.0012534 mē/kg; those values,
    with predicted space weather, would advance the estimated date of decay by about one day, to Jan 14.
    I extracted the actual and predicted space weather data required by STOAG from data provided by, which
    compiles data issued by official sources (mainly NOAA) and presents it in a convenient format:
    Ted Molczan
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